Gender differences in basal protein kinetics in young adults

Sex as a Biological Variable in Nutrition Research: From Human Studies to Animal Models

Biological sex is a fundamental source of phenotypic variability across species. Males and females have different nutritional needs and exhibit differences in nutrient digestion and utilization, leading to different health outcomes throughout life. With personalized nutrition gaining popularity in scientific research and clinical practice, it is important to understand the fundamentals of sex differences in nutrition research. Here, we review key studies that investigate sex dimorphism in nutrition research: sex differences in nutrient intake and metabolism, sex-dimorphic response in nutrient-restricted conditions, and sex differences in diet and gut microbiome interactions. Within each area above, factors from sex chromosomes, sex hormones, and sex-specific loci are highlighted.

Plasma amino acid profile, a biomarker for visceral adipose tissue that can substitute for waist circumference in Japanese Americans

BACKGROUND

Greater visceral fat area (VFA) is associated with cardiometabolic outcomes. We sought to identify cross-sectional and longitudinal associations between amino acid (AA) levels and VFA in Japanese-Americans.

METHODS

From the cohort of 342 Japanese-American participants (51% men) in a study of diabetes risk factors who were free from diabetes, we measured levels of 20 AA by mass spectrometry, height, weight, waist circumference (WC), VFA, subcutaneous fat area by single-slice CT at the umbilicus. Using AA significantly associated with VFA in univariate analyses, we created a VFA prediction index, termed the 4A index. We compared area under receiver-operating characteristic curve (AUROC) of the 4A index to WC and an existing AA index (Yamakado et al. Clin Obes 2012) in classifying VFA at different cutoff values. We fit age-adjusted linear regression models to evaluate associations between AA levels and change in VFA over 5 years.

RESULTS

All 20 AA levels significantly detected VFA excess, but WC was better. The 4A index performed better than Yamakado index at classifying VFA ≥ 100 cm² (0.798, 0.807 vs. 0.677, 0.671 for men and women, p < 0.0033) and VFA ≥ sex-specific median values (0.797, 0.786 vs. 0.676, 0.629 for men and women, p < 0.0017). AA significantly associated with change in VFA over 5 years were asparagine, glutamate, glutamine, glycine, methionine, proline, threonine in men; and histidine, isoleucine, tyrosine in women (p < 0.05).

CONCLUSIONS

The 4A index can serve as a biomarker for VFA in Japanese-Americans and be considered for this purpose when WC is not available.

Feeding female soldiers: Consideration of sex-specific nutrition recommendations to optimise the health and performance of military personnel

Appropriate nutrition recommendations are required to optimise the health and performance of military personnel, yet limited data are available on whether male and female military personnel have different nutrition requirements.

OBJECTIVES

To consider the evidence for sex-specific nutrition requirements to optimise the health and performance of military personnel.

DESIGN

Narrative review.

METHODS

Published literature was reviewed, with a focus on sex-specific requirements, in the following areas: nutrition for optimising muscle mass and function, nutrition during energy deficit, and nutrition for reproductive and bone health.

RESULTS

There are limited data on sex differences in protein requirements but extant data suggest that, despite less muscle mass, on average, in women, sex-specific protein feeding strategies are not required to optimise muscle mass in military-aged individuals. Similarly, despite sex differences in metabolic and endocrine responses to energy deficit, current data do not suggest a requirement for sex-specific feeding strategies during energy deficit. Energy deficit impairs health and performance, most notably bone and reproductive health and these impairments are greater for women. Vitamin D, iron and calcium are important nutrients to protect the bone health of female military personnel due to increased risk of stress fracture.

CONCLUSIONS

Women have an increased incidence of bone injuries, less muscle mass and are more susceptible to the negative effects of energy deficit, including compromised reproductive health. However, there are limited data on sex differences in response to various nutrition strategies designed to improve these elements of health and performance. Future studies should evaluate whether sex-specific feeding recommendations are required.

Influence of Second Generation Oral Contraceptive Use on Adaptations to Resistance Training in Young Untrained Women

