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The Journal of Clinical Endocrinology & Metabolism Vol. 85, No. 11 4028-4035
Copyright © 2000 by The Endocrine Society

From the Clinical Research Centers

Homeostatic Joint Amplification of Pulsatile and 24-Hour Rhythmic Cortisol Secretion by Fasting Stress in Midluteal Phase Women: Concurrent Disruption of Cortisol-Growth Hormone, Cortisol-Luteinizing Hormone, and Cortisol-Leptin Synchrony1

M. Bergendahl, A. Iranmanesh, C. Pastor, W. S. Evans and J. D. Veldhuis

Departments of Pediatrics and Physiology, University of Turku (M.B.), FIN-20520 Turku, Finland; Endocrinology Section, Medicine Service, Salem Veterans Affairs Medical Center (A.I.), Salem, Virginia 24513; and Division of Endocrinology and Metabolism, Department of Internal Medicine, General Clinical Research Center, and the National Science Foundation Center for Biological Timing, University of Virginia Health Sciences Center (C.P., W.S.E., J.D.V.), Charlottesville, Virginia 22908

Address all correspondence and requests for reprints to: Dr. J. D. Veldhuis, Division of Endocrinology and Metabolism, Department of Internal Medicine, Box 202, McKim Hall, University of Virginia Health Sciences Center, Charlottesville, Virginia 22908. E-mail: jdv{at}virginia.edu


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Short-term fasting as a metabolic stress evokes prominent homeostatic reactions of the reproductive, corticotropic, thyrotropic, somatotropic, and leptinergic axes in men and women. Although reproductive adaptations to fasting are incompletely studied in the female, nutrient deprivation can have major neuroendocrine consequences in the follicular phase. Unexpectedly, a recent clinical study revealed relatively preserved sex steroid and gonadotropin secretion during short-term caloric restriction in the midluteal phase of the menstrual cycle. This observation suggested that female stress-adaptive responses might be muted in this sex steroid-replete milieu. To test this hypothesis, we investigated the impact of fasting on daily cortisol secretion in healthy young women during the midluteal phase of the normal menstrual cycle. Eight volunteers were each studied twice in separate and randomly ordered short-term (2.5-day) fasting and fed sessions. Pulsatile cortisol secretion, 24-h rhythmic cortisol release, and the orderliness of cortisol secretory patterns were quantified. Within-subject statistical comparisons revealed that fasting increased the mean serum cortisol concentration significantly from a baseline value of 8.0 ± 0.61 to 12.8 ± 0.85 µg/dL (P = 0.0003). (For Systeme International conversion to nanomoles per L, multiply micrograms per dL value by 28.) Pulsatile cortisol secretion rose commensurately, viz. from 101 ± 11 to 173 ± 16 µg/dL/day (P = 0.0025). Augmented 24-h cortisol production was due to amplification of cortisol secretory burst mass from 8.2 ± 1.5 to 12.9 ± 2.0 µg/dL (P = 0.017). In contrast, the estimated half-life of endogenous cortisol (104 ± 9 min), the calculated duration of underlying cortisol secretory bursts (16 ± 7 min) and their mean frequency (14 ± 2/day) were not altered by short-term fasting. The quantifiable orderliness of cortisol secretory patterns was also not influenced by caloric restriction. Nutrient deprivation elevated the mean of the 24-h serum cortisol concentration rhythm from 12.4 ± 1.3 to 18.4 ± 1.9 µg/dL (P = 0.0005), without affecting its diurnal amplitude or timing. Correlation analysis disclosed that fasting reversed the positive relationship between cortisol and LH release evident in the fed state, and abolished the negative association between cortisol and GH as well as between cortisol and leptin observed during nutrient repletion (P < 0.001). Pattern synchrony between cortisol and GH as well as that between cortisol and LH release was also significantly disrupted by fasting stress.

In summary, short-term caloric deprivation enhances daily cortisol secretion by 1.7-fold in healthy midluteal phase young women by selectively amplifying cortisol secretory burst mass and elevating the 24-h rhythmic cortisol mean. Augmentation of daily cortisol production occurs without any concomitant changes in cortisol pulse frequency or half-life or any disruption of the timing of the 24-h rhythmicity or orderliness of cortisol release. Fasting degrades the physiological coupling between cortisol and LH, cortisol and GH, and cortisol and leptin secretion otherwise evident in calorie-sufficient women. We conclude that the corticotropic axis in the young adult female is not resistant to the stress-activating effects of short-term nutrient deprivation, but, rather, evinces strong adaptive homeostasis both monohormonally (cortisol) and bihormonally (cortisol paired with GH, LH, and leptin).


