help button home button Endocrine Society JCEM JCEM Call for Nominations for EIC
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a related Letter to the Editor
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow Request Copyright Permission
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Williams, N. I.
Right arrow Articles by Cameron, J. L.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Williams, N. I.
Right arrow Articles by Cameron, J. L.
The Journal of Clinical Endocrinology & Metabolism Vol. 86, No. 11 5184-5193
Copyright © 2001 by The Endocrine Society

Endocrine Care

Evidence for a Causal Role of Low Energy Availability in the Induction of Menstrual Cycle Disturbances during Strenuous Exercise Training

Nancy I. Williams1, Dana L. Helmreich2, David B. Parfitt2, Anne Caston-Balderrama3 and Judy L. Cameron

Departments of Psychiatry (N.I.W., A.C.-B., J.L.C.), Cell Biology and Physiology (N.I.W., A.C.-B., J.L.C.), and Neuroscience (D.L.H., D.B.P., J.L.C.), University of Pittsburgh, Pittsburgh, Pennsylvania 15213

Address all correspondence and requests for reprints to: Judy L. Cameron, Ph.D., Department of Psychiatry, University of Pittsburgh, 3811 O’Hara Street, Pittsburgh, Pennsylvania 15213. E-mail: cameronj{at}ohsu.edu

Abstract

Cross-sectional and short-term prospective studies in humans support the concept that low energy availability, and not other factors associated with exercise, causes the development of exercise-induced reproductive dysfunction. To rigorously test this hypothesis, we performed a longitudinal study, examining the role of low energy availability on both the development and the reversal of exercise-induced amenorrhea, using a monkey model (Macaca fascicularis). Eight adult female monkeys developed amenorrhea (defined as absence of menses for at least 100 d, with low and unchanging concentrations of LH, FSH, E2, and P4) after gradually increasing their daily exercise to 12.3 ± 0.9 km/d of running over a 7- to 24-month period. Food intake remained constant during exercise training. To test whether amenorrhea is caused by low energy availability, four of the eight amenorrheic monkeys were provided with supplemental calories (138–181% of calorie intake during amenorrhea) while they maintained their daily training. All four monkeys exhibited increased reproductive hormone levels and reestablished ovulatory cycles, with recovery times for circulating gonadotropin levels ranging from 12–57 d from the initiation of supplemental feeding. The rapidity of recovery within the reproductive axis in a given monkey was directly related to the amount of energy that was consumed during the period of supplemental feeding (r = -0.97; P < 0.05). Repeated measurements of plasma T3 concentrations, a marker of cellular energy availability, revealed a tight correlation between the changes in reproductive function and T3 levels, such that T3 significantly decreased (27%) with the induction and significantly increased (18%) with the reversal of amenorrhea (P < 0.05). These data provide strong evidence that low energy availability plays a causal role in the development of exercise-induced amenorrhea.

MANY FEMALE ATHLETES participating in strenuous exercise training develop various forms of reproductive dysfunction, including oligomenorrhea, amenorrhea, and luteal phase defects (1, 2). Over the past 2 decades, studies that have attempted to identify the mechanism(s) underlying exercise-induced reproductive dysfunction have examined a number of potential causative factors, including the physical stress of exercise, the psychological stress of competition, nutrient deficiencies, and low body weight or low body fat (1, 2). Although these factors may partially contribute to the cause of exercise-induced reproductive dysfunction, evidence accumulated to date indicates that negative energy balance (i.e. a chronic deficit in energy intake relative to energy expenditure) is the primary cause of exercise-induced impairment of normal reproductive function.

The concept that negative energy balance is tightly associated with a suppression of reproductive function originated from numerous studies in humans and a variety of animal species showing that decreases in food intake [both short-term (3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13) and long-term (14, 15, 16, 17, 18, 19, 20, 21)] lead to a suppression of reproductive hormone secretion and eventually to infertility after sustained periods of undernutrition (17, 18). Negative energy balance resulting from increased energy expenditure due to thermoregulation has also been shown to be associated with a suppression of reproduction (22). Moreover, studies in mice examining alterations in nutritional intake, thermoregulation, and energy required to search for food, have shown that negative energy balance caused by any combination of these factors is associated with a suppression of reproductive function (23).

The hypothesis that negative energy balance underlies exercise-induced suppression of reproductive function in female athletes was proposed by Warren (24), who reported a close correlation between the onset of menarche and temporary decreases in the volume of exercise performed by adolescent female dancers at times of injury or vacation. Studies aimed at estimating energy balance of young exercising women consistently find that energy deficits are greater in athletes than in sedentary women, and most (25, 26, 27, 28), but not all (29, 30), studies find that the energy deficits in amenorrheic athletes are even greater than those in eumenorrheic athletes. Additionally, many endocrine adaptations associated with exercise-induced amenorrhea are similar to those occurring in response to chronic caloric restriction and are thus reflective of a state of energy deficit (31, 32). Further support for this concept was provided by a prospective longitudinal study by Bullen et al. (33) showing that women who underwent strenuous exercise training and dieting for two menstrual cycles were more likely to show menstrual disturbances than women who only exercised. However, it is unclear whether the results of this study indicate that decreased energy availability caused by exercise led to menstrual disturbances or simply that exposure of women to multiple stresses leads to an increased probability of developing menstrual disturbances.

Although cross-sectional studies such as those discussed above support the concept that a negative energy balance may play a critical role in exercise-induced suppression of reproductive function, such studies only indicate a correlation between negative energy balance and reproductive dysfunction. Such studies do not provide firm evidence that negative energy balance causes exercise-induced reproductive dysfunction. A secondary concern in interpreting the existing cross-sectional studies is the potential for self-selection of individuals with a preexisting susceptibility to reproductive disturbances in studies examining exercise- induced reproductive dysfunction.

To further elucidate the mechanisms underlying exercise-induced menstrual disturbances, we first developed a nonhuman primate model of exercise-induced amenorrhea in which we could randomly assign individuals to the exercise regimen and in which we had complete control over the exercise schedule and dietary energy intake throughout experimentation. Using this monkey model, we recently characterized the progressive changes in reproductive hormones as exercise-induced amenorrhea developed in a group of female monkeys (34). In the current study we examined the role of low energy availability in the development of exercise-induced reproductive dysfunction by conducting two experiments. First, we tested whether reproductive suppression is due to the increased energy utilization occurring with daily chronic vigorous exercise or to some other aspect of exercise by determining whether increased energy intake could reverse the state of exercise-induced amenorrhea. Second, we examined the relationship between changes in plasma levels of the metabolic hormone, T3, and changes in reproductive function during the development and subsequent reversal of exercise-induced amenorrhea. Our findings indicate that low energy availability associated with strenuous exercise training plays an important role in causing exercise-induced amenorrhea.

