For many decades, most of the research regarding substrate metabolism during exercise had been conducted predominately with male subjects. Given that an early report had suggested that men and women respond similarly to endurance exercise (12) and have similar muscle fiber types (12), there was little reason to question the generalization of results derived in men to women. Consequently, there was a sex bias in the exercise physiology literature, with a disproportionate number of male subjects, until research began to question the canonical concept that there were no sex differences in metabolism in response to endurance exercise (3,44). At one time, it was even considered that endurance exercise might be harmful for women, and it was not until 1984 that women could officially compete in the Olympic marathon. In fact, data showing that women can oxidize more fat during endurance exercise (see below) would suggest that women would be ideally suited to complete long-duration endurance exercise and, possibly, even outperform men in ultraendurance events. Two studies have found that women outperformed men for a distance of 85 km when matched for their marathon pace (29,42). These data indicate that any exclusion of women from endurance athletics has not been based on scientific evidence. The current review summarizes a presentation from an ACSM/APS symposium focusing on sex differences in substrate metabolism during endurance exercise from a biological and molecular perspective, with emphasis on the potential mediating effects of 17-β-estradiol.
WHOLE-BODY SUBSTRATE OXIDATION
Whole-body substrate metabolism has been evaluated during endurance exercise, using indirect calorimetry for estimation of whole-body fat and carbohydrate (CHO) use (3,9,35,39,44,51). Stable isotope tracers and urea excretion have been used to estimate the relative contribution of amino acid oxidation to intermediary metabolism during endurance exercise (32,35,36). There were several conflicting reports regarding sex differences in metabolism in the 1970s and 1980s (3,12,20). The discrepancies in the findings were not clear; however, there are important potential confounding variables in any between-group study that are magnified by inherent sex differences. Studies evaluating substrate metabolism must control for antecedent diet, habitual training status, time since the last training bout, and body composition. When matching men and women for training status, V˙O2max per kilogram of body mass is higher in men; however, it is usually identical when expressed relative to fat-free mass, given that women have approximately 5-8% body fat more than men (9,32,35,49). It has been suggested that matching on the basis of V˙O2max per kilogram of fat-free mass is the most appropriate way to match men and women for levels of fitness (23,25,35,44). Indeed, our group (9,15,32,35) and others (23,38) have found that when men and women are selected for training background, V˙O2max per kilogram of fat-free mass is the same between the sexes. Furthermore, when men and women are matched for V˙O2max per kilogram of fat-free mass, the lactate threshold is the same between sexes (23,35). Another potential confounding variable is the menstrual cycle phase and menstrual irregularities such as oligoamenorrhea and amenorrhea. For example, amenorrhea reduces the pulsatility of LH and FSH, which lowers total estrogen and progesterone concentrations to those seen in men (testosterone would still be approximately tenfold greater in men). Menstrual cycle phase does have a small influence on substrate selection during endurance exercise; however, these differences are rather small when compared with the differences between the sexes (11,15,18,24). Oral contraceptive use has a slightly greater effect on substrate selection during endurance exercise, with an increase in glycerol rate of appearance (Ra, lipolysis) being observed (18). Together, a failure to consider the aforementioned factors can increase variance and lead to false conclusions regarding sex differences.
When most of the potential confounding factors are considered, and men and women are compared within the same study, there are a number of reports concluding that the RER for women is lower than for men during endurance exercise. A summary from the mean RER for the majority of the studies published in the English literature comparing substrate oxidation in men and women during moderate-duration endurance exercise (at least 60 min) clearly shows that the RER is lower for women than for men (higher fat and lower CHO oxidation) during endurance exercise (Table 1). It is important to note that the data in Table 1 are a compilation of all reported studies, with no weighting for the aforementioned factors that could increase variance and lead to an inability to detect a difference (type II error). Nevertheless, when all of the evidence is considered together, sex differences at the level of whole-body substrate oxidation are a robust finding across studies. In addition to this meta-analysis approach, a recent summary of exercise testing results from 157 men and 147 women in the same laboratory also found higher fat oxidation in women compared with men (51). Finally, the fact that fat oxidation is higher for women than men during endurance exercise before and after a supervised endurance exercise training program (9,32) further suggests that the sex-based differences in substrate selection are attributable to sex, and not matching issues related to unequal training status. Although a higher proportionate area of type I muscle fibers in women compared with men can partially explain the higher fat-oxidation rates seen for women during endurance exercise (10,39), the fact that acute 17-β-estradiol administration to men shifts fuel selection toward higher fat oxidation (14) implies that fiber type is not the only explanation for the sex difference in metabolism.