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
INADEQUATE NUTRITIONAL intake can profoundly alter neuroendocrine function, e.g. by suppressing reproductive capability; inhibiting TSH, PRL, and leptin production; and activating hypothalamo-pituitary drive of the corticotropic and somatotropic axes (1, 2, 3, 4, 5, 6, 7). However, the neuroendocrine mechanisms underlying and the possible linkages coupling the foregoing multiaxis adaptive responses are not well defined. In relation to neuroregulation of the male gonadal axis, fasting in healthy young men suppresses the frequency and mass of LH secretory bursts in a GnRH-reversible fashion (5, 6, 8). In the case of the GH-insulin-like growth factor I (IGF-I) axis, short-term fasting in both sexes augments pulsatile GH production and in women concomitantly reduces leptin release (9, 10). Unexpectedly, the female reproductive axis appears to be relatively resistant to acute nutrient deprivation in the midluteal phase, inasmuch as the pulsatile release, diurnal rhythmicity, and pattern orderliness of LH secretion were largely unaffected by fasting (10). The preservation of gonadotropin and gonadal sex steroid secretion during profound metabolic stress at this phase of the menstrual cycle raises the question of whether the ACTH-cortisol axis shows attenuated stress-adaptive responses in this sex steroid-sufficient context. This issue arises because output of the corticotropic axis reciprocally impacts that of LH in the human and experimental animal (1).

The corticotrope-adrenal axis in men and follicular phase women is highly responsive to fasting stress (11, 12, 13, 14, 15), protein-calorie malnutrition (16, 17), and the metabolic sequelae of anorexia nervosa (18, 19, 20, 21, 22). In particular, short-term nutritional deficiency in men and follicular phase women elevates the serum cortisol concentration (14, 19, 20, 22), increases the 24-h urinary excretion of free cortisol (17, 21, 22, 23), impairs dexamethasone’s suppression of cortisol release (12, 20), and blunts the incremental rise in cortisol stimulated acutely by ACTH or CRH (16, 23, 24). In one study of women with anorexia nervosa, some 24-h rhythmicity of cortisol release was retained (19), and in another, the pulsatile mode of cortisol secretion was preserved with an accelerated frequency (25). However, there are limited corresponding insights into the nature of corticotropic axis reactivity to fasting stress in midluteal phase, steroidogenically sufficient young women. Indeed, to our knowledge, no studies have elucidated the neuroendocrine effects of hypocaloric stress on the 3-fold regulated modes (pulsatile, 24-h rhythmic, and pattern orderliness) of cortisol secretion in the healthy female.

The present clinical investigation evaluates the impact of short-term fasting on cortisol dynamics in healthy young women studied in the midluteal phase of the normal menstrual cycle. To this end, we quantitated 24-h pulsatile cortisol production (26, 27, 28), diurnal cortisol rhythmicity, the orderliness of the cortisol release process (29, 30, 31), and the synchrony between cortisol and GH, LH, and leptin secretion.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Clinical protocol

Eight young healthy women within ±25% of normal body weight (body mass index, 21 ± 1.3 kg/m2) and aged 21–28 yr were studied after provision of written informed consent. This study was approved by the human investigation committee of the University of Virginia. Volunteers were not smokers and were not taking birth control pills or any other systemic medications. None had undertaken transmeridian travel across three or more time zones in the previous 2 weeks. Each had an unremarkable clinical history and physical examination. Volunteers had regular (28 ± 3 day) menstrual cycles; normal screening biochemical tests of renal, hepatic, metabolic, and hematological function; and unremarkable serum concentrations total and free T4, TSH, GH, PRL, testosterone, progesterone, estradiol, immunoreactive LH and FSH, and IGF-I. GH, LH, and leptin secretion in this paradigm was described previously (7, 8, 9, 10).

Subjects were admitted to the General Clinical Research Center of the University of Virginia during the midluteal phase of the normal menstrual cycle on the night before blood sampling. Ovulation was documented ultrasonographically during each of the two study cycles by the development of a normal preovulatory follicle (mean diameter, 20 mm; range, 16–26 mm), followed by its disappearance, as characterized by daily or alternate day transvaginal ovarian sonograms. Five to 8 days later, when serum progesterone concentrations exceeded 7 ng/mL (22 nmol/L), fed and fasting admissions were assigned in randomized order.