Materials and Methods

Animals

Eight adult female cynomolgus monkeys (Macaca fascicularis), weighing 2.10–3.45 kg, were used for this study. Monkeys were housed at the University of Pittsburgh Primate Research Laboratory in individual cages and were maintained on a controlled lighting schedule, with lights on from 0700–1900 h. Temperature was maintained at 24 ± 2 C. Animals in this study were maintained on the standard diet used at the Primate Research Laboratory, which consisted of a single daily meal of approximately 300 kcal (15 pellets) of Purina High Protein monkey chow (no. 5045, Ralston Purina Co., St. Louis, MO), supplemented with one quarter piece of fresh fruit (~25 kcal). The macronutrient content of the monkey chow is approximately 48% carbohydrate, 25% protein, and 5% fat by weight. This meal size was chosen because in our laboratory adult female cynomolgus monkeys generally maintain stable body weight on this level of food availability when they are housed in single cages. Not all monkeys ate the entire number of calories provided on a daily basis (calorie intake at various points in the study is presented in Tables 1Go and 2Go). However, some monkeys did eat all of the food provided each day (see examples in Figs. 3Go and 4Go). Monkeys were fed at 1600 h each day. Water was available ad libitum. Food intake was recorded daily by subtracting the number of pellets that were not consumed each morning (0700–0800 h) from the total number of pellets provided to the monkeys the preceding day. Body weight was monitored every other day. Monkeys also received novel items, such as toys or noncaloric foods on a regular basis, as part of a psychological enrichment program, in accordance with the USDA guidelines. All experiments were performed in compliance with the regulations of the animal care and use committee of the University of Pittsburgh.


View this table:
[in this window]
[in a new window]
  		Table 1. Changes in training, dietary parameters, and reproductive hormones during the reversal of amenorrhea in four monkeys

 

View this table:
[in this window]
[in a new window]
  		Table 2. Training, dietary, and hormonal changes during the development of exercise-induced amenorrhea in eight monkeys

 


View larger version (14K):
[in this window]
[in a new window]
  		Figure 3. Data from one monkey (no. 1887), showing changes in reproductive hormones, menstrual cyclicity, body weight, energy intake, and exercise in response to the overfeeding protocol in Exp 1.

 


View larger version (18K):
[in this window]
[in a new window]
  		Figure 4. The association among changes in energy intake, body weight, and plasma LH concentrations in one monkey (no. 1886) in response to the overfeeding protocol in Exp 1.

 
Blood sampling procedures

Blood samples for the measurement of serum LH, FSH, E2, and progesterone (P4) were collected from unanesthetized animals every other day. Samples were obtained before feeding and before the animals exercised. For collection of blood samples, each monkey was trained to jump from its cage into a transport box and enter a specially designed cage that allowed immobilization of the monkey’s leg, so a blood sample could be obtained from the femoral region by venipuncture. Blood was collected into sterile syringes, transferred into glass tubes, and allowed to clot. Samples were then centrifuged at 2500 rpm for 10 min, and serum was collected and stored at -20 C in glass vials until assays were performed. Every 6 wk, hematocrit was measured, and animals were given a 0.5-cc im injection of iron dextran. Hematocrits were maintained within the normal range in all monkeys throughout the study.

Monitoring reproductive function

Before the study all animals were accustomed to blood sampling procedures and daily checks for menses, which involved swabbing the vaginal area with a cotton-tipped applicator. The occurrence of several normal menstrual cycles was documented in each monkey before the initiation of the study. The first day of menses was designated the first day of the menstrual cycle. A menstrual cycle was considered normal if it was ovulatory, 25–35 d in length, and exhibited typical cyclic changes in reproductive hormones, including midcycle surges of LH and FSH and a rise in serum P4 concentrations during the luteal phase to levels greater than 2 ng/ml. A monkey was considered to be amenorrheic if she did not have menses for a period equivalent to at least three of her normal menstrual cycles (~90–110 d), and if she exhibited low, noncyclic levels of E2 and P4 with no evidence of ovulation.

Exercise training

Animals were trained to run on standard human size treadmills (model 910e, Precor, Inc., Bothell, WA). Each treadmill was covered by a Plexiglas box, which had numerous air holes in the front and back panels to allow adequate ventilation (34). Monkeys were slowly adapted to the treadmill by first being allowed to sit on the treadmill and explore it for several days and then being allowed to walk slowly. Speed and duration were then increased in an individualized manner until the animals were running approximately 8–16 km/d. When fully trained, monkeys ran 7 d/wk for a total of 2 h/d, with a 3-min break after each 30-min period of exercise. This training regimen was previously shown to lead to a high proportion of monkeys developing exercise-induced amenorrhea (34).

Hormone assays

Serum LH, FSH, E2, and P4 were measured by RIA at the RIA Core Laboratory of the Center for Research in Reproductive Physiology, University of Pittsburgh, using previously described methods (34, 35, 36, 37). The sensitivities of the LH assays ranged from 7.5–13.6 ng/ml, and the intra- and interassay coefficients of variation for the LH assays used in these studies were 7.1% and 9.2%, respectively. The maximum detectable dose was 50 ng/ml. Data points estimated to be above this level by the RIA analysis algorithm (38) were assigned a value of 50 ng/ml for statistical analyses. The sensitivities of the FSH assays ranged from 1.4–3.4 ng/ml, and the intra- and interassay coefficients of variation for the FSH assays used in these studies were 6.3% and 7.9%, respectively. The sensitivities of the E2 assays ranged from 2.1–4.2 pg/ml, and the intra- and interassay coefficients of variation for the E2 assays used in these studies were 5.1% and 7.6%, respectively. The sensitivities of the P4 assays ranged from 0.08–0.12 ng/dl, and the intra- and interassay coefficients of variation were 4.1% and 7.9%, respectively.

Experimental design

Exp 1: reversal of exercise-induced amenorrhea with increased food intake (overfeeding). This experiment was designed to test the hypothesis that the increased utilization of energy occurring with exercise, and not some other aspect of exercise, causes exercise-induced amenorrhea. We reasoned that if the primary cause of exercise-induced amenorrhea is the energy utilization occurring during regular vigorous exercise training, then it would be possible to reverse the amenorrheic state by providing an increased number of calories to a monkey while the monkey continued to exercise. We further reasoned that if some other aspect of exercise suppresses reproductive function, then providing increased energy intake while continuing the exercise regimen would not restore normal reproductive function.

For this experiment, four female cynomolgus monkeys, who had undergone strenuous exercise training and developed exercise-induced amenorrhea, were used. Monkeys had run 7–24 months at the time that amenorrhea developed (mean, 14.3 ± 2.2 months). Changes in reproductive hormone secretion with the development of amenorrhea in these monkeys have been previously published as part of a larger study characterizing changes in reproductive hormone secretion occurring with exercise training (34). Before the initiation of the current experiment, these four monkeys had each exhibited exercise-induced amenorrhea for the equivalent of at least three menstrual cycles (~90–110 d) and showed low noncyclic patterns of circulating LH, FSH, E2, and P4. For the present experiment each of these monkeys was provided with an increased number of calories every day (~400 extra kcal) while they continued their exercise regimen (running ~12 km/d). Supplemental food was provided at the regular meal time in the form of granola bars, dried fruit, fresh fruit, and commercial monkey chow. Monkeys ate a variable amount of the extra food each day, and exact daily food intake was tabulated and is presented in Table 1Go. Throughout the period of training and continuing during the period of training plus extra food intake (overfeeding), blood samples were collected every other day to monitor reproductive hormone secretion, and monkeys were checked daily for menses. The experiment was terminated when monkeys exhibited menses after the initiation of overfeeding. The other four monkeys that had developed exercise-induced amenorrhea in the previous experiment (34) were used in a different experiment and thus were not available for this study.

