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Gender Differences in Basal Protein Kinetics in Young Adults

Department of Internal Medicine, Endocrine and Metabolic Sciences, University of Perugia, 06126 Perugia, Italy

Address all correspondence and requests for reprints to: Pierpaolo De Feo, M.D., Department of Internal Medicine, Endocrine and Metabolic Sciences, Università di Perugia, Via E. dal Pozzo, 06126 Perugia, Italy. E-mail: defeo{at}dimisem.med.unipg.it

	Abstract

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Gender affects energy expenditure and influences the relative utilization of carbohydrate and fat as fuels. However, little is known about the possible effects of gender on protein metabolism. Thus, we compared whole body and plasma (albumin and fibrinogen) protein kinetics in the basal postabsorptive state in young, untrained volunteers divided into two groups according to gender (women: n = 17; age, 24 ± 4 yr; men: n = 17; age, 25 ± 2 yr). The two groups were matched for body mass index. Protein kinetics were measured by means of L-[1-¹⁴C]leucine infusion. The leucine whole body rate of appearance, an index of proteolysis, and nonoxidative rate of disappearance, an index of protein synthesis, were similar in the two groups. However, the leucine oxidation rate was significantly lower in women compared to men (0.23 ± 0.07 vs. 0.31 ± 0.08 µmol/kg·min; P = 0.0062). Similar results were obtained when data were adjusted for estimated body composition. Albumin and fibrinogen fractional secretion rates were not different in the two groups. In conclusion, in the basal state leucine oxidation is lower in women than in men regardless of body composition. This could be one of the factors contributing to the lower metabolic rate in women.

	Introduction

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GENDER is one of the major determinants of body composition and energy expenditure in humans. Fat-free body mass and body cell mass are less in women than in men (1, 2). As the fat-free body mass is considered to be the metabolically active compartment of the body, the gender-related differences in body composition could be responsible for the lower energy expenditure observed in women (3). However, recent data show that the metabolic rate is lower in women than in men even when adjusted for body composition (1, 4). Thus, factors other than body composition must be responsible for the differences in metabolic rate between women and men.

Different metabolic rates can result from a differential utilization of the three major fuels: carbohydrates, fat, and proteins. Whereas the relative contribution of carbohydrates and fat utilization to energy expenditure is usually accounted for when the metabolic rate is measured with indirect calorimetry, the specific role of protein metabolism in the variability in energy expenditure has often been neglected and/or considered constant. However, a large part of the fat-free body mass is comprised of proteins that turn over at a fairly high rate. Every day, 5–6 g proteins/kg body mass are metabolized (5). The protein turnover process requires energy for both protein breakdown (6, 7) and protein synthesis (8). In addition, amino acid oxidation is one of the direct determinants of oxygen consumption, but its contribution is usually considered constant or negligible. Overall, it has been estimated that the contribution of protein turnover to the resting metabolic rate is approximately 20% (9). Thus, it is possible that the differences in energy consumption between women and men are due at least in part to differences in protein metabolism.

Therefore, we have analyzed the effect of gender on whole body and plasma protein kinetics in healthy young adults divided into two groups according to gender. As the amount of body fat affects the protein turnover rate (9), we matched the two groups for body mass index.

	Subjects and Methods

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Subjects

We analyzed the protein kinetic parameters of 17 young women and 17 young men, matched for body mass index (see Table 1 for subject characteristics). All of the subjects had been studied for different protocols (5, 10, 11, 12, 13, 14), which included the measurement of protein kinetics in the basal postabsorptive state. The subjects were healthy and untrained and had no history of chronic illness or family history of diabetes. They had given informed consent before the study. The protocols had been approved by the regional ethical committee. In the group of women, the phase of the ovarian cycle was assessed at the time of the study by both a clinical interview and measurements of estradiol, progesterone, FSH, and LH.