Repetitive blood sampling was carried out at 10-min intervals for 24 h beginning at 0800 h at least 1 h after initial venipuncture. In the fasting admission, blood samples were withdrawn during 32–56 h of the fast, i.e. starting approximately 1 1/3 days into the fast. Samples were obtained via a forearm iv catheter and were allowed to clot at room temperature. Sera were frozen at -20 C for later assays. Subjects remained in bed or a chair during the sampling procedure, except for bathroom privileges. In the fed state, three isocaloric meals were provided daily (at 0800, 1200, and 1800 h). During the 2.5-day fast, volunteers received caffeine- and calorie-free liquids, potassium chloride (40 mEq daily), and water-soluble vitamins. Participants slept at the Clinical Research Center. Urinary ketones were monitored daily to corroborate compliance with the fast.

Assays

Serum cortisol concentrations were measured by solid phase RIA (Diagnostic Products, Los Angeles, CA) with an assay sensitivity of 0.5 ng/dL (14 nmol/L), as described previously (29, 32). The median inter- and intraassay coefficients of variation were less than 6.5% and 5.6%, respectively. All 145 samples in each admission were assayed together.

Deconvolution analysis

A simple burst model of hormone secretion (multiparameter deconvolution analysis) was used to quantitate condition- and subject-specific features of daily pulsatile cortisol production and its endogenous half-life (26, 27, 28). This technique resolves the entire serum hormone concentration profile into its constituent secretory bursts and simultaneously estimates the hormone half-life. The daily (24-h) pulsatile cortisol secretion rate is the product of secretory burst frequency and the mean mass of cortisol released per secretory event. A basal secretion term was not required to model the present data. The mass of hormone released per burst is calculated as the analytical integral of the underlying (computer-resolved) secretory pulse. Deconvolution analysis was carried out at 95% joint statistical confidence intervals based on all values of calculated secretory burst mass simultaneously. The technician was blinded to the randomized order of the fed vs. fasting admissions. A common half-life and homogeneous secretory-burst half-duration were calculated for each woman’s time series. This approach has been validated independently for cortisol secretion (33).

24-h cortisol rhythmicity

The diurnal 24-h rhythmic secretion of cortisol was appraised by cosinor analysis, as described previously (29, 32). This entails trigonometric regression of a 1440-min cosine function on the profiles of serum cortisol concentrations or secretory measures vs. time.

Pattern orderliness

Approximate entropy (ApEn) was used as a scale- and model-independent statistic to quantify the orderliness or regularity of cortisol release patterns over 24 h. Normalized ApEn parameters of m = 1 and r = 20% of the intraseries SD were used, as previously described (30, 31, 34, 35, 36, 37, 38). For this parameter set, ApEn is designated ApEn (1, 20%). ApEn quantifies the reproducibility of subordinate (nonpulsatile) patterns in a time series, and therefore yields information complementary to both pulse analysis and cosine fitting. Higher absolute ApEn values denote greater disorderliness or irregularity of neurohormone release, as observed in acromegaly (31), Cushing’s disease (36, 37), aldosteronoma (38), and the aging LH (30), GH (39), and insulin (40) axes and for GH secretion in women compared with men (7, 10, 34).

To test pairwise (conditional) pattern orderliness between two hormone series, cross-ApEn was applied analogously. Cross-ApEn allows two-variable lag-independent synchrony analysis after the time series are subjected to z-score transformation (30).

Statistical analyses

Due to non-Gaussian distributions, derived measures of pulsatile or rhythmic cortisol release were compared statistically after logarithmic transformation using a paired, unequal variance, two-tailed Student’s t test. Mean and integrated (24-h) serum cortisol concentrations and ApEn values were compared without transformation. Results are presented as the mean ± SEM (median). Statistical significance was accepted for P < 0.05, except for cross-correlations (conservatively P < 0.01, due to multiple comparisons).

Cross-correlation analysis was applied to test for significant time-lagged (linear) synchrony between successive serum concentrations of cortisol and GH, cortisol and LH, and cortisol and leptin considered pairwise, as described previously (41).