Exp 2: characterization of metabolic status by measurement of circulating T3 during the induction of amenorrhea and the reversal of amenorrhea with increased food intake. To test whether changes in reproductive function with exercise correlate with an index of energy availability, we retrospectively examined plasma T3 levels in eight monkeys over the period of time in which they developed exercise-induced amenorrhea [reproductive hormone data for these monkeys during the development of exercise-induced amenorrhea was previously published (34)]. We also measured T3 levels in the four monkeys used in Exp 1 during the period when they were overfed, and we observed a reversal of exercise-induced amenorrhea. We reasoned that if changes in energy availability are the primary mechanism by which strenuous exercise training causes reproductive dysfunction, then the timing of both the onset of reproductive dysfunction and the restoration of normal reproductive function would be tightly correlated with changes in metabolic status. We chose to measure plasma T3 concentrations as an index of energy availability because there is strong evidence that slight changes in energy balance are reflected by changes in circulating T3 levels for both humans and monkeys (8, 39, 40). To assess T3 levels, a subset of blood samples, collected every other day throughout these studies for measurement of reproductive hormone levels, was analyzed for T3.

Data analysis

Exp 1. For statistical analyses, the data for LH, FSH, and E2 were divided into three time periods (Fig. 1Go): 1) late amenorrhea represents the last 60 d before the first day monkeys began receiving extra calories; 2) early overfeed represents the time period from the first day of overfeeding (i.e. first day of receiving extra calories) to the day before either the preovulatory E2 surge (used for E2 and P4 analyses) or the day before the LH surge (used for LH and FSH analyses); and 3) surge represents the day of the LH and FSH surges (used for LH and FSH analyses) or the day of the preovulatory E2 peak (used for E2 and P4 analyses). Data for P4 were divided into these same three time periods plus a fourth time period, postsurge, which represents the period from the day after the LH surge until the day before menses.



View larger version (22K):
[in this window]
[in a new window]
  		Figure 1. Schematic diagram of the design for Exp 1. Data for reproductive hormone levels were analyzed during four periods of the experiment: late amenorrhea (the last 60 d of the amenorrheic period before the initiation of overfeeding), early overfeed (starting with the initiation of the overfeed period and ending the day before the first E2 surge), surge (the day of the E2, or the LH/FSH surges), and postsurge (starting with the day after the first LH surge in the overfeed period and ending with the first day of menses in the overfeed period). The times when LH and FSH were measured are indicated by the solid black bar at the top of the diagram, the times when E2 was measured are indicated by the horizontal-lined bar, and the times when P4 was measured are indicated by the cross-hatched bar. Serum T3 was measured in pooled samples at four times during this study: twice in the late amenorrhea period, at the beginning of the early overfeed, and just before the first LH surge in the overfeed period. For reference, schematic diagrams of LH, menses, E2, kilometers per d, and kilocalories per d over the course of this study are also shown.

 
Exp 2. The data for Exp 2 were examined using two separate analyses for the induction of amenorrhea and the reversal of amenorrhea with increased energy intake. Progressive changes in LH, FSH, E2, and P4 during the development of amenorrhea have been previously reported (34). To assess changes in circulating T3 during the development of amenorrhea, blood samples were pooled during the same six time periods over which we had previously analyzed LH, FSH, E2, and P4 (Fig. 2Go): during a control month when monkeys were sedentary (Sed), during the two menstrual cycles preceding the onset of amenorrhea (cycle 2 and cycle 1), and then at three points during amenorrhea (AM1, AM2, and AM3; each equivalent to the monkey’s average menstrual cycle length). To obtain a measurement of mean T3 levels during Sed, cycle 2, and cycle 1, the first three blood samples collected at the beginning of each menstrual cycle were pooled for the measurement of T3. During AM1, AM2, and AM3, three blood samples were pooled at intervals equivalent to the monkey’s average menstrual cycle length.



View larger version (13K):
[in this window]
[in a new window]
  		Figure 2. Schematic diagram of the design for Exp 2. Data for reproductive hormone levels were analyzed during six periods of the experiment: Sed (during the control period when animals were sedentary), cycle 2 (the second to the last menstrual cycle before the development of amenorrhea), cycle 1 (the last menstrual cycle before the development of amenorrhea), and AM1, AM2, and AM3 (periods during exercise-induced amenorrhea that are each equivalent to the average menstrual cycle length). T3 was measured in pooled plasma samples at the times indicated. Schematic diagrams of LH secretion, menses, and kilometers per d are shown for reference.

 
To assess changes in T3 during the reversal of amenorrhea with overfeeding, we measured T3 in pooled samples (three samples per pool) at four different time points: at two times during late amenorrhea (60 and 30 d before the first day of overfeeding), at the beginning of the early overfeed period, and just before the LH surge (see Fig. 1Go). In all cases data for serum T3 are expressed and were analyzed as percentages relative to Sed values for each monkey to control for the fact that there was considerable variability in serum T3 levels among individuals during the control phase of the study.

To quantify energy intake and daily exercise training during the induction of amenorrhea, kilocalories per d and kilometers per d were averaged for each animal over each of the six time periods described above, i.e. Sed, cycle 2, cycle 1, AM1, AM2, and AM3, for Exp 2 (Fig. 2Go). To quantify energy intake and daily exercise training during the reversal of amenorrhea, kilocalories per d and kilometers per d were averaged for each animal over the four time periods described above, i.e. late amenorrhea, early overfeed, surge, and postsurge, for Exp 1 (Fig. 1Go). To determine kilocalories per kg, the average kilocalories for each time period were divided by the average body weight for the corresponding time period.

Statistical analyses

To determine whether significant changes occurred over time in each experiment, one-way ANOVAs for repeated measures were performed on serum hormone concentrations and training and dietary parameters over each entire experiment. When a significant main effect was detected, t tests were performed, using the Bonferroni correction, to detect specific differences between the control sedentary cycle and each time period during training. To test whether there was a correlation between training and dietary parameters and the number of months it took to develop amenorrhea, correlation coefficients were calculated using simple regression analyses. The significance of r was determined using an F test (SuperAnova, Abacus Concepts, Inc., Berkeley, CA). All data are reported as the mean ± SEM.

Results

Exp 1

Table 1Go shows the mean training, body weight, energy intake, and hormone data during late amenorrhea and throughout the period of overfeeding until the occurrence of menses for the four monkeys in Exp 1. Overfeeding resulted in a significant increase (P < 0.05) in daily energy intake of approximately 58% compared with energy intake during AM2 and AM3. Mean body weight increased significantly during the overfeeding period compared with that during the period of amenorrhea (by ANOVA main effect of time, P < 0.04) by approximately 6%. LH, FSH, and E2 levels increased significantly from late amenorrhea to the surge (P < 0.003, late amenorrhea vs. surge), and postsurge levels of P4 were significantly greater than levels measured during late amenorrhea, early overfeed, and surge (P < 0.0001).