Dalgaard, LB, Jørgensen, EB, Oxfeldt, M, Dalgaard, EB, Johansen, FT, Karlsson, M, Ringgaard, S, and Hansen, M. Influence of second generation oral contraceptive use on adaptations to resistance training in young untrained women. J Strength Cond Res XX(X): 000-000, 2020-The study purpose was to determine effects of using second generation oral contraceptives (OC) on muscle adaptations to resistance training in young untrained women. Twenty users and 18 nonusers of OC completed a 10-week supervised progressive resistance training program. Before and after the intervention, muscle cross-sectional area (mCSA) of the quadriceps was measured using magnetic resonance imaging and muscle fiber CSA (fCSA) was determined by immunohistochemistry. In addition, body composition (DXA, fat mass/fat-free mass), maximal isometric muscle strength (dynamometry), 5 repetition maximum (5RM) leg press strength, counter movement jump (CMJ) height, and average power using a modified Wingate test were determined. Serum hormone analysis ensured OC compliance and 4-day food records documented dietary intake. After the training period, quadriceps mCSA (OC: 11.0 ± 6.0% vs. non-OC: 9.2 ± 5.0%, p = 0.001), type II fCSA (OC: 19.9 ± 7.9% vs. non-OC: 16.6 ± 7.2%, p = 0.05), muscle strength (knee extension, knee flexion and 5RM, p < 0.001), and functional power (CMJ, AP, p < 0.001) were significantly increased with no significant difference between the groups. However, a tendency toward a greater increase in fat-free mass (FFM) in the OC group was observed (OC: 3.7 ± 3.8% vs. non-OC: 2.7 ± 3.5%, p = 0.08). Collectively, use of second generation OCs in young untrained women did not significantly improve adaptations to 10 weeks of resistance training compared with nonusers. The trend toward greater gains in FFM in the OC group warrant future studies.

Effect of acute total sleep deprivation on plasma melatonin, cortisol and metabolite rhythms in females

Disruption to sleep and circadian rhythms can impact on metabolism. The study aimed to investigate the effect of acute sleep deprivation on plasma melatonin, cortisol and metabolites, to increase understanding of the metabolic pathways involved in sleep/wake regulation processes. Twelve healthy young female participants remained in controlled laboratory conditions for ~92 hr with respect to posture, meals and environmental light (18:00-23:00 hr and 07:00-09:00 hr <8 lux; 23:00-07:00 hr 0 lux (sleep opportunity) or <8 lux (continuous wakefulness); 09:00-18:00 hr ~90 lux). Regular blood samples were collected for 70 hr for plasma melatonin and cortisol, and targeted liquid chromatography-mass spectrometry metabolomics. Timepoints between 00:00 and 06:00 hr for day 1 (baseline sleep), day 2 (sleep deprivation) and day 3 (recovery sleep) were analysed. Cosinor analysis and MetaCycle analysis were performed for detection of rhythmicity. Night-time melatonin levels were significantly increased during sleep deprivation and returned to baseline levels during recovery sleep. No significant differences were observed in cortisol levels. Of 130 plasma metabolites quantified, 41 metabolites were significantly altered across the study nights, with the majority decreasing during sleep deprivation, most notably phosphatidylcholines. In cosinor analysis, 58 metabolites maintained their rhythmicity across the study days, with the majority showing a phase advance during acute sleep deprivation. This observation differs to that previously reported for males. Our study is the first of metabolic profiling in females during sleep deprivation and recovery sleep, and offers a novel view of human sleep/wake regulation and sex differences.

Muscle metabolism and atrophy: let's talk about sex

Skeletal muscle health is a strong predictor of overall health and longevity. Pathologies affecting skeletal muscle such as cancer cachexia, intensive care unit treatment, muscular dystrophies, and others are associated with decreased quality of life and increased mortality. Recent research has begun to determine that these muscular pathologies appear to present and develop differently between males and females. However, to our knowledge, there has yet to be a comprehensive review on musculoskeletal differences between males and females and how these differences may contribute to sex differences in muscle pathologies. Herein, we present a review of the current literature on muscle phenotype and physiology between males and females and how these differences may contribute to differential responses to atrophic stimuli. In general, females appear to be more susceptible to disuse induced muscle wasting, yet protected from inflammation induced (such as cancer cachexia) muscle wasting compared to males. These differences may be due in part to differences in muscle protein turnover, satellite cell content and proliferation, hormonal interactions, and mitochondrial differences between males and females. However, more works specifically examining muscle pathologies in females are necessary to more fully understand the inherent sex-based differences in muscle pathologies between the sexes and how they may correspond to different clinical treatments.

Lack of sexual dimorphism on the inhibitory effect of alcohol on muscle protein synthesis in rats under basal conditions and after anabolic stimulation

Previous studies indicate women have a higher blood alcohol (i.e., ethanol) and acetaldehyde concentration after consuming an equivalent amount of alcohol, and that women are more susceptible to the long-term negative health effects of alcohol. However, there is a paucity of data pertaining to whether there is a sexual dimorphic response in skeletal muscle to alcohol. Adult male and female Sprague-Dawley rats were used and the primary endpoint was in vivo determined muscle (gastrocnemius) protein synthesis (MPS). The initial study indicated MPS did not differ in female rats during proestrus, estrus, metestrus, or diestrus; hence, subsequent studies used female rats irrespective of estrus cycle phase. There was no difference in MPS between male and female rats under basal fasted conditions, and the time- and dose-responsiveness of both groups to the inhibitory effect of acute alcohol did not differ. The ability of alcohol to suppress MPS was comparable in male and female rats pretreated with alcohol dehydrogenase inhibitor 4-methylpyrazol. Chronic alcohol feeding for 6 weeks decreased MPS in male but not in female rats; however, MPS was reduced in both sexes at 14 weeks. Finally, oral gavage of leucine increased MPS similarly in male and female rats and chronic alcohol feeding for 14 weeks prevented the anabolic effect in both sexes. These data suggest normal fluctuations in ovarian hormones do not significantly alter MPS in female rats, and that there is no sexual dimorphic response to the effects of acute alcohol intoxication on MPS. While chronic alcohol consumption appeared to decrease MPS at an early time point in male compared to female rats, there was no sex difference in the suppressive effect of alcohol at a later time point. Overall, these data do not support the prevailing belief that females are more susceptible than males to alcohol's catabolic effect on MPS.