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Mean daily serum cortisol concentrations

All serum cortisol concentrations in the eight women were detectable during both admissions. The 2.5-day fast elicited a 1.5-fold increase in the 24-h mean serum cortisol concentration (i.e. 12.3 ± 1.3 fed vs. 18.2 ± 1.9 µg/dL fasting; P = 0.00004; Fig. 1Go; multiply cortisol concentrations in micrograms per dL by 28 to obtain corresponding values in nanomoles per L).



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  		Figure 1. Twenty-four hour mean (A) and integrated (B) serum cortisol concentrations in young healthy normal-weight women studied in the ad libitum fed vs. fasting state (a 2.5-day water-only fast) in the midluteal phase of the normal menstrual cycle. P values define paired parametric statistical contrasts. Data are the mean ± SEM (n = 8 women).

 
Deconvolution analysis of pulsatile cortisol secretion

All 16 24-h profiles of serum cortisol concentrations were visibly pulsatile in both the fed and fasting states. Observed cortisol profiles (Fig. 2AGo) and deconvolution-calculated secretory rates (Fig. 2BGo) are illustrated for three women in Fig. 2Go. Specific measures of pulsatile cortisol secretion and half-life are summarized in Table 1Go. The half-duration (the duration in minutes at half-maximal amplitude) of computed cortisol secretory bursts and the endogenous cortisol half-life did not change significantly during fasting. The number of statistically significant cortisol secretory pulses also remained constant [14 ± 1.6 (median, 13) fasting and 14 ± 0.8 (median, 14) secretory bursts/24 h fed; P = NS]. The mean cortisol intersecretory burst interval averaged 104 ± 12.2 (median, 96) min fed and 100 ± 7.6 (median 97) min fasting (P = NS; Table 1Go). The mass of cortisol secreted per burst (area of the calculated cortisol secretory pulse) increased significantly from 8.2 ± 1.5 (median, 8.0) µg/dL fed to 12.9 ± 2.0 (median, 10.9) µg/dL fasting (P = 0.017; Fig. 3Go). Twenty-four hour pulsatile (total) cortisol production rose from 101 ± 11 (median, 106) in the fed state to 173 ± 16 (median, 165) µg/dL/day in the fasting state (P = 0.0025).



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  		Figure 2. Illustrative 24-h serum cortisol concentration and secretion profiles in healthy young women studied in the fed and fasting states in the midluteal phase of the normal menstrual cycle. Blood samples were collected at 10-min intervals for 24 h when the volunteers were nutritionally replete and also during the last 24 h of a 2.5-day fast. Fed and fasting sessions were assigned in randomized order at least 4 weeks apart. The continuous curves through the observed serum cortisol concentrations (A) are predicted by deconvolution analysis (see Subjects and Methods). Vertical bars through the serum cortisol concentration measurements denote the dose-dependent intrasample SDs. Punctuated bursts of cortisol release (B) were identified by deconvolution analysis to give rise to the pulsatile serum cortisol concentration profiles. Deconvolution estimates of the number, duration, mass, and amplitude of underlying cortisol secretory bursts and the half-lives of endogenous cortisol in all eight subjects in the fed and fasting states are summarized in Table 1Go and Fig. 3Go.

 

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  		Table 1. Deconvolution and approximate entropy analyses of cortisol secretion and half-life in fed and fasting states in healthy young women studied in the midluteal phase of the normal menstrual cycle

 


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  		Figure 3. Deconvolution-estimated cortisol secretory burst mass in young normal weight women studied in the midluteal phase of the menstrual cycle (n = 8) in the fed vs. fasting state (2.5-day water-only fast). Blood was collected at 10-min intervals for 24 h when volunteers were nutritionally replete and during the last 24 h of a 2.5-day fast. Numerical values are the mean ± SEM. P values were determined via paired two-tailed Student’s t test.

 
24-h cortisol rhythms

Twenty-four-hour rhythmic serum cortisol concentrations were maximal (acrophase values) at 0953 h (0822–1124 h clocktimes, 95% confidence intervals) in the fed state and at 0946 h (0831–1101 h) in the fasting state (P = NS). Fasting elevated the mesor (average value about which the 24-h rhythm oscillates) significantly from 12.4 ± 1.3 to 18.4 ± 1.9 µg/dL (P = 0.0005). Caloric restriction did not alter the amplitude (half the absolute difference between the nadir and peak value) of the 24-h cortisol rhythm (fed, 5.8 ± 0.7; fasting, 5.9 ± 1.6 µg/dL; P = NS).