Two monkeys recovered very rapidly, with preovulatory LH surges occurring only 12 and 16 d, respectively, after the overfeeding period began. Figure 3Go shows the individual data from one of these animals. The other two animals required 50 and 57 d, respectively, before they experienced a preovulatory LH surge. In the two animals that recovered more quickly, energy intake during the period of overfeeding averaged 163% and 181% of their intake while they were amenorrheic, whereas in the animals that recovered more slowly, average energy intake during the period of overfeeding was 138% and 141% of their energy intake while they were amenorrheic. The rate of recovery of menstrual cyclicity was related to the energy intake; there was a significant negative correlation (r = -0.98; P < 0.024) between the calories consumed during the overfeeding period and the number of days until the LH surge. There was also a significant correlation between body weight during the AM3 period and the time to recovery, i.e. the lighter monkeys recovered more quickly than the heavier monkeys (r = 0.96; P < 0.05). Overall, body weight increased from 3–11% during the overfeeding period. An example of the relationship among serum LH, body weight, and energy intake during the recovery period in one monkey that recovered in 12 d is shown in Fig. 4Go.

Exp 2

Table 2Go shows the data for training, energy intake, and serum LH, FSH, and T3 during the development of exercise-induced amenorrhea in eight monkeys [the training and reproductive hormone changes have been previously reported (34) and are shown again here for comparison with the T3 data]. There were no significant changes in body weight or caloric intake over the course of training and the development of amenorrhea. Both LH and FSH levels decreased significantly over time (P < 0.05, by ANOVA main effect of time), with the highest mean plasma concentrations occurring during Sed, and the lowest occurring during AM1–AM3. T3 levels remained relatively stable during the time when the monkeys were training vigorously, but were not yet amenorrheic (cycle 2 and cycle 1). During the period when monkeys were amenorrheic, T3 levels decreased by approximately 20% relative to Sed levels and were significantly lower than the Sed values during AM2 and AM3.

The reversal of amenorrhea that occurred in all four overfed monkeys was accompanied by a concomitant increase in circulating T3 levels (see Table 1Go). Levels (expressed as a percentage of T3 levels for each monkey during the Sed period) increased significantly from 72.6 ± 9.6% during late amenorrhea to 118 ± 24% during early overfeed (P < 0.05 vs. late amenorrhea). Figure 5Go shows data from a representative monkey during both the induction and the reversal of amenorrhea.



View larger version (19K):
[in this window]
[in a new window]
  		Figure 5. Changes in plasma T3 concentrations during the induction of exercise-induced amenorrhea and the restoration of normal menstrual cyclicity with overfeeding in one monkey (no. 1886). Data are presented as the percentage of T3 values measured during the Sed period.

 
Discussion

The combined results of Exp 1 and 2 provide strong evidence that exercise-induced amenorrhea results from a state of low energy availability created by the increased energy expenditure of a vigorous and regular exercise regimen. In the first experiment we found that monkeys in a state of exercise-induced amenorrhea resumed normal menstrual cycles while receiving supplemental energy in the form of calories. We have not previously seen any spontaneous recovery of menstrual cyclicity in animals that have developed exercise-induced amenorrhea while they have continued their daily exercise regimen. As recovery occurred while the animals continued their daily exercise regimen, one can conclude that exercise-induced suppression of reproductive function is due primarily to the energy cost associated with exercise, rather than being the result of other exercise-associated factors, such as physical or psychological stress. If other factors played a major role in the development of exercise-induced amenorrhea, it seems unlikely that caloric supplementation alone would have been able to reverse the amenorrheic state. Further support for the concept that low energy availability causes exercise-induced disturbances in reproductive function was provided by the results of our second experiment, showing that both the development and the reversal of exercise-induced amenorrhea are tightly correlated with endocrine changes indicative of alterations in energy availability. Specifically, circulating levels of T3, an endocrine factor linked to metabolic rate, proved to be more closely correlated with changes in reproductive status than did body weight, or an index of energy intake such as kilcalories per kg BW. Levels of T3 were significantly decreased in association with the onset of amenorrhea and significantly increased when monkeys were provided with calorie supplementation that reinstated normal menstrual cyclicity. In contrast, neither body weight nor kilocalories per kg changed significantly with the onset of amenorrhea, and there was no correlation between these variables and the time to onset of amenorrhea.

In humans the results from several different types of studies have supported the hypothesis that a state of negative energy balance resulting from the increased energy expenditure of exercise plays a causal role in the development of exercise-induced disturbances in menstrual cyclicity. First, there have been numerous cross-sectional reports suggesting that exercising women consume far fewer calories than would be estimated for the maintenance of their body weight (25, 26, 41). Further, most (25, 26, 27, 28), but not all (29, 30), studies report that amenorrheic athletes consume even fewer calories than eumenorrheic athletes when they are matched for training level, size, and body weight. Although these studies collectively suggest that female athletes, and amenorrheic athletes in particular, are chronically exposed to a state of low energy availability, they have been criticized because of the propensity for study subjects to underreport food intake (31). Moreover, although such studies indicate a correlation between a state of low energy availability and an exercise-associated state of reproductive dysfunction, they do not establish a causal relationship between these states.

Second, prospective studies examining the short-term impact of low energy availability on circulating reproductive hormone levels provide evidence that the energy cost of exercise can result in a suppression of reproductive hormone secretion when exercise-induced increases in energy expenditure are not offset by supplemental caloric intake. Williams et al. (42) showed that a 21% reduction in midfollicular LH pulse frequency occurred after a short-term (4-d) increase in training, but only when subjects were fed fewer calories than were necessary to maintain body weight. In contrast, no change in LH pulse frequency occurred in the same subjects when they trained for 4 d but were given adequate food intake to maintain body weight. Similar results were subsequently reported by Loucks et al. (43). These investigators also showed that 1 d of increased feeding produced a significant increase in LH pulse frequency in subjects who exercised while being fed a diet that provided fewer calories than were necessary to maintain body weight (44). Although these short-term studies show that circulating levels of reproductive hormones respond to small changes in energy availability created by manipulating exercise, it cannot be assumed that similar mechanisms are operative when reproduction function is completely suppressed after chronic training.

A third source of evidence linking low energy availability to exercise-induced reproductive disturbances comes from Loucks et al. (29, 31) and Laughlin et al. (32, 45), who conducted several studies demonstrating differences in circulating levels of metabolic substrates and hormones among amenorrheic athletes, eumenorrheic athletes, and sedentary controls. Loucks et al. (29) reported higher 24-h circulating cortisol levels in amenorrheic athletes compared with both eumenorrheic athletes and sedentary controls. The two athletic groups in this study did not differ with respect to body composition, body weight, exercise training, scores on psychometric tests, or calorie consumption. In a similar study Loucks et al. (31) found that serum concentrations of free T3 and T4 were significantly lower in amenorrheic compared with eumenorrheic athletes. Laughlin et al. (32) have since characterized an overall hypometabolic state in both eumenorrheic and amenorrheic athletes compared with sedentary control subjects, but the magnitude of differences in circulating levels of metabolic hormones and substrates is greatest in the amenorrheic athletes. This group reported a decrease in body temperature, reduced plasma glucose, a decreased ratio of IGF-I/IGF-binding protein-1, an accelerated GH pulse frequency, increased insulin sensitivity, and elevated cortisol levels in athletes compared with control subjects. They conclude that the endocrine and metabolic profile of athletes is indicative of a hypometabolic state, i.e. a cascade of glucoregulatory adaptations to repartition fuels for conservation of protein. The findings from these cross-sectional studies indicate that endocrine adaptations similar to those occurring during chronic undernutrition occur as a result of exercise training and appear to be more profound in amenorrheic than in eumenorrheic athletes.