Sex hormones and macronutrient metabolism

The biological differences between males and females are determined by a different set of genes and by a different reactivity to environmental stimuli, including the diet, in general. These differences are further emphasized and driven by the exposure to a different hormone flux throughout the life.

These differences have not been taken into appropriate consideration by the scientific community. Nutritional sciences are not immune from this “bias” and when nutritional needs are concerned, females are considered only when pregnant, lactating or when their hormonal profile is returning back to “normal,” i.e., to the male-like profile.

The authors highlight some of the most evident differences in aspects of biology that are associated with nutrition.

This review presents and describes available data addressing differences and similarities of the “reference man” vs. the “reference woman” in term of metabolic activity and nutritional needs. According to this assumption, available evidences of sex-associated differences of specific biochemical pathways involved in substrate metabolism are reported and discussed. The modulation by sexual hormones affecting glucose, amino acid and protein metabolism and the metabolization of nutritional fats and the distribution of fat depots, is considered targeting a tentative starting up background for a gender concerned nutritional science.

Metabolite Profiles of Male and Female Humboldt Penguins

We examined 185 metabolites in 30 adult Humboldt Penguins (Spheniscus humboldti) nesting at the Punta San Juan Marine Protected Area, Peru, in order to examine gender differences in metabolome profiles, particularly those involved in metabolism and energetics. The majority of the compounds identified were fatty (26% of total identified compounds), organic (19%), and amino (16%) acids. We were able to differentiate male and female penguins with 96.6% accuracy on the basis of 12 metabolites, most of which are involved in lipid and carbohydrate metabolism. These included 2-oxoglutarate, erythronic acid, GABA, mannitol, sedoheptulose 7-phosphate, and serine and six metabolites present in higher concentrations in females compared to males (2-aminoadipic acid, O-phosphorylethanolamine, glycerol 2-phosphate, glycerol 3-phosphate, pantothenic acid, and creatinine). Of these, 2-oxoglutarate and glycerol 3-phosphate were key metabolites distinguishing gender. Our results indicated that male and female Humboldt Penguins were characterized by differing metabolic states. Such differences could be important to individual and brood survival in times of environmental stress.

Comparison of the effects of 52 weeks weight loss with either a high-protein or high-carbohydrate diet on body composition and cardiometabolic risk factors in overweight and obese males

BACKGROUND

A high-protein (HP), low-fat weight-loss diet may be advantageous for improving cardiometabolic health outcomes and body composition. To date, only limited research has been conducted in male participants.

OBJECTIVE

To evaluate the medium to long-term effects of two, low-fat, hypocaloric diets differing in carbohydrate:protein ratio on body composition and cardiometabolic health outcomes in overweight and obese males.

DESIGN

One hundred and twenty males (age 50.8±9.3 (s.d.) years, body mass index 33.0±3.9 kg m(-2)) were randomly assigned and consumed a low-fat, isocaloric, energy-restricted diet (7 MJ per day) with either HP (protein:carbohydrate:fat %energy, 35:40:25) or high carbohydrate (HC; 17:58:25). Body weight, body composition and cardiometabolic risk factors were assessed at baseline and after 12 and 52 weeks.

RESULTS

Sixty-eight participants completed the study (HP, n=33; HC, n=35). At 1 year both the groups experienced similar reductions in body weight (HP, -12.3±8.0 kg (-12%); HC, -10.9±8.6 kg (-11%); P=0.83 time × group interaction) and fat mass (-9.9±6.0 kg (-27%) vs -7.3±5.8 kg (-22%); P=0.11). Participants who consumed the HP diet lost less fat-free mass (-2.6±3.7 kg (-4%) vs -3.8±4.7 kg (-6%); P<0.01). Both groups experienced similar increases in high-density lipoprotein cholesterol (8%) and reductions in total cholesterol (-7%), low-density lipoprotein cholesterol (-9%), triglycerides (-24%), glucose (-3%), insulin (-38%), blood pressure (-7/-12%) and C-reactive protein (-29%), (P0.14).

CONCLUSION

In overweight and obese men, both a HP and HC diet reduced body weight and improved cardiometabolic risk factors. Consumption of a HP diet was more effective for improving body composition compared with an HC diet.