To avoid possible diet-related variations in basal protein oxidation, for the 3 days preceding the study the subjects consumed an isocaloric (35 Cal/kg·day) diet containing 55% carbohydrate, 30% fat, and 15% protein. The morning of the study, at about 0730 h, the subjects were admitted to the Clinical Research Unit of the Department of Internal Medicine, Endocrine and Metabolic Sciences of the University of Perugia. At approximately 0800 h, a Teflon catheter was placed in a forearm vein for the infusion (Harvard syringe pump, Harvard Apparatus, Ealing, South Natick, MA) of [1-¹⁴C]leucine (SA, 54 mCi/mmol; Amersham International, Aylesbury, UK) and saline (0.5 mL/min; Vial Médical pump, Grenoble, France). A butterfly needle was inserted in retrograde fashion in a vein of the opposite hand, and the hand was maintained at 65 C in a thermostatic Plexiglas box for sampling of arterialized venous blood (15). At approximately 0900 h (0 min), a primed constant iv infusion of L-[1-¹⁴C]leucine (prime, 9 µCi; infusion rate, 0.3 µCi/min) was started and continued for 4 h (240 min).

Blood and breath samples were collected at -15, 0, 60, 120, 180, 200, 220, and 240 min to measure the plasma specific activity of leucine and -ketoisocaproic acid (KIC), the specific activity of leucine incorporated into albumin and fibrinogen, the ¹⁴CO₂ excretion rate, and the CO₂ specific activity. The blood samples were collected in ethylenediamine tetraacetate-containing vials, and plasma was separated by centrifugation and stored at -80 C until analysis.

Analytical methods

One milliliter of plasma was used to measure in women estradiol, progesterone, FSH, and LH by the means of chemiluminescence assays (Ortho-Clinical Diagnostics, Johnson & Johnson, New Brunswick, NJ). Plasma amino acids and ketoacids were separated by cation exchange chromatography (16). The plasma specific activities of leucine and KIC were measured by the means of high performance liquid chromatography and liquid scintillation (17). The ¹⁴CO₂ excretion rate and CO₂ SA were measured as previously described (18).

The plasma proteins albumin and fibrinogen were purified from 2 mL plasma as previously described (10). Subsequently, plasma proteins were hydrolyzed in 6 N HCl at 110 C, the amino acids derived from the hydrolyzed proteins were processed as the plasma amino acids, and the protein-bound leucine specific activity was measured with high performance liquid chromatography and liquid scintillation (17).

Calculations

Total body water (liters) was calculated from the subject’s weight and height according to Hume and Weyer (19). These researchers found that in healthy adults the total body water, measured by means of tritiated water, strongly correlates with the body surface area, i.e. with height and weight. The relationship between body surface and measured total body water is linear, but it differs between women and men. Therefore, separate coefficients of linear regression were provided. Total body water was then calculated as follows: for women: total body water = (weight x 0.183809) + (height x 0.34454) - 35.270121 (Eq I); for men: total body water = (weight x 0.296785) + (height x 0.194786) - 14.01934 (Eq II), where the weight is in kilograms, and the height is in centimeters.

The fat-free body mass (kilograms) was then calculated from the total body water as follows (19): fat-free body mass = total body water/0.73 (Eq III). The rate of administration of the leucine isotope was measured by multiplying the disintegrations per min/ml infusate, by the infusion rate (milliliters per min). The estimates of whole body leucine kinetics were determined from the data obtained between 180–240 min at the isotopic and metabolic steady state, using the four-compartment model (20). The rate of leucine appearance (Ra; micromoles per kg/min) was calculated using the following formula: Ra = i/SA_KIC (Eq IV), where i is the labeled leucine infusion rate (disintegrations/kg·min), and SA_KIC is the plasma KIC specific activity (disintegrations/µmol).

The rate of leucine oxidation (Ox; micromoles/kg·min) was calculated using the precursor-product model: Ox = ( CO₂/SA_KIC) x (1/) (Eq V), where CO₂ is the ¹⁴CO₂ excretion rate (disintegrations/kg·min), and is the correction factor for the CO₂ recovery, assuming a value of 0.70 in the postabsorptive state (21). We used the same coefficient of correction for women and men because there is no evidence to hypothesize a different CO₂ fixation between genders.