In addition to the above-mentioned cross-sectional studies describing the metabolic and endocrine characteristics of amenorrheic athletes, one prospective study has been performed in humans to test whether increasing food intake in combination with a reduction in training could reverse athletic amenorrhea. As reported in a case study and subsequently in three of four amenorrheic athletes, Dueck et al. (46) and Kopp-Woodroffe et al. (47) have purported to achieve a resumption of menses and ovulation after a 20-wk diet and exercise intervention. The latter consisted of a daily caloric supplement in the form of a sport beverage supplying 360 kcal and a reduction in training from 7 to 6 d/wk.

The results from the current study extend the findings of the aforementioned studies and substantially strengthen the argument that exercise-induced decreases in energy availability cause exercise-induced amenorrhea. The prospective nature of our study, with random assignment of individual animals to the exercise and control groups, avoided the potential problem of self-selection that has plagued the interpretation of many studies of exercise-induced reproductive dysfunction in human populations. More importantly, we were able to very accurately measure daily calorie intake and exercise so that we could be sure that exercise level was maintained constant during the period of time that energy intake was increased. Getting well trained athletes to agree to change their calorie intake is usually difficult because they are often training for competitive events. Moreover, obtaining very accurate measures of food intake and exercise level is difficult with people, unless they are in a completely supervised environment. Lastly, by using monkeys we were able to carry out this study for a prolonged duration, allowing for an examination of factors responsible for the development of complete reproductive suppression with exercise training and examination of relationships between changes in reproductive function and metabolic status of an individual. Carrying out longitudinal studies in humans over a period of several years with frequent experimental measurements is rarely, if ever, possible.

Our results can be most closely compared with those of Bullen et al. (33), who studied strenuous exercise training over a duration of 2 menstrual cycles in women randomly assigned to either a weight loss or a weight maintenance group. This protocol produced dramatic effects on menstrual cyclicity; only 4 of 28 subjects had a normal cycle during the study. Clinical disorders such as abnormal bleeding, luteal phase disturbances, and delayed menses occurred in both groups, with a significantly higher incidence of delayed menses in those in the weight loss group (-0.45 kg/wk; -4.0 ± 0.3 kg during the 2-month study). Our study differed from that by Bullen et al. (33) in that our training regimen was initiated more gradually, and the end point for the experiment for each individual was the development of amenorrhea. Another difference between our study and that of Bullen et al. was that our animals did not lose a significant amount of weight in association with the development of exercise-induced reproductive dysfunction.

Ours is not the first study to show that changes in body weight are not necessarily associated with the development of reproductive dysfunction caused by states of low energy availability. Several studies have reported that neither body weight nor body fat differ between amenorrheic and eumenorrheic exercising women (31, 32, 48). In addition, a number of animal studies have shown that changes in reproductive hormone secretion resulting from short-term changes in nutritional intake occur without significant weight loss or gain (6, 49, 50, 51, 52). Thus, although there is often a correlation between changes in body weight and reproductive function, such that states of low energy availability are associated with both a decrease in body weight and a loss of reproductive function and states of increased energy availability are associated with both an increase in body weight (as we report here in the overfeed study) and an increase in reproductive function, it appears that changes in the activity of the reproductive axis resulting from changes in energy availability are not necessarily linked to changes in body weight. One explanation for the lack of association between a loss of body weight and the development of exercise-induced reproductive disturbances in this and other studies (31, 32, 48) is that other adaptive mechanisms may occur so that energy is conserved and body weight is maintained. Support for this hypothesis is provided by data from cross-sectional studies that have documented reduced levels of circulating T3 (31, 32) in amenorrheic vs. eumenorrheic athletes and in the results of the present study, in which we found that changes in circulating T3 levels corresponded to both the development and the reversal of exercise-induced amenorrhea. A close association between reproductive hormone secretion and plasma T3 levels has also been reported in short-term studies by Loucks et al. (43), who showed that a significant decrease in T3 occurs in association with a reduction in LH pulse frequency during experiments in which low energy availability is produced by a combination of exercise and caloric restriction. In addition, work in our laboratory has shown that there is a close association between changes in plasma T3 levels and the frequency of pulsatile LH secretion during periods of short-term fasting and refeeding in adult male monkeys (53). Despite the apparent link between plasma T3 levels and activity of the reproductive axis, there is strong evidence that T3 itself does not regulate the neural systems driving the reproductive axis. It is known that LH secretion is maintained after thyroidectomy (54, 55), and we have previously reported that the fasting-induced decrease in the frequency of pulsatile LH secretion in male rhesus monkeys cannot be prevented with an infusion of exogenous T3 (40).

Plasma T3 concentrations are largely determined by the rate of conversion of T4 to T3, a process that is influenced by energy availability (39). Changes in serum T3 levels also correlate with changes in metabolic rate (56), and lower T3 levels indicate an increased metabolic efficiency. It is possible that the metabolic rate provides a critical signal to the reproductive axis to link reproductive function to energy availability within the body. In agreement with this, Myerson et al. (57) and most recently Lebenstedt et al. (58) found that athletes with exercise-induced menstrual disturbances have lower resting metabolic rates than either eumenorrheic athletes or sedentary women. In contrast, however, Wilmore et al. (30) failed to find indications of energy conservation in eumenorrheic and amenorrheic elite distance runners; no differences were observed in these women compared with controls in resting metabolic rate, the energy cost of a fixed pace run, or the thermic effect of food. However, the hormonal status of the eumenorrheic group in this study could not be discerned from the endocrine data presented, and it is possible that many of the eumenorrheic women were either anovulatory or had luteal phase deficiency. The latter two conditions have recently been associated with a reduced resting metabolic rate (kilocalories per kg fat-free mass) (58).

It is also possible that some other metabolic factor, which changes in parallel with changes in circulating T3 concentrations and metabolic rate, provides the critical cue linking the activity of the reproductive axis to metabolic status. A number of metabolic substrates and hormones have been proposed to play such a role, including plasma glucose levels (53, 59, 60, 61, 62), circulating leptin concentrations (63, 64, 65, 66), GH and IGF-I (67, 68), and insulin (69, 70, 71, 72). In the case of low metabolic fuel availability caused by low calorie intake, there is evidence that both supports and argues against each of these factors as playing a critical role in linking the activity of the reproductive axis to the metabolic status of the body (53, 59, 69, 72). However, there is very little information regarding the role that these factors may play in mediating the effects of exercise on the activity of the reproductive axis. Further studies of this nature are needed, and we believe that prospective studies of exercise-induced reproductive dysfunction in monkeys will provide an excellent model system in which to determine the mechanism(s) by which metabolic signals regulate the activity of the reproductive axis.