Age and sex affect protein metabolism at protein intakes that span the range of adequacy: comparison of leucine kinetics and nitrogen balance data

Research suggests that changes in leucine oxidation (leuox) with feeding may reflect adult protein requirements. We evaluated this possibility by assessing the effects of age, sex, and different protein intakes on whole-body leucine kinetics and nitrogen balance. Thirty-four young (n=18, 22-46 years) and old (n=16, 63-81 years) men and women completed three 18-day trials with protein intakes of 0.50, 0.75 and 1.00 g protein·kg body weight(-1)·d(-1). Fasting and fed-state leucine kinetics were quantified on day 12 of each trial using a primed, constant infusion of L-[1-13C]leucine. Protein requirement was estimated using classical nitrogen balance measurements and calculations. Leucine kinetics parameters were influenced by age and sex across all protein intakes. With feeding, leuox increased more in old vs. young adults. Independent of age, fasting and fed-state leuox were lower, and net leucine balance (fasting+fed-state) was higher in women vs. men. Among all subjects and protein intakes, nitrogen balance was correlated with fed-state leuox (r=0.39), fed-state leucine balance (r=0.60), net leucine balance (r=0.49) and the change in leuox from the fasting to fed state (r=0.49) (P<.05 for all results). At the highest protein intake, the change in leuox with feeding was inversely correlated with protein requirement (r=-0.39). These findings indicate that leucine kinetics, especially leuox, reflect nitrogen balance-based estimates of the need for dietary protein and generally support the view that protein requirement is comparable between young and old adults.

Protein metabolism in women and men: similarities and disparities

PURPOSE OF REVIEW

To provide an objective and comprehensive review of the recent literature addressing the effects of sex on protein metabolism. We also evaluate whether these differences can be attributed to physiology or methodology. Because of the developmental changes in hormonal milieu and body composition that occur across life, the literature has been examined in a longitudinal manner across the lifespan.

RECENT FINDINGS

Throughout most points of life, men and women of similar health status and BMI display fairly similar protein turnover rates. However, some investigators have reported sexual dimorphism in protein metabolism, which may be partly attributable to differences in fat-free mass and/or methodology. In periods of significant changes in the hormonal milieu (puberty and menopause), sex differences may become more evident. Finally, anabolic stimuli such as feeding and exercise may help highlight any discrepancies in protein turnover between men and women.

SUMMARY

Sex differences in protein metabolism, if any, are most evident during the main phases of hormonal changes, and may be also due to differences in body composition. However, methodological issues and sample size must be considered when designing and evaluating these studies.

Resistance exercise increases leg muscle protein synthesis and mTOR signalling independent of sex

AIM

Sex differences are evident in human skeletal muscle as the cross-sectional area of individual muscle fibres is greater in men than in women. We have recently shown that resistance exercise stimulates mammalian target of rapamycin (mTOR) signalling and muscle protein synthesis in humans during early post-exercise recovery. Therefore, the aim of this study was to determine if sex influences the muscle protein synthesis response during recovery from resistance exercise.

METHODS

Seventeen subjects, nine male and eight female, were studied in the fasted state before, during and for 2 h following a bout of high-intensity leg resistance exercise. Mixed muscle protein fractional synthetic rate was measured using stable isotope techniques and mTOR signalling was assessed by immunoblotting from repeated vastus lateralis muscle biopsy samples.

RESULTS

Post-exercise muscle protein synthesis increased by 52% in the men and by 47% in the women (P < 0.05) and was not different between groups (P > 0.05). Akt phosphorylation increased in both groups at 1 h post-exercise (P < 0.05) and returned to baseline during 2 h post-exercise with no differences between groups (P > 0.05). Phosphorylation of mTOR and its downstream effector S6K1 increased significantly and similarly between groups during post-exercise recovery (P < 0.05). eEF2 phosphorylation decreased at 1- and 2 h post-exercise (P < 0.05) to a similar extent in both groups.

CONCLUSION

The contraction-induced increase in early post-exercise mTOR signalling and muscle protein synthesis is independent of sex and appears to not play a role in the sexual dimorphism of leg skeletal muscle in young men and women.

Fibrinogen kinetics and protein turnover in obese non-diabetic males: effects of insulin

BACKGROUND

Although hyperfibrinogenemia and insulin resistance are common in obesity and diabetes mellitus, the impact of obesity per se on fibrinogen turnover and the insulin effects on fibrinogen and protein kinetics is unknown.

METHODS

We measured fibrinogen and albumin fractional (FSR) and absolute (ASR) synthesis rates, as well as protein turnover, in non-diabetic, obese and in control male subjects both before and following an euglycemic, euaminoacidemic, hyperinsulinemic clamp, using L-[(2)H(3)]-Leucine isotope infusion.