Cosinor analysis of deconvolution-calculated cortisol secretory measures documented strong day-night rhythms in the mass of cortisol secreted per burst, with no significant diurnal variations in cortisol secretory burst frequency (an inverse function of interburst interval; Fig. 4Go). The mesor (mean) of the diurnal rhythmicity in the mass of cortisol secreted per burst was increased by fasting (P < 0.01).



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  		Figure 4. Diurnal rhythms of deconvolution-calculated cortisol secretory measures in young normal-weight women (n = 8) in the fed and fasting (a 2.5-day water-only) states. Upper panels depict the mass of cortisol released per secretory episode as a function of clocktime; lower panels present cortisol interburst interval vs. clocktimes. Cosine regression was employed to quantitate diurnal rhythms in each measure. Data are the mean and 95% statistical confidence intervals. Acrophase denotes the clocktime at which the value is maximal, the mesor is the average value about which the diurnal rhythm oscillates, and the amplitude is half the absolute difference between the nadir and peak values. Cortisol interburst interval is approximately the reciprocal of secretory burst frequency. P = NS denotes an amplitude not distinguishable from zero at the 0.05 level.

 
ApEn

Cortisol ApEn (1, 20%) averaged 1.080 ± 0.078 during the fed admission. The mean was unaffected by fasting (1.217 ± 0.063; P = NS; Table 1Go). Two women showed a decrease and six showed an increase in cortisol ApEn with fasting.

Cross-correlation of cortisol time series with those of GH, LH, and leptin

Cross-correlation analysis disclosed a significantly positive (P < 0.01) time-lagged relationship between successively paired serum cortisol and LH concentrations in the group of eight women in the fed state. Correlation maxima occurred at time lags of +10, 0, and -10 min, whereby changes in serum cortisol concentrations preceded, coincided with, and/or followed directionally similar changes in serum LH concentrations from 0–10 min (Fig. 5AGo). The cortisol-LH relationship became significantly negative in the fasting state, and the window of the correlation maxima was shifted to -120- and -80-min lags, such that a rise in LH concentrations preceded a fall in cortisol levels (and vice versa) by 80–120 min (P < 0.01; Fig. 5BGo). Cross-correlation r values were also significantly negative between serum cortisol and GH concentration time series in the fed state, with maxima across the broader window of a -60- to +40-min time lag, i.e. opposite changes in cortisol concentrations preceded by 40 min and followed by up to 60 min those of GH (Fig. 5CGo). This inverse cortisol-GH relationship was abolished by fasting. Cross-correlation was further negative for pairwise cortisol and leptin release in the fed state, with maxima at +70 to +140-min lags (i.e. leptin changes preceded those of cortisol by 70–140 min). This group association vanished during fasting.



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  		Figure 5. Cross-correlation plots of paired profiles of serum cortisol and LH (A), GH (B), and leptin (C) concentrations in eight young healthy women. Volunteers were studied in the midluteal phase of the normal menstrual cycle during randomly ordered fed (upper panel) and short-term (2.5-day water-only) fasting (lower panel) sessions. Cross-correlation analysis was used to test for significant linear synchrony between changes in the two hormone concentrations considered at the indicated time lags (41 ). The continuous curves give the absolute range of the observed group r values at any given lag time, and the solid points are the median values. The horizontal line through zero defines the null hypothesis of no significant relationship between the paired hormones. The lag is the time in minutes separating the paired (two-hormone) concentrations being compared. A negative time lag (left) indicates that changes in the concentration of the first-named hormone follow those of the second. Conversely, a positive lag (right) denotes that changes in the first-named hormone precede those in the second. A positive r denotes a feedforward, and a negative r a feedback, relationship. *, P < 0.01; NS, P > 0.05.

 
Cross-ApEn

Cross-ApEn analysis was used to assess a lag-independent pairwise synchrony of secretory patterns (see Subjects and Methods). This appraisal revealed significant fasting-induced increases in mean cortisol-GH and cortisol-LH cross-ApEn values (see Fig. 6Go, A and B). In contrast, cortisol-leptin cross-ApEn remained unchanged. Higher cross-ApEn denotes loss of joint (bihormonal) synchrony or greater pairwise pattern randomness.