In summary, our findings provide strong evidence that exercise-induced suppression of reproductive function is caused by the energy utilization associated with vigorous regular exercise training and not other factors associated with exercise. We have extended the findings of previous investigators by showing that the reversal of amenorrhea can be accomplished simply by providing supplemental calories, and we have documented that both the development and the reversal of exercise-induced amenorrhea are accompanied by corresponding changes in plasma T3, a marker of energy balance that has been shown to correlate with changes in reproductive hormone levels in other paradigms. In the absence of significant weight loss during the development of exercise-induced amenorrhea, we have hypothesized that changes in T3 may correlate with adaptive changes in energy expenditure, such that metabolic efficiency is increased. To address this issue, future studies that prospectively document metabolic changes that are tightly correlated with the development of exercise-induced reproductive dysfunction and test the role that each of these factors plays in mediating the function of the reproductive axis are needed. We believe that our monkey model of exercise-induced amenorrhea will be particularly useful in such future studies.

Acknowledgments

We are grateful for the assistance of the technical staff, whose dedication was necessary to train monkeys to run for many hours each day, including Karen Church, Dawn Murphy, Cindy Heilman, Lisa Mattern, and Sally Kuhn. The resources and assistance of the staff of the Primate and RIA Core Laboratories of the Center for Research in Reproductive Physiology at the University of Pittsburgh are also greatly appreciated.

Footnotes

This work was supported by NIH Grants HD-20789, HD-25929, and HD-08610.

1 Present address: Department of Kinesiology and Noll Physiological Research Center, Pennsylvania State University, University Park, Pennsylvania 16803. Back

2 Present address: Department of Biology, Middlebury College, Middlebury, Vermont 05753. Back

3 Present address: Department of Obstetrics and Gynecology, Harbor-University of California-Los Angeles Medical Center, 1124 West Carson Street, Torrance, California 90502. Back

Abbreviations: AM1, AM2, and AM3, Three points during amenorrhea; cycles 2 and 1, the two menstrual cycles preceding the onset of amenorrhea; P4, progesterone; Sed, sedentary.

Received March 28, 2001.

Accepted July 25, 2001.