RESULTS

In the obese, basal fibrinogen concentrations was approximately 25% greater (p < 0.035), and fibrinogen pool approximately 45% greater (p < 0.005), than in controls. Both FSR and ASR of fibrinogen were similar to control values. With hyperinsulinemia, although fibrinogen FSR and ASR were not significantly modified with respect to baseline in either group, fibrinogen ASR resulted to be approximately 50% greater in the obese than in controls (p < 0.015). Hyperinsulinemia equally stimulated albumin synthesis and suppressed leucine appearance from endogenous proteolysis in both groups. Amino acid clearance was also similar. In the obese, the insulin-mediated glucose disposal was approximately 50% lower (p < 0.03) than in controls, and it was inversely correlated with fibrinogen ASR during the clamp in both groups (r = - 0.58).

CONCLUSIONS

In obese, non-diabetic males, post absorptive fibrinogen production is normal. Whole-body amino acid disposal, basal and insulin-responsive protein degradation, and albumin synthesis are also normal. However, the greater fibrinogen ASR in the obese with hyperinsulinemia, and the inverse relationship between insulin sensitivity and clamp fibrinogen production, suggest a role for hyperinsulinemia and/or insulin resistance on fibrinogen production in obesity.

Response of fractional synthesis rate (FSR) of fibrinogen, concentration of D-dimer and fibrinolytic balance to physical activity-based intervention in obese children

BACKGROUND

Physical activity-induced reduction in obesity-related hyperfibrinogenemia in children has been reported. The underlying mechanisms remain elusive. Further, the effect of such interventions on fibrinolysis in children is scarce.

OBJECTIVES

To investigate in obese children, before and after a physical activity-based intervention: (i) the mechanistic role of fractional synthesis rate (FSR) of fibrinogen in the reduction of hyperfibrinogenemia; and (ii) the changes in fibrinolytic factors.

METHODS

Subjects included 21 (age > 14 < 18 years; Tanner stage, IV-V) children (15 obese, BMI >95%tile for age and sex and six lean, BMI <85%tile). After baseline measurements of FSR of fibrinogen, and concentrations of fibrinogen, D-dimer, PAI-1 and t-PA in all children, studies were repeated after a 3-month randomized controlled physical activity-based lifestyle intervention in obese children only.

RESULTS

FSR of fibrinogen was higher (P = 0.002) in the obese (vs. lean) group, which was reduced (P = 0.001) after intervention. This almost completely accounted for the reduction in obesity-related hyperfibrinogenemia. High levels of D-dimer decreased (P = 0.001) after intervention, whereas fibrinolysis was not enhanced.

CONCLUSIONS

The direct reduction in the FSR of fibrinogen and the remarkable correlation between the magnitudes of reduction in fibrinogen FSR and concentration signify a mechanistic role for FSR in the regulation of physical activity-induced reversal of hyperfibrinogenemia in obese children. The congruent reductions in the FSR of fibrinogen and the concentrations of fibrinogen and D-dimer in response to intervention despite depressed fibrinolysis suggest an overall improvement in the hypercoagulable state in obese children with physical activity-based lifestyle intervention.

Basal muscle intracellular amino acid kinetics in women and men

Sexual dimorphism in skeletal muscle mass is apparent, with men having more muscle mass and larger individual muscle cells. However, no sex-based differences have been detected in blood forearm phenylalanine turnover, although whole body leucine oxidation has been reported to be greater in men than in women. We hypothesized that sex differences in intracellular amino acid turnover may account for these discrepancies, with men having a higher intracellular turnover than women. We studied young, healthy women (women, n = 8) and men (men, n = 10) following an overnight fast. Phenylalanine, leucine, and alanine muscle intracellular kinetics were assessed using stable isotope methodologies, femoral arteriovenous blood sampling, and muscle biopsies. Muscle intracellular amino acid kinetics were reported relative to both leg volume and lean leg mass because of sex differences in leg volume and in muscle and fat distribution. When expressed per leg volume (nmol.min(-1).100 ml leg volume(-1)), phenylalanine net balance (women: -16 +/- 4, men: -31 +/- 5), release from proteolysis in the blood (women: 46 +/- 9, men: 75 +/- 10) and intracellular availability (women: 149 +/- 23, men: 241 +/- 35), and alanine production, utilization, and intracellular availability were higher in men (P < 0.05). However, when the kinetic parameters were normalized per unit of lean leg mass, all differences disappeared. Muscle fractional synthetic rate was also not different between women and men. We conclude that there are no sex-based differences in basal muscle intracellular amino acid turnover when the data are normalized by lean mass. It remains to be determined if there are sex differences in intracellular amino acid metabolism following anabolic or catabolic stimuli.