With respect to glycogen metabolism, there does not seem to be a sex difference in basal muscle glycogen content in trained or untrained individuals (32,49). We have found attenuated glycogen use during treadmill running in women compared with men (44), and similar sex differences have been noted in rats (26,27,40). In contrast, we and others have not found a sex difference in net glycogen use during cycling exercise when using muscle biopsies (32,38). However, estimated glycogen use from data using tracer methodology has suggested glycogen sparing (9), and there was evidence of glycogen sparing during endurance cycling in the luteal phase of the menstrual cycle (15). In contrast to the variable results with respect to muscle glycogen, there is consensus showing a lower glucose Ra and rate of disappearance in women compared with men during endurance exercise (9,15,18,23). This may relate to lower sympathetic activation in women than in men (23,32,43).
Animal studies have robustly demonstrated that the administration of 17-β-estradiol attenuates glycogen use in skeletal muscle, heart, and liver (26,40). Furthermore, ovariectomy leads to an increase in muscle and hepatic glycogen use during endurance exercise in female rats (27). In humans, 17-β-estradiol administration is associated with a reduction in hepatic glucose Ra (8,14,41) and with lower epinephrine (41) during endurance exercise; however, we have not seen any effect on net glycogen use using the muscle biopsy technique (14). We have recently shown that 17-β-estradiol administration to men slightly, but significantly, reduced basal muscle glycogen concentration (14). Importantly, the administration of 17-β-estradiol to men does not seem to work indirectly through a suppression of endogenous testosterone given that short-term testosterone supplementation does not appear to alter substrate oxidation during exercise (4). Finally, one study has used pharmacological suppression of endogenous sex hormone secretion and selective replacement of 17-β-estradiol with and without progesterone in exercising women; the study found that 17-β-estradiol reduced total CHO oxidation by reducing both glucose Ra and estimated muscle glycogen use (13).
We recently compared skeletal muscle mRNA abundance in men and women, and before and after 17-β-estradiol administration, for a variety of transcripts involved in glucose transport (GLUT4), glucose phosphorylation (hexokinase II), glycogen synthesis (glycogenin, glycogen synthase 1, glycogen synthase kinase 3-α), glycogenolysis (glycogen phosphorylase), and glycolysis (phosphofructokinase). We found significantly higher resting skeletal muscle hexokinase II and GLUT4 mRNA content during the follicular phase and lower mRNA abundance for phosphofructokinase during the luteal phase (Table 2). Overall, these mRNA changes were not directionally consistent with any of the observed metabolic substrate-use data described above or with our previous observation that hexokinase II enzyme activity is similar between men and women (49). We also found that 17-β-estradiol administration to men did not alter basal mRNA abundance for any of the aforementioned genes involved in glucose metabolism (Table 2). Overall, it is likely that neither sex nor 17-β-estradiol directly mediates glycogen use in skeletal muscle, and that the sex differences in metabolism are likely responding to a sex difference in fat metabolism occurring during endurance exercise (see below).
Most studies have reported higher intramyocellular lipid (IMCL) in women compared with men (38,39,48). We have used electron microscopy to show that the higher IMCL abundance in women is attributable to more lipid droplets, and not a larger size of individual droplets (48). Biochemical assays have shown a greater use of IMCL during endurance exercise in women (38,39,43). These findings are consistent with the higher hormone-sensitive lipase mRNA and protein (PRO) content reported in women, but not with the fact that the activation of hormone-sensitive lipase after 90 min of exercise was higher in men (39). Given that IMCL content is often reported not to decrease significantly in men in response to endurance exercise (39,43), the role of hormone-sensitive lipase activation in modulating IMCL breakdown during endurance exercise is in question.