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  		Figure 6. Loss of two-hormone secretory pattern synchrony as monitored by the cross-approximate entropy (cross-ApEn, X-ApEn) statistic applied to 24-h paired serum cortisol and GH (A) and cortisol and LH (B) concentration time series. Data were obtained in eight women studied in the midluteal phase of the normal menstrual cycle. Higher cross-ApEn values denote a loss of pattern synchrony between the indicated hormone pairs. Cross-ApEn provides a lag-independent and nonlinear (conditional probability) measure of bivariate pattern consistency, and hence is complementary to cross-correlation analysis (see Fig. 5Go). Data are the mean ± SEM. P values depict statistical comparisons made parametrically (see Subjects and Methods).

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The present study demonstrates that a 2.5-day fast evokes a 1.5-fold increase in 24-h integrated (and mean) serum cortisol concentrations and an estimated 1.7-fold elevation in daily pulsatile cortisol production in young women studied in the midluteal phase of the normal menstrual cycle. Further analyses established that fasting-induced hypercortisolism was achieved mechanistically by selective amplification of the mass of cortisol secreted per burst with no alteration in the number or duration of cortisol secretory bursts or the cortisol half-life. Although no other analogous clinical data exist at present in the healthy female, augmentation of cortisol secretory burst mass would accord with earlier fasting studies in young (2) and older (42) men. In the male, the degree of fasting-enhanced cortisol secretion was related to the duration of the fasting interval (5, 42). The current findings in healthy women thus extend earlier reports describing relative hypercortisolism in response to nutrient restriction (13, 14, 15, 43, 44), protein-calorie malnutrition (17), and eating disorders (18, 21, 22).

The calculated half-life of endogenous cortisol remained unchanged during fasting in the young midluteal phase women studied here, in agreement with earlier deconvolution-based observations in young (2, 42) and older (42) men. In contrast, two other studies estimated a prolonged cortisol half-life after 3 weeks of complete food deprivation in healthy women and patients with anorexia nervosa (12, 19). Whereas deconvolution analysis of cortisol kinetics was recently validated independently by waveform-independent methods (33), the cortisol half-life obtained by conventional graphical techniques (12, 19) tends to be overestimated, especially when consecutive secretory pulses are confluent (26, 27, 28).

Analyses of the nonpulsatile facets of cortisol secretion using the ApEn statistic disclosed no significant effect of fasting on the mean orderliness of the adrenal secretory process. However, six of eight women showed fasting-induced erosion of orderly cortisol release. We recently noted that a 3.5-day fast more consistently disrupts the quantifiable regularity of cortisol secretory patterns in young men (42). The foregoing difference may be due to a gender distinction in the neuroregulation of cortisol release during fasting, unequal statistical power, and/or the different durations of fasting imposed in the two studies.

Short-term fasting increased the mesor of the 24-h rhythm in cortisol release in young women. Neither the amplitude nor the timing of the maximum 24-h cortisol rhythm was altered. These data in healthy women are consistent with earlier observations in patients with severe anorexia nervosa (45) and in fasting young men (42). However, in older men, fasting induces a marked (~90 min) further phase advance in diurnal cortisol release (42). As the phase advance was restricted to older men, it is probably due to an age-dependent difference in cortisol-axis reactivity to metabolic stress. Thus, the nature of 24-h rhythmic adaptations of the corticotropic axis to fasting stress is influenced by the duration of the metabolic stress and by age, but not so evidently by gender.

Assay of concurrent 24-h serum cortisol, GH, LH, and leptin concentration profiles allowed an appraisal of the possible coordinate secretion of cortisol with each of these three stress-related hormones. Under calorically replete conditions, midluteal phase women maintained inverse cortisol-GH as well as cortisol-leptin relationships, as assessed by linear time-lagged cross-correlation analysis. The first reciprocal association might be explained by the known ability of glucocorticoids to suppress GH secretion, which may be mediated via heightened somatostatinergic tone (46). The inverse cortisol-leptin correlation may reflect the ability of leptin to inhibit CRH drive and block adrenal glucocorticoid production (47, 48, 49, 50, 51, 52, 53, 54). Both the cortisol-GH and the cortisol-leptin correlations were abolished by fasting. Moreover, the positive correlation observed between cortisol and LH release in the fed state was inverted and time-shifted by food withdrawal. These observations collectively identify marked loss of interaxis synchrony in response to metabolic stress, which was corroborated by (lag-independent) cross-ApEn analysis. The latter new technique of pattern synchrony quantitation for paired time series documented prominent fasting-induced disruption of the evident pattern coordination between cortisol and GH as well as cortisol and LH release. Whether similar inferences of 3-fold synchrony failure apply in other stress states and/or to the male cortisol-GH, cortisol-leptin, and cortisol-LH neuroendocrine axes is not known.