Synthesis rates of total liver protein and albumin are both increased in patients with an acute inflammatory response

The general perception that catabolism and inflammation are associated with a high synthesis rate of total liver protein and a low albumin synthesis rate has been challenged in recent years by several studies in man, indicating that the synthesis rate of albumin in response to a catabolic insult is increased rather than decreased. Thus changes in liver protein synthesis rates in conjunction with catabolism and acute inflammation in man need to be characterized better. The aim of the present study was to measure protein synthesis rates of total liver protein and albumin during a state of acute inflammation. Patients (n = 10) undergoing acute laparoscopic cholecystectomy due to acute cholecystitis were investigated. FSRs (fractional synthesis rates) of total liver protein (liver biopsy specimens) and albumin (plasma samples) were investigated as early as possible during the surgical procedure, using a flooding dose of L-[2H5]phenylalanine. The results were compared with a reference group of patients without cholecystitis undergoing elective laparoscopic cholecystectomy (n = 17). FSR of total liver protein was 60% higher (P < 0.001) and the FSR of albumin was 45% higher (P < 0.01) in the cholecystitis patients compared with the control group. In conclusion, the synthesis rates of total liver protein and albumin are both increased in patients with an acute general inflammatory reaction undergoing laparoscopic cholecystectomy.

No effect of menstrual cycle on myofibrillar and connective tissue protein synthesis in contracting skeletal muscle

We tested the hypothesis that acute exercise would stimulate synthesis of myofibrillar protein and intramuscular collagen in women and that the phase of the menstrual cycle at which the exercise took place would influence the extent of the change. Fifteen young, healthy female subjects were studied in the follicular (FP, n=8) or the luteal phase (LP, n=7, n=1 out of phase) 24 h after an acute bout of one-legged exercise (60 min of kicking at 67% W(max)), samples being taken from the vastus lateralis in both the exercised and resting legs. Rates of synthesis of myofibrillar and muscle collagen proteins were measured by incorporation of [(13)C]leucine. Myofibrillar protein synthesis (means+/-SD; rest FP: 0.053+/-0.009%/h, LP: 0.055+/-0.013%/h) was increased at 24-h postexercise (FP: 0.131+/-0.018%/h, P<0.05, LP: 0.134+/-0.018%/h, P< 0.05) with no differences between phases. Similarly, muscle collagen synthesis (rest FP: 0.024+/- 0.017%/h, LP: 0.021+/- 0.006%/h) was elevated at 24-h postexercise (FP: 0.073+/- 0.016%/h, P<0.05, LP: 0.072+/- 0.015%/h, P<0.05), but the responses did not differ between menstrual phases. Therefore, there is no effect of menstrual cycle phase, at rest or in response to an acute bout of exercise, on myofibrillar protein synthesis and muscle collagen synthesis in women.

In vivo whole body and organ arginine metabolism during endotoxemia (sepsis) is dependent on mouse strain and gender

Arginine metabolism involves various organs such as the kidney, the intestines, and the liver, which act together in an interorgan axis. Major pathways for arginine production are protein breakdown and de novo arginine production from citrulline; disposal of arginine is mainly used for protein synthesis or used by the enzymes arginase and nitric oxide synthase (NOS). To assess in vivo organ arginine metabolism under normal conditions and during endotoxemia we used a mouse model, and analyzed for gender and strain differences. Male and female inbred FVB and C57BL6/J mice were anesthetized and catheterized to study whole body, gut, liver, renal and muscle metabolism, using a stable isotope infusion protocol. Animals were treated with saline or lipopolysaccharide. Plasma arginine levels tended to be higher in female mice, although levels were not significantly different from male mice (P = 0.09). Although not all significantly different, whole body arginine production and arginine clearance tended to be higher in C57BL6/J mice (P < 0.1), while citrulline (P = 0.05), NO (P = 0.08), and de novo arginine (P < 0.01) production were higher in FVB mice. During endotoxemia, NO production increased in general (P < 0.05), while whole body arginine clearance increased in FVB mice, but decreased in C57BL6/J mice (P < 0.01). At the organ level, portal-drained viscera (PDV) arginine metabolism was higher in FVB than in C57BL6/J mice (P < 0.05). During endotoxemia, liver arginine metabolism decreased in general (P < 0.05), while strain differences existed for PDV, muscle, and renal arginine metabolism. In conclusion, stable isotope techniques in multicatheterized mice allow measurements of arginine metabolism on whole body and organ level. Strain and gender differences are present in arginine metabolism under physiological conditions and during endotoxemia.