References

  1. Loucks AB 1990 Effects of exercise training on the menstrual cycle: existence and mechanisms. Med Sci Sport Exerc 22:275–280[Medline]
  2. Loucks AB, Vaitukaitis J, Cameron JL, Rogol AD, Skrinar G, Warren MP, Kendrick J, Limacher MC 1992 The reproductive system and exercise in women. Med Sci Sports Exerc 24:S288–S293
  3. McClure TJ, Saunders J 1985 Effect of witholding food for 0–72 hours on mating, pregnancy rate and pituitary function in female rats. J Reprod Fertil 74:57–64[Abstract]
  4. Morin LP 1986 Environment and hamster reproduction: responses to phase-specific starvation during the estrous cycle. Am J Physiol 251:R663–R669
  5. Rojdmark S 1987 Influence of short-term fasting on the pituitary-testicular axis in normal men. Horm Res 25:140–146[Medline]
  6. Cameron JL, Nosbisch C 1991 Suppression of pulsatile luteinizing hormone and testosterone secretion during short term food restriction in the adult male rhesus monkey (Macaca mulatta). Endocrinology 128:1532–1540[Abstract]
  7. Veldhuis JD, Iranmanesh A, Evans WS, Lizarralde G, Thorner MO, Vance ML 1993 Amplitude suppression of the pulsatile mode of immunoradiometric luteinizing hormone release in fasting-induced hypoandrogenemia in normal men. J Clin Endocrinol Metab 76:587–593[Abstract]
  8. Schreihofer DA, Parfitt DB, Cameron JL 1993 Suppression of LH secretion during short-term fasting in male rhesus monkeys: the role of metabolic vs stress signals. Endocrinology 132:1881–1889[Abstract]
  9. Cagampang FRA, Maeda K-I, Yokoyama A, Ota K 1990 Effect of food deprivation on the pulsatile LH release in cycling and ovariectomized female rats. Horm Metab Res 22:269–272[Medline]
  10. Cagampang FRA, Maeda K-I, Ohkura S, Ota K 1991 Involvement of ovarian steroids and endogenous opioids in the fasting-induced suppression of pulsatile LH release in ovariectomized rats. J Endocrinol 129:321–328[Abstract]
  11. Loucks AB, Heath EM 1994 Dietary restriction reduces luteinizing hormone (LH) pulse frequency during waking hours and increases LH pulse amplitude during sleep in young menstruating women. J Clin Endocrinol Metab 78: 910–915
  12. Olson BR, Cartledge T, Sebring N, Defensor R, Nieman L 1995 Short-term fasting affects luteinizing hormone secretory dynamics but not reproductive function in normal-weight sedentary women. J Clin Endocrinol Metab 80:1187–1193[Abstract]
  13. Alvero R, Kimzey L, Sebring N, Reynolds J, Loughran M, Nieman L, Olson BR 1998 Effects of fasting on neuroendocrine function and follicle development in lean women. J Clin Endocrinol Metab 83:76–80[Abstract/Free Full Text]
  14. Zubiran S, Gomez-Mont F 1953 Endocrine disturbances in chronic human malnutrition. In: Harris P, Marion UF, Thiman KV, eds. Vitamins and hormones. New York: Academic Press; 97–132
  15. Warren MP, Van de Wiele RJ 1973 Clinical and metabolic features of anorexia nervosa. Am J Obstet Gynecol 117:435–449[Medline]
  16. Smith SR, Chhetri MK, Johanson AJ, Radfor N, Migeon CJ 1975 The pituitary-gonadal axis in men with protein-calorie malnutrition. J Clin Endocrinol Metab 41:60–69[Abstract]
  17. Knuth UA, Hull MG, Jacobs HS 1977 Amenorrhea and loss of weight. Br J Obstet Gynaecol 84:801–807[Medline]
  18. Vigersky RA, Andersen AE, Thompson RH, Loriaux DL 1977 Hypothalamic dysfunction in secondary amenorrhea associated with simple weight loss. N Engl J Med 297:1141–1145[Abstract]
  19. Dubey AK, Cameron JL, Steiner RA, Plant TM 1986 Inhibition of gonadotropin secretion in castrated male rhesus monkeys (Macaca mulatta) induced by dietary restriction: analogy with the prepubertal hiatus of gonadotropin release. Endocrinology 118:518–525[Abstract]
  20. Foster DL, Ebling FJP, Micka AF, Vannerson LA, Bucholtz DC, Wood RI, Suttie JM, Fenner DE 1989 metabolic interfaces between growth and reproduction. I. Nutritional modulation of gonadotropin, prolactin, and growth hormone secretion in the growth-limited female lamb. Endocrinology 125:342–350[Abstract]
  21. Wade GN, Schneider JE, Hui-Yun L 1996 Control of fertility by metabolic cues. Am J Physiol 270:E1–E9
  22. Schneider JE, Wade GN 1990 Effects of diet and body fat content on cold-induced anestrus in Syrian hamsters. Am J Physiol 259:R1198–R1204
  23. Perrigo G, Bronson FH 1983 Behavioral and physiological responses of female house mice to foraging variation. Physiol Behav 34:437–440
  24. Warren MP 1980 The effects of exercise on pubertal progression and reproductive function in girls. J Clin Endocrinol Metab 51:1150–1157[Abstract]
  25. Drinkwater BL, Nilson K, Chesnut CH, Bremner WJ, Shainholtz S, Southworth MB 1984 Bone mineral content of amenorrheic and eumenorrheic athletes. N Engl J Med 311:277–281[Abstract]
  26. Nelson ME, Fisher EC, Catsos PD, Meredith CN, Turksoy RN, Evans WJ 1986 Diet and bone status in amenorrheic runners. Am J Clin Nutr 43:910–916[Abstract/Free Full Text]
  27. Deuster PA, Kyle SB, Mosler PB, Vigersky RA, Singh A, Shoomaker EB 1986 Nutritional intakes and status of highly trained amenorrheic and eumenorrheic women runners. Fertil Steril 46:636–643[Medline]
  28. Kaiserauer S, Snyder AC, Sleeper M, Zierath J 1989 Nutritional, physiological, and menstrual status of distance runners. Med Sci Sports Exerc 21:120–125[Medline]
  29. Loucks, AB 1989 Alterations in the hypothalamic-pituitary-ovarian and the hypothalamic-pituitary-adrenal axes in athletic women. J Clin Endocrinol Metab 68:402–411[Abstract]
  30. Wilmore JH, Wambsgans KC, Brenner M, Broeder CE, Paijmans I, Volpe JA, Wilmore KM 1992 Is there energy conservation in amenorrheic compared with eumenorrheic distance runners? J Appl Physiol 72:15–22[Abstract/Free Full Text]
  31. Loucks AB, Laughlin GA, Mortola JF, Girton L, Nelson JC, Yen SSC 1992 Hypothalamic-pituitary-thyroidal function in eumenorrheic and amenorrheic athletes. J Clin Endocrinol Metab 75:514–518[Abstract]
  32. Laughlin GA, Yen SSC 1996 Nutritional and endocrine-metabolic aberrations in amenorrheic athletes. J Clin Endocrinol Metab 81:4301–4309[Abstract]
  33. Bullen BA, Skrinar GS, Beitins IZ, Mering G, Turnbull BA, McArthur JW 1985 Induction of menstrual disorders by strenuous exercise in untrained women. N Engl J Med 312:1349–1353[Abstract]
  34. Williams NI, Caston-Balderrama AL, Helmreich DL, Parfit DB, Nosbisch C, Cameron JL 2001 Longitudinal changes in reproductive hormones and menstrual cyclicity in cynomolgus monkeys during strenuous exercise training: abrupt transition to exercise-induced amenorrhea. Endocrinology 142:2381–2389[Abstract/Free Full Text]
  35. Karsch FJ, Weick RF, Butler WR, Dierschke DJ, Krey LC, Weiss G, Hotchkiss J, Yamaji T, Knobil E 1973 Induced LH surges in the rhesus monkey: strength-duration characteristics of the estrogen stimulus. Endocrinology 92:1740–1747[Medline]
  36. Plant TM, Dubey AK 1984 Evidence from the rhesus monkey (Macaca mulatta) for the view that negative feedback control of luteinizing hormone secretion by the testis is mediated by a deceleration of hypothalamic gonadotropin-releasing hormone pulse frequency. Endocrinology 115:2145–2153[Abstract]
  37. Goodman RL 1978 A quantitative analysis of the physiological role of estradiol and progesterone in the control of tonic and surge secretion of luteinizing hormone in the rat. Endocrinology 102:142–150[Medline]
  38. Rodbard D, Lewald JE 1970 Computer analysis of radioligand assay and radioimmunoassay data. Acta Endocrinol (Copenh) 64:79–103
  39. Danforth Jr E, Burger AG 1989 The impact of nutrition on thyroid hormone physiology and action. Annu Rev Nutr 9:201–227[CrossRef][Medline]
  40. Williams NI, Cameron JL 1996 Stimulation of luteinizing hormone (LH) secretion in male rhesus monkeys by food intake: role of circulating triiodothyronine (T3) [Abstract P2–505]. Proc of the Endocrine Soc/Int Congr of Endocrinol. 1996
  41. Marcus R, Cann C, Madvig P, Minkoff J, Goddard M, Bayer M, Martin L, Gaudiani L, Haskell W, Genant H 1985 Menstrual function and bone mass in elite women distance runners. Ann Intern Med 102:158–163
  42. Williams NI, Young JL, McArthur JW, Bullen BA, Skrinar GS, Turnbull BA, Young 1995 Strenuous exercise with caloric restriction: effects on luteinizing hormone secretion. Med Sci Sports Exerc 27: 390–1398
  43. Loucks AB, Verdun M, Heath EM 1998 Low energy availability, not stress of exercise, alters LH pulsatility in exercising women. J Appl Physiol 84:37–46[Abstract/Free Full Text]
  44. Loucks AB, Verdun M 1998 Slow restoration of LH pulsatility by refeeding in energetically disrupted women. Am J Physiol 275:R1218–R1226
  45. Laughlin GA, Yen SSC 1997 Hypoleptinemia in women athletes: absence of a diurnal rhythm with amenorrhea. J Clin Endocrinol Metab 82:318–321[Abstract/Free Full Text]
  46. Dueck CA, Matt KS, Manore MM, Skinner JS 1996 Treatment of athletic amenorrhea with a diet and training intervention program. Int J Sports Nutr 6:24–40[Medline]
  47. Kopp-Woodroffe SA, Manore MM, Dueck CA, Skinner JS, Matt KS 1999 Energy and nutrient status of amenorrheic athletes participating in a diet and exercise training intervention program. Int J Sports Nutr 9:70–88[Medline]
  48. Sanborn CF, Albrecht BH, Wagner WW 1987 Athletic amenorrhea: lack of association with body fat. Med Sci Sports Exerc 19:207–212[Medline]
  49. Cameron JL 1989 Influence of nutrition on the hypothalamic-pituitary-gonadal axis in primates. In: Pirke KM, Wuttke W, Schweiger U, eds. The menstrual cycle and its disorders: influences of nutrition, exercise and neurotransmitters. Berlin: Springer-Verlag; 66–78
  50. Bronson FH 1986 Food-restricted, prepubertal, female rats: rapid recovery of luteinizing hormone pulsing with excess food, and full recovery of luteinizing hormone pulsing with excess food, and full recovery of pubertal development with gonadotropin-releasing hormone. Endocrinology 118:2483–2487[Abstract]
  51. Howland BE 1975 The influence of feed restriction and subsequent re-feeding on gonadotropin secretion and serum testosterone levels in male rats. J Reprod Fertil 44:429–436[Abstract]
  52. Parfitt DB, Church KR, Cameron JL 1991 Restoration of luteinizing hormone secretion after fasting in rhesus monkeys (Macaca mulatta): dependence on the size of the refeed meal. Endocrinology 129:749–756[Abstract]
  53. Schreihofer DA, Renda F, Cameron JL 1996 Feeding-induced stimulation of luteinizing hormone secretion in male rhesus monkeys is not dependent on a rise in blood glucose concentration. Endocrinology 137:3770–3776[Abstract]
  54. Mentoer SM, Woodfill CJI, Karsch FJ 1991 Role of the thyroid gland in seasonal reproduction: thyroidectomy blocks seasonal suppression of reproductive neuroendocrine activity in ewes. Endocrinology 128:1337–1344[Abstract]
  55. Follet BK, Nicholls TJ 1985 Influences of thyroidectomy and T4 replacement on photoperiodically controlled reproduction in quail. J Endocrinol 107: 211–221
  56. Guernsey DL, Edleman IS 1983 Regulation of thermogenesis by thyroid hormones. In: Oppenheimer JH, Samuels HH eds. Molecular basis of thyroid hormone action. New York: Academic Press; 293–294
  57. Myerson M, Gutin B, Warren MP, May MT, Contento I, Lee M, Pi-Sunyer FX, Pierson RN, Brooks-Gunn J 1991 Resting metabolic rate and energy balance in amenorrheic and eumenorrheic runners. Med Sci Sports Exerc 23:15–22[Medline]
  58. Lebenstedt M, Platte P, Pirke K 1999 Reduced resting metabolic rate in athletes with menstrual disorders. Med Sci Sports Exerc 31:1250–1256[Medline]
  59. Schneider JE, Friedenson DG, Hall AJ, Wade GN 1993 Glucoprivation induces anestrus and lipoprivation may induce hibernation in Syrian hamsters. Am J Physiol 264:R573–R577
  60. Schneider JE, Wade GN 1989 Availability of fuels controls estrous cyclicity of Syrian hamsters. Science 244:1326–1328[Abstract/Free Full Text]
  61. Murahashi K, Bucholtz DC, Nagatani S, Tsukamura H, Foster DL, Maeda K-I 1996 Suppression of luteinizing hormone pulses by restriction of glucose availability is mediated by sensors in the brain stem. Endocrinology 137:1171–1176[Abstract]
  62. Bucholtz DC, Vidwans NM, Herbosa CG, Schillo KK, Foster DL 1996 Metabolic interfaces between growth and reproduction. V. Pulsatile luteinizing hormone secretion is dependent on glucose availability. Endocrinology 137:601–607[Abstract]
  63. Aubert ML, Pierroz DD, Gruaz NM, d’Alleves V, Vuagnat BA, Pralong FP, Blum WF, Sizonenko PC 1998 Metabolic control of sexual function and growth; role of neuropeptide Y and leptin. Mol Cell Endocrinol 140:107–113[CrossRef][Medline]
  64. Foster DL, Nagatani S 1999 Physiological perspectives on leptin as a regulator of reproduction: role in timing puberty. Biol Reprod 60:205–215[Abstract/Free Full Text]
  65. Nagatani S, Guthikonda P, Thompson RC, Tsukamura H, Maeda K-I, Foster DL 1998 Evidence for GnRH regulation by leptin: leptin administration prevents reduced pulsatile LH secretion during fasting. Neuroendocrinology 67:370–376[CrossRef][Medline]
  66. Ahima RS, Prabakaran D, Mantzoros C, Qu D Lowell B, Maratos-Flier E, Flier JS 1996 Role of leptin in the neuroendocrine response to fasting. Nature 382:250–252[CrossRef][Medline]
  67. Hiney JK, Ojega SR, Dees WL 1991 Insulin-like growth factor 1: a possible metabolic signal involved in the regulation of female puberty. Neuroendocrinology 54:420–423[Medline]
  68. Richards MW, Wettemann RP, Spicer LJ, Morgan GL 1991 Nutritional anestrus in beef cows: effects of body condition and ovariectomy on serum luteinizing hormone and insulin-like growth factor-I. Biol Reprod 44:961–966[Abstract]
  69. Williams NI, Lancas MJ, Cameron JL 1996 Stimulation of luteinizing hormone (LH) secretion in male rhesus monkeys by food intake: Evidence against a role for insulin. Endocrinology 137:2565–2571[Abstract]
  70. Hileman SM, Schillo KK, Hall JB 1993 Effects of acute, intracerebroventricular administration of insulin on serum concentrations of LH, insulin, and glucose in ovariectomized lambs during restricted and ad libitum feed intake. Biol Reprod 48:117–124[Abstract]
  71. Harrison LH, Randel RD 1986 Influence of insulin and energy intake on ovulation rate, LH and progesterone in beef heifers. J Anim Sci 63:1228–1235
  72. Wade GN, Schneider JE, Friedman MI 1991 Insulin-induced anestrus in Syrian hamsters Am J Physiol 260:R148–R152