The rate of nonoxidative leucine disposal (NOLD; micromoles/kg·min), an index of whole body protein synthesis, was estimated as follows: NOLD = Ra - Ox (Eq VI).

The fractional secretory rate (FSR; percent/h) of plasma pro-teins was calculated using the precursor-product model (22): FSR = [(SA_{Leu protein}/t)/SA_{KIC plasma}] x 60 x 100 (Eq VII), where SA_{Leu protein}/t is the incorporation rate of labeled leucine into pro-teins from 180–240 min, and SA_KICplasma is the mean plasma KIC specific activity during the same time period. The use of plasma KIC specific activity as a precursor pool specific activity for hepatic protein syn-thesis in the postabsorptive state has been recently validated (12).

Statistical analysis

The comparisons between the two groups were carried out for each variable using the two-tailed unpaired t test. Differences were considered significant at P < 0.05. The linear relationships between variables were measured with Pearson’s correlation coefficient.

	Results

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Subjects

Height, body weight, estimated total body water, and fat-free body mass were significantly lower in women than in men (Table 1). On the basis of the clinical history and the estradiol, progesterone, FSH, and LH measurements, women were distributed among the phases of the menstrual cycle as follows: follicular phase, n = 5 (30%); and luteal phase, n = 6 (35%). The other six women (35%) were taking low dose oral contraceptives.

Whole body protein kinetics

Between 180–240 min, the plasma specific activities of leucine, KIC, and expired CO₂ were at steady state, as was the CO₂ excretion rate (Fig. 1). The whole body leucine Ra, an index of proteolysis, was not different between women and men (Table 2). The NOLD, an index of protein synthesis, also did not differ between women and men (Table 2). However, the leucine oxidation rate was significantly lower in women than in men (Table 2). The results were similar when expressed per kg of either total body weight or estimated fat-free body mass (Table 2). Plasma leucine concentrations were similar in the two groups (women, 112 ± 33; men, 121 ± 18 µmol/L). The leucine oxidation rate per kilogram total body weight was significantly correlated (r = 0.3654; P = 0.0336) to the estimated fat-free body mass (Fig. 2), but such a relationship did not hold when leucine oxidation was expressed per kg estimated fat-free mass (r = 0.3017; P = 0.083).

View larger version (12K): [in this window] [in a new window]   Figure 1. Time course of 14CO2 excretion. During the sampling period (180–240 min), the 14CO2 excretion rate was at a plateau.

View larger version (15K): [in this window] [in a new window]   Figure 2. Correlation between leucine oxidation and estimated fat-free mass. The leucine oxidation rate per kg total body weight (micromoles per kg/min) was significantly, although weakly, correlated with fat-free mass (kilograms). However, such a correlation was not significant when leucine oxidation was corrected per kg fat-free mass (r = 0.3017; P = 0.083).

The plasma concentrations of albumin and fibrinogen were not different between women and men (data not shown). The specific activity of protein-bound leucine increased linearly between 180–240 min. The fractional secretory rates of albumin and fibrinogen were not different between women and men (Table 2).

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Our data show that in the basal postabsorptive state, protein oxidation is lower in women than in men. The other protein kinetic parameters measured in the present study, whole body proteolysis, whole body protein synthesis, and albumin and fibrinogen secretion rates, showed no gender differences.

The sex-related difference we observed for protein oxidation is independent of body composition, i.e. it is still significant when the data are corrected per the estimated fat-free body mass. The independence of leucine oxidation from body composition is also indicated by the weakness of the relationship (r = 0.3) between leucine oxidation (expressed per kg of either total body weight or estimated fat-free mass) and estimated fat-free mass. Our results are consistent with previous data obtained in trained female athletes during endurance exercise using both nitrogen balance (23, 24) and leucine kinetics (24) and indicate that the gender-related differences in protein oxidation are essentially due to a direct effect of gender on basal protein metabolism.