Amino acid ingestion improves muscle protein synthesis in the young and elderly

We recently demonstrated that muscle protein synthesis was stimulated to a similar extent in young and elderly subjects during a 3-h amino acid infusion. We sought to determine if a more practical bolus oral ingestion would also produce a similar response in young (34 +/- 4 yr) and elderly (67 +/- 2 yr) individuals. Arteriovenous blood samples and muscle biopsies were obtained during a primed (2.0 micromol/kg) constant infusion (0.05 micromol.kg(-1).min(-1)) of L-[ring-2H5]phenylalanine. Muscle protein kinetics and mixed muscle fractional synthetic rate (FSR) were calculated before and after the bolus ingestion of 15 g of essential amino acids (EAA) in young (n = 6) and elderly (n = 7) subjects. After EAA ingestion, the rate of increase in femoral artery phenylalanine concentration was slower in elderly subjects but remained elevated for a longer period. EAA ingestion increased FSR in both age groups by approximately 0.04%/h (P < 0.05). However, muscle intracellular (IC) phenylalanine concentration remained significantly higher in elderly subjects at the completion of the study (young: 115.6 +/- 5.4 nmol/ml; elderly: 150.2 +/- 19.4 nmol/ml). Correction for the free phenylalanine retained in the muscle IC pool resulted in similar net phenylalanine uptake values in the young and elderly. EAA ingestion increased plasma insulin levels in young (6.1 +/- 1.2 to 21.3 +/- 3.1 microIU/ml) but not in elderly subjects (3.0 +/- 0.6 to 4.3 +/- 0.4 microIU/ml). Despite differences in the time course of plasma phenylalanine kinetics and a greater residual IC phenylalanine concentration, amino acid supplementation acutely stimulated muscle protein synthesis in both young and elderly individuals.

Age and aerobic exercise training effects on whole body and muscle protein metabolism

Aging in humans is associated with loss of lean body mass, but the causes are incompletely defined. Lean tissue mass and function depend on continuous rebuilding of proteins. We tested the hypotheses that whole body and mixed muscle protein metabolism declines with age in men and women and that aerobic exercise training would partly reverse this decline. Seventy-eight healthy, previously untrained men and women aged 19-87 yr were studied before and after 4 mo of bicycle training (up to 45 min at 80% peak heart rate, 3-4 days/wk) or control (flexibility) activity. At the whole body level, protein breakdown (measured as [13C]leucine and [15N]phenylalanine flux), Leu oxidation, and protein synthesis (nonoxidative Leu disposal) declined with age at a rate of 4-5% per decade (P < 0.001). Fat-free mass was closely correlated with protein turnover and declined 3% per decade (P < 0.001), but even after covariate adjustment for fat-free mass, the decline in protein turnover with age remained significant. There were no differences between men and women after adjustment for fat-free mass. Mixed muscle protein synthesis also declined with age 3.5% per decade (P < 0.05). Exercise training improved aerobic capacity 9% overall (P < 0.01), and mixed muscle protein synthesis increased 22% (P < 0.05), with no effect of age on the training response for either variable. Fat-free mass, whole body protein turnover, and resting metabolic rate were unchanged by training. We conclude that rates of whole body and muscle protein metabolism decline with age in men and women, thus indicating that there is a progressive decline in the body's remodeling processes with aging. This study also demonstrates that aerobic exercise can enhance muscle protein synthesis irrespective of age.

Effect of lifestyle changes on whole-body protein turnover in obese adolescents

OBJECTIVE

To investigate the effect of lifestyle changes on whole-body protein turnover (WBPT) in obese adolescents.

DESIGN/METHODS

Randomized and controlled nonpharmacological intervention study of WBPT in obese adolescents using stable isotope dilution techniques.

SUBJECTS AND MEASUREMENTS

We studied a total of 21 adolescents (11 boys and 10 girls, matched for their pubertal status) of which 15 were obese (age=15.8+/-0.4 y old and BMI=38.6+/-3.3 kg/m(2)) and six were lean controls (age=16.0+/-0.4 y old and BMI=21.3+/-1.2 kg/m(2)). The obese subjects were subjected to a randomized controlled lifestyle intervention program that involved moderate physical activity and diet changes for 3 months. A group of lean age-matched subjects was also studied at baseline to compare the WBPT in obese and lean adolescents. The studies were performed during a primed, continuous infusion of L-[1-(13)C]leucine. Leucine appearance rate (Leu Ra) was used as an index of whole protein breakdown and the nonoxidative portion of leucine disposal (NOLD) as an index of whole-body protein synthesis.

RESULTS

The obese groups showed significantly higher body mass index (BMI), fat mass (FM), percent body fat (%BF), fat-free mass (FFM), resting energy expenditure (REE) and WBPT compared to the lean controls. The intervention program resulted in a redistribution of the parameters of body composition without apparent changes in BMI or body weight. There was a significant decrease in WBPT in the obese intervention group, but not in the obese control group. Insulin levels also decreased significantly in the obese group after intervention but not in the obese control group, whereas the glucose concentrations remained normal in all groups at baseline and also after intervention/or control.

CONCLUSIONS

Results from the current study suggest: (i). abonormalities of protein metabolism occur early in the clinical course of obesity and (ii). these abnormalities are modifiable by moderate lifestyle changes in obese adolescents. The mechanism for these changes in WBPT in obese adolescents as well as their impact on specific cardiovascular risk factors and turnover of specific proteins will require further investigation.