There also seems to be a sex difference in whole-body lipolysis, because we (9) and others (18) have found a higher glycerol Ra for women compared with men during endurance exercise. Importantly, the aforementioned sex differences were apparent when the glycerol Ra values were expressed relative to total body mass, and the difference would be even greater if it were expressed relative to the smaller fat-free mass inherent in women. Furthermore, arterial glycerol concentration is higher during endurance cycling in women than in men during the first 30 min (38). Animal studies have shown that 17-β-estradiol administration to male rats led to increased skeletal muscle and lower adipocyte lipoprotein lipase (LPL) activity, which could indicate a preferred storage of lipids in the skeletal muscle for immediate availability during exercise (17). In contrast to the robust effects of 17-β-estradiol on whole-body lipolysis in resting and exercised rats (17), the administration of 17-β-estradiol to men (8) and amenorrheic women (41) was not associated with a higher exercise-stimulated lipolysis (glycerol Ra) during endurance exercise; however, plasma FFA concentrations were higher after 17-β-estradiol in women (41). Studies using testosterone supplementation to postmenopausal women (4) suggest that testosterone may attenuate exercise-stimulated lipolysis, which would make lipolysis higher for women than for men.
In contrast to the lack of directional consistency between the mRNA content for genes involved in CHO metabolism and measured substrate use, the mRNA abundance for a wide variety of genes in skeletal muscle involved in fat metabolism are directionally consistent with the observation of enhanced fat use by women compared with men during endurance exercise. For example, skeletal muscle hormone-sensitive lipase (HSL) mRNA and PRO are all higher in women than in men (39). Women also have much higher skeletal muscle LPL mRNA content compared with men, with no sex differences in LPL activity (28). Fatty acid translocase (FAT/CD36) PRO is about 50% higher in women than in men (28), and women have about twofold more sarcolemmal fatty acid transporter (FATP-1) mRNA compared with men (2). Given the recent observation that FAT/CD36 may also localize to the mitochondrial membrane (22), it will be of interest to see whether there are sex differences in this localization that could indicate greater fatty acyl group transport into mitochondria in women. There does not seem to be a sex difference in carnitine palmitoyl transferase-1 in humans (1); however, 17-β-estradiol supplementation in ovariectomized rats demonstrated an increase in the maximal enzyme activity of carnitine palmitoyl transferase-1 and short-chain β-3-OH-acyl-CoA-dehydrogenase (7). We have not found a sex difference in short-chain β-3-OH-acyl-CoA-dehydrogenase or several components of the electron-transport chain enzyme activity (10,32); however, very-long-chain acyl-CoA dehydrogenase (VLCAD) is significantly higher in women than in men according to Western blot analysis of total PRO content (Fig. 1).
We have recently conducted an evaluation of many of the transcripts involved in fat metabolism in skeletal muscle, and we found a coordinate higher abundance in skeletal muscle of women compared with men (Table 2). Although mRNA abundance may not always indicate PRO abundance or activity, the directional consistency of these findings, and the fact that another laboratory has found identical findings (28,37), suggest that some aspect of sex leads to an increase in fat oxidation through a regulated process at the transcriptome level. From a regulatory perspective, it does not seem that AMP-kinase is involved in the pathway responsible for the increase in fat oxidation in women (39) (Fig. 1). As further evidence that there is some pretranslational control of fat-oxidation pathways, we have administered 17-β-estradiol to men and shown increases in lipid oxidation (14) and in many of the aforementioned mRNA species involved in fat metabolism (Table 2).