This article has been cited by other articles:


Home page
 EndocrinologyHome page
N. R Vulliemoz, E. Xiao, L. Xia-Zhang, J. Rivier, and M. Ferin
Astressin B, a Nonselective Corticotropin-Releasing Hormone Receptor Antagonist, Prevents the Inhibitory Effect of Ghrelin on Luteinizing Hormone Pulse Frequency in the Ovariectomized Rhesus Monkey
Endocrinology, March 1, 2008; 149(3): 869 - 874.
[Abstract] [Full Text] [PDF]

Home page
 J. Appl. Physiol.Home page
A. B. Loucks
New animal model opens opportunities for research on the female athlete triad
J Appl Physiol, November 1, 2007; 103(5): 1467 - 1468.
[Full Text] [PDF]

Home page
 Am. J. Physiol. Endocrinol. Metab.Home page
N. I. Williams, S. L. Berga, and J. L. Cameron
Synergism between psychosocial and metabolic stressors: impact on reproductive function in cynomolgus monkeys
Am J Physiol Endocrinol Metab, July 1, 2007; 293(1): E270 - E276.
[Abstract] [Full Text] [PDF]

Home page
 Obstet GynecolHome page
A. B. Loucks, M. J. De Souza, and N. I. Williams
Effects of Lifetime Exercise on the Outcome of in Vitro Fertilization
Obstet. Gynecol., February 1, 2007; 109(2): 456 - 457.
[Full Text] [PDF]

Home page
 Br. J. Sports. Med.Home page
A B Loucks
Refutation of "the myth of the female athlete triad"
Br. J. Sports Med., January 1, 2007; 41(1): 55 - 57.
[Full Text] [PDF]

Home page
 Br. J. Sports. Med.Home page
M J De Souza, J Alleyne, J D Vescovi, N I Williams, J L VanHeest, and M P Warren
Correction of misinterpretations and misrepresentations of the female athlete triad
Br. J. Sports Med., January 1, 2007; 41(1): 58 - 59.
[Full Text] [PDF]

Home page
 J. Nutr.Home page
K. L. Campbell and A. McTiernan
Exercise and Biomarkers for Cancer Prevention Studies
J. Nutr., January 1, 2007; 137(1): 161S - 169S.
[Abstract] [Full Text] [PDF]

Home page
 J. Clin. Endocrinol. Metab.Home page
A. B. Loucks
The Response of Luteinizing Hormone Pulsatility to 5 Days of Low Energy Availability Disappears by 14 Years of Gynecological Age
J. Clin. Endocrinol. Metab., August 1, 2006; 91(8): 3158 - 3164.
[Abstract] [Full Text] [PDF]

Home page
 Br. J. Sports. Med.Home page
C L Zanker and M P Warren
Regulation of reproductive function in athletic women: an investigation of the roles of energy availability and body composition * Commentary
Br. J. Sports Med., June 1, 2006; 40(6): 489 - 490.
[Full Text] [PDF]

Home page
 Br. J. Sports. Med.Home page
L DiPietro and N S Stachenfeld
The myth of the female athlete triad
Br. J. Sports Med., June 1, 2006; 40(6): 490 - 493.
[Full Text] [PDF]

Home page