Effects of fatty acids on hepatic amino acid catabolism and fibrinogen synthesis in young healthy volunteers

Increased synthesis rate of fibrinogen, an independent risk factor for cardiovascular disease, was recently reported in obese insulin-resistant female adolescents with chronic elevated nonesterified fatty acids (NEFA). It is unknown whether a short-term change of NEFA concentrations controls hepatic fibrinogen synthesis. Therefore, 10 healthy male volunteers (24.5 +/- 3.3 yr, body mass index 23.5 +/- 2.9 kg/m2) were investigated in random order under basal and elevated NEFA for 8 h. Leucine metabolism, the fractional synthesis rates (FSR) of plasma fibrinogen, and endogenous urea production rates were measured during primed, continuous infusion of [1-13C]leucine and [15N2]urea, respectively. Plasma alpha-[13C]ketoisocaproic acid and [15N2]urea enrichment values were measured with GC-MS. Plasma fibrinogen was isolated with the beta-alanine method, and fibrinogen-related [13C]leucine enrichment was analyzed by GC-CIRMS. Lipofundin infusion and subcutaneous heparin tripled NEFA and triglycerides in the tests. Plasma glucose, circulating insulin, human C-peptide, and plasma glucagon were not changed by the study procedure. Fibrinogen FSR were significantly lower in tests with NEFA elevation (18.44 +/- 4.67%) than in control tests (21.48 +/- 4.32%; P < 0.05). Plasma fibrinogen concentrations measured were not significantly different (NEFA test subjects: 1.85 +/- 0.33, controls: 1.97 +/- 0.54 g/l). Parameters of leucine metabolism, such as leucine rate of appearance, leucine oxidation, and nonoxidative leucine disposal, were not influenced by NEFA elevation, and endogenous urea production remained unchanged. NEFA contributes to short-term regulation of fibrinogen FSR in healthy volunteers under unchanged hormonal status, leucine metabolism, and overall amino acid catabolism. Its contribution might be of relevance at least after fat-rich meals, counteracting by reduction of FSR the blood viscosity increase implied by hyperlipidemia.

Increased synthesis rate of fibrinogen as a basis for its elevated plasma levels in obese female adolescents

Increased concentrations of plasma fibrinogen, an independent risk factor for cardiovascular disease (CVD), in obese children have been reported. The underlying mechanism for this, however, remains to be defined. In the current study, we measured the fractional synthesis rates (FSR) of plasma fibrinogen in six healthy postpubertal obese girls [body mass index (BMI) 36.6 +/- 1.8 kg/m(2); age 16.6 +/- 0.5 yr] and six age-matched lean normal control girls (BMI 20.8 +/- 0.7 kg/m(2); age 16.4 +/- 0.4 yr) during a primed, continuous infusion of L-[1-(13)C]leucine in the postabsorptive state. The method involved purification of plasma fibrinogen by use of immunoaffinity chromatography followed by measurement of [(13)C]leucine enrichment using gas chromatography-combustion-isotope ratio mass spectrometry. The FSR of fibrinogen in obese girls (35.06 +/- 2.61%/day) was almost double that in lean girls (17.02 +/- 1.43%/day), and this increase was associated with a relative increase in plasma concentration of fibrinogen as well as BMI in the subjects studied. Obese subjects had high fasting insulin levels (138 +/- 47 pmol/l) compared with lean subjects (54 +/- 11 pmol/l), whereas their glucose concentrations were similar (4.5 +/- 0.3 mmol/l in obese and 4.4 +/- 0.4 mmol/l in lean subjects), suggesting insulin resistance. The doubling of the FSR of fibrinogen provides novel insight into the mechanism of elevated levels of plasma fibrinogen and suggests a primary role for increased synthesis in producing the hyperfibrinogenemia associated with obesity. This finding may have important implications in the design of therapies for modulating plasma fibrinogen levels in obesity and/or CVD in childhood.

Gender related differences in the effect of aging on blood amino acid compartmentation*

This work has been focused on the study of the variations in blood amino acid compartmentation (plasma and blood cells) with aging, both in men and women. Aging is a situation which, under the influence of gender, involves a decline in body weight functions and variations in energy metabolism with a deterioration of muscular metabolism leading to changes in amino acid handling. We determined the blood levels of individual amino acids in whole blood, plasma compartment and blood cell compartment of 51 men and 51 women. Subjects were classified in three age groups-AG1 (18 to 35 y), AG2 (35-50 y) and AG3 (more than 50 y). Aging was accompanied by significant changes in blood levels of amino acids showing gender-linked differences which were distinct for both blood compartments (plasma and blood cells). In men, aging was accompanied by a drop in blood levels of several amino acids, due mainly to the plasma compartment, whereas in women aging brought about a rise in blood levels of various amino acids mainly in blood cell compartment. This paper contributes to enhancing the physiological importance of the blood cell pool in the handling of amino acids.