From a hypothalamo-pituitary regulatory perspective, the precise timing of ACTH-GH, ACTH-leptin, and ACTH-LH coupling is likely to reflect a 10- to 20-min closer association than that reported here for cortisol, as ACTH release precedes that of cortisol consistently by 10 or 20 min (55, 56, 57, 58, 59). This consideration does not exclude coordinate interaxis control at other levels as well, e.g., of adrenal gland output (above). Indeed, the exact nature of the putative CNS neurotransmitter pathways that couple ACTH output to the secretion of LH, leptin, and GH is not established. Experiments in the rodent suggest that among other mediators, neuropeptide Y, galanin, and nitric oxide may be plausible hypothalamic integrators of such interaxis signaling (46, 60, 61).

In conclusion, fasting amplifies daily pulsatile cortisol secretion in healthy midluteal phase young women via neuroregulatory mechanisms that selectively augment glucocorticoid secretory burst mass and the 24-h rhythmic cortisol mean without altering the burst frequency, duration, half-life, timing, or orderliness of cortisol release patterns. Fasting abolished the negative linear time-lagged linkages between cortisol and GH as well as those between cortisol and leptin release and disrupted the positive cortisol-LH relationship. Moreover, nutrient withdrawal markedly eroded the conditional pattern synchrony expected between paired cortisol-GH and cortisol-LH secretory profiles, as assessed in an analytically separate lag-independent (nonlinear) manner. Together, the present new findings point to multiaxis neuroendocrine adaptations to fasting stress in the midluteal phase of the normal human menstrual cycle.


   Acknowledgments
 
We thank Patsy Craig for expert assistance with manuscript preparation, Paula Azimi for deconvolution and statistical analyses and skillful artwork, Brenda Grisso for performance of the immunoassays, and Sandra Jackson and the expert nursing staff at the University of Virginia Clinical Research Center for conduct of the research protocols.


   Footnotes
 
1 This work was supported in part by NIH Grant MO1-RR-00847 to the Clinical Research Center of the University of Virginia, the NIH U-54 Specialized Center for Reproduction Research (NICHHD Grant U54-HD-28934; to J.D.V. and W.S.E.), 1-FO5-TW-O5080 from the NIH Fogarty International Center (to M.B.), V.A. Merit Review Medical Research Funds (to A.I.), the Academy of Finland (to M.B.), the Yrjö Jahnsson Foundation (to M.B.), the Emil Aaltonen Foundation (to M.B.), the National Science Foundation Center for Biological Timing (to J.D.V. and W.S.E.), the Center for Biomathematical Technology (to J.D.V.), and NIH NIA Grant AG-14799 (to J.D.V.). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of any of the above agencies. Back

Received February 15, 2000.

Revised July 18, 2000.

Accepted July 27, 2000.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Bergendahl M, Veldhuis JD. 1995 Altered pulsatile gonadotropin signaling in nutritional deficiency in the male. Trends Endocrinol Metab. 6:145–159.
  2. Bergendahl M, Vance ML, Iranmanesh A, Thorner MO, Veldhuis JD. 1996 Fasting as a metabolic stress paradigm selectively amplifies cortisol secretory burst mass and delays the time of maximal nyctohemeral cortisol concentrations in healthy men. J Clin Endocrinol Metab. 81:692–699.[Abstract]
  3. Bergendahl M, Evans WS, Veldhuis JD. 1996 Current concepts on ultradian rhythms of luteinizing hormone secretion in the human. Hum Reprod Update. 2:507–518.[Abstract/Free Full Text]
  4. Hartman ML, Pezzoli SS, Hellmann PJ, Suratt PM, Thorner MO. 1996 Pulsatile growth hormone secretion in older persons is enhanced by fasting without relationship to sleep stages. J Clin Endocrinol Metab. 81:2694–2701.[Abstract]
  5. Aloi JA, Bergendahl M, Iranmanesh A, Veldhuis JD. 1997 Pulsatile intravenous gonadotropin-releasing hormone administration averts fasting-induced hypogonadotropism and hypoandrogenemia in healthy, normal-weight men. J Clin Endocrinol Metab. 82:1543–1548.[Abstract/Free Full Text]
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