The possible mediators of the sex regulation of protein oxidation are numerous. Differences in the sex hormone milieu might provide an appealing explanation for the different protein oxidation rates in women and men. However, the results of previous studies on the effects of sex hormones on protein metabolism do not support this hypothesis. In fact, testosterone administration reduced protein oxidation when given at pharmacological doses (25, 26), whereas when the hormone was given at physiological doses in hypogonadal men no effect on protein oxidation was observed (27). On the other hand, in women, protein oxidation is influenced by ovarian hormones, namely progesterone, whose increase in the luteal phase of the ovarian cycle increases leucine oxidation (28). The cyclic variation in the progesterone concentration could have affected our results because we included women at all phases of the ovarian cycle. In addition, we included women taking low dose oral contraceptives, some of which have been shown to slightly increase nitrogen excretion (29). Nonetheless, this could have resulted only in an underestimation of the impact of gender on protein oxidation due to the fact that protein oxidation is enhanced in the luteal phase and during low dose oral contraceptives. In any case, none of the effects of sex hormones can explain the lower protein oxidation in women compared to men found in our study. Other hormones, such as thyroid hormones and GH, could influence protein oxidation, but it is unlikely that they are the mediators of the gender-related differences in basal protein oxidation due to the fact that thyroid function and GH secretion are similar in normal adult women and men (30, 31). As the gender-related differences in protein oxidation cannot be accounted for by the different sex hormone milieu, other gender-related factors must be considered. In vitro data suggest that amino acid utilization for oxidation might differ according to the muscle fiber type (32). Considering that gender has been shown to affect muscle fiber composition and muscle enzyme activity (33), this could provide an explanation for the differences in amino acid oxidation between the sexes.

In any case, although the underlying mechanism is still unclear, the lower basal protein oxidation rate in women compared to men can justify at least in part the lower metabolic rate previously described for women (1). This hypothesis would not be in contrast with a recent report suggesting that the difference between genders in resting energy expenditure is due to a lower fat oxidation in women (34). In fact, fat oxidation was estimated by means of indirect calorimetry, assuming that the changes in the respiratory quotient were not significantly affected by variations in protein oxidation, which was considered constant. Therefore, combining our data with those from that study, we suggest that part of the difference in the respiratory quotient between sexes could be due to differences in protein oxidation. However, assuming that body proteins contain about 8% leucine and that the energy content of proteins is 4.07 Cal/g, we estimate that in women the energy saved in the basal state due to the lower protein oxidation is about 0.70 Cal/kg·day. Considering that the basal metabolic rate in adults is approximately 21–23 Cal/kg·day (1, 4, 34), this amount of calories accounts almost entirely for the approximately 3% difference (4) in basal metabolic rate between women and men.

The absence of an apparent effect of gender on either whole body protein synthesis or proteolysis suggests that protein turnover is not influenced by gender, i.e. the rates at which proteins are degraded and synthesized are not different between women and men. The fact that protein oxidation is lower in women but protein synthesis and breakdown are similar in the two genders can be due to two reasons. Whole body protein synthesis was not directly measured, but was estimated indirectly from the rate of appearance and the oxidation rate. Therefore, we cannot exclude that the lack of a significant effect of gender on protein synthesis, i.e. a higher protein synthesis rate in women, is due to a low sensitivity of the method. However, over the long term a higher protein synthesis rate would result in an increase in the bulk of body proteins in women, which is clearly not the case. A more plausible explanation is that the amino acids saved from oxidation in the women in the basal state temporarily increase the free amino acid pool and are subsequently oxidized in other physiological states. As the concentrations of plasma leucine were similar in the two groups, we hypothesize that the amino acids not oxidized must have been retained within the cells. However, this hypothesis needs to be demonstrated with specific studies addressing this issue.

Finally, the absence of a difference in plasma protein fractional secretory rates is not surprising, as several studies have shown no differences in albumin and fibrinogen concentrations in normal adults (35, 36). This finding is also consistent with the lack of a significant effect of gender on basal whole body protein synthesis.

In conclusion, we found that the only gender-related difference in protein kinetics is a difference in protein oxidation. This finding could explain at least part of the difference in energy expenditure between women and men. In addition, we caution researchers of protein metabolism to take into account the gender composition of the experimental groups to avoid a possible gender-related bias.

Received June 11, 1998.

Revised August 13, 1998.

Accepted August 24, 1998.

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