There is no question that there is an increase in the oxidation of several amino acids during endurance exercise; however, this rarely represents more than about 5% of the total energy cost. The first evidence that there may be a sex difference in oxidation of amino acids during exercise came from the observation that urinary urea excretion was higher on an exercise day (15.1-km run) versus a rest day for men, but this pattern was not seen in women (32). Subsequently, several other studies have also shown less of an increase in urea excretion for women compared with men in response to endurance exercise (30-32,35,36). There also seems to be a sex difference in amino acid oxidation, with women oxidizing less leucine (30-32,35,36). The increase in leucine oxidation is attributable to dephosphorylation and subsequent activation of branched chain 2-oxo acid dehydrogenase (BCOAD). Our group has shown that the basal activation of BCOAD was lower in women than in men, but the activation after exercise was similar between the sexes (32). Interestingly, after a 2-month period of training, there was an attenuation of BCOAD activation, which corresponded to a suppression of leucine oxidation during exercise for men and women (32). The latter observation would predict that PRO requirements would be lower after a period of training. Consistent with the latter observation, studies in recreationally trained subjects (predominantly men) have found no influence, or a minimal influence, of this type of activity on PRO requirements (50), and modest increases in better-trained athletes (33). In contrast, well-trained to top sport athletes have much higher volumes of training at higher intensities and are not always able to maintain optimal CHO and energy balance. These factors could raise PRO requirements to above that of a sedentary person (5,47).
Three studies used nitrogen balance to determine the dietary PRO requirements for very highly trained male endurance athletes (runners, triathletes, cyclists, nordic skiers), and these estimates came out to approximately 1.4-1.7 g·kg−1·d−1 (5,19,47). We have found that well-trained men and women were both in negative nitrogen balance while consuming a PRO intake that was considered by the Canadian and U.S. government bodies to be adequate to meet the needs of all adults (men = 0.94 g·kg−1·d−1; women 0.8 g·kg−1·d−1) (35). Even though the women were consuming less PRO than the men, they were in a less negative nitrogen balance (35). Carol Meredith and colleagues (33) also have shown that dietary PRO requirements were higher than the U.S. recommended daily intake for young and middle-aged male runners who were training at about the same level as those in our latter study (35).
The most likely candidate for the lower rates of amino acid oxidation and the less negative impact on whole-body PRO balance would be 17-β-estradiol. Consequently, we administered 17-β-estradiol to 12 men in a randomized, double-blind fashion for 8 d, and we found that leucine oxidation during 90 min of cycling was significantly lower by 16%, and this was associated with an increase in fat oxidation (44%) and a reduction in CHO oxidation (16%) (21). We have recently measured the skeletal muscle mRNA content for both BCOAD and its kinase (inactivator), BCOADK, and found that BCOADK was significantly higher for women in both phases of the menstrual cycle, and although mRNA content went up 33% in men on 17-β-estradiol, it was not statistically significant (Fu et al., PhD thesis, 2007).
Nutritional implications of sex differences in metabolism.
On the basis of our observations that women have lower CHO oxidation compared with men during endurance exercise, we evaluated whether this would attenuate their ability to CHO load and derive an ergogenic benefit (45). We randomly allocated athletic men and women to follow a CHO-loading strategy for 4 d while consuming one of two diets (habitual = 55% energy from CHO; high = 75% energy from CHO). Muscle biopsies were taken, and a performance ride (60 min at 75% V˙O2peak → time to exhaustion at 85% V˙O2peak) was completed after each loading session. As expected, the men showed a significant increase in muscle glycogen concentration (+41%), with a corresponding increase in performance (+45%); however, the women showed neither an increase in muscle glycogen concentration (0%) nor an increase in performance (+5%) (45). The mechanism behind the latter result was unclear; however, the relatively low energy intake by the women limited the amount of dietary CHO, such that even on the 75% CHO diet, only 6.5 g·kg−1·d−1 were consumed (45). In contrast, the hallmark study by Sherman and colleagues that set the standard for CHO loading used men where the high-CHO diet represented >8.0 g·kg−1·d−1 (16). For many women, attaining a level of > 8.0 g CHO·kg−1·d−1 would require that 100% of the diet came from CHO; consequently, the only practical method to achieve this would be to increase total energy intake. Consequently, we repeated our CHO-loading study, using men and women with three trials (habitual = 55% energy from CHO; high CHO = 75% energy from CHO; high CHO + extra energy = 75% energy from CHO + 34% more energy (> 8.0 g CHO·kg−1·d−1)). For men, there was a progressive increase in muscle glycogen concentration on each of the three diets, and for women, we again found no muscle glycogen increase in the 75% CHO diet, yet they did show an increase with the high CHO + extra energy diet, albeit to levels that were about 50% that of men (49). Others have found that CHO-loading protocols do not result in the same level of glycogen supercompensation as demonstrated in studies done with men (52). From a practical perspective, attempting to improve exercise performance through CHO loading is futile unless women can get CHO intake to > 8.0 g·kg−1·d−1, which requires an energy intake of 2500 kcal·d−1 with 70% CHO for a 55-kg woman.
The amount of exogenous glucose that is oxidized from sports drinks is similar (53) and, in some cases, slightly higher (7,36), for women compared with men. In contrast to the limited ability for women to CHO load, the dietary recommendations for men and women with respect to sport drink consumption during exercise (36,53) seem to be similar. In the early postexercise recovery period, the rate of muscle glycogen resynthesis from either a CHO or CHO + PRO drink provided immediately and at 1 h after endurance exercise is similar for men and women (46). Given that the rate of muscle glycogen resynthesis during the first 4 h after endurance exercise is negligible (46), it is prudent for both sexes to provide CHO and PRO in the early postexercise period if a subsequent training session or race is scheduled later in the same day, to maximize glycogen resynthesis.
Although no study has specifically calculated PRO requirements for top sport women athletes, the available nitrogen-balance data in men (5,19,47), and the nitrogen-balance data that we have in endurance-trained men and women (35), would suggest that requirements for women are about 25% less than for men (i.e., top sport women = 1.2-1.3 g·kg−1·d−1). It is important to note that most athletes will get this level of dietary PRO intake from an energy-adequate habitual diet containing 10-15% PRO; however, we and others have found that some female and a few male endurance athletes can have intake levels as low as 0.45 g·kg−1·d−1 (32,34,35). Consequently, it is important to evaluate PRO intake on a gram-per-kilogram basis, as opposed to a percentage of the diet, to avoid low intakes that can be seen in energy-restricting athletes. This is particularly important because a low-energy intake is well known to negatively impact PRO requirements (6).
Women oxidize more fat and less CHO and amino acids during endurance exercise as compared with men. At least some of the sex difference is attributable to the higher levels of 17-β-estradiol in women, because administration of 17-β-estradiol to men leads to lower amino acid and CHO (and higher fat) oxidation during endurance exercise. The skeletal muscle mRNA transcript abundance for genes involved in fat metabolism are coordinately consistent with the higher fat oxidation seen in women than in men, and some of the transcripts are altered in a directionally similar pattern with the administration of 17-β-estradiol to men. Given that sex differences in the mRNA abundance for genes involved in CHO and PRO metabolism are not directionally consistent with the observed sex differences in exercise metabolism or the effect of 17-β-estradiol in animals or humans, it is likely that sex/17-β-estradiol primarily influence fat oxidation during exercise and that CHO and PRO metabolism follow by metabolic demand.
The majority of the author's work in the current study was funded by a grant in aid from NSERC Canada. Some of the research was conducted using equipment purchased through a donation from Mr. Warren Lammert and Kathy Corkins. A number of current and former graduate students and postdoctoral fellows have contributed to the work presented from the author's laboratory (Dr. Mazen Hamadeh, Ms. Michaela Devries, Mr. Minghua Fu, Ms. Amy Maher, Mr. Stuart Lowther, Dr. Sherry Carter, Dr. Scott McKenzie, Dr. Stuart Phillips, Dr. Courtney Rennie). None of the work completed in Dr. Tarnopolsky's laboratory involving any aspect of sex differences in metabolism or sex differences in nutrition have been funded by a company.
1. Berthon PM, Howlett RA, Heigenhauser GJ, Spriet LL. Human skeletal muscle carnitine palmitoyltransferase I activity determined in isolated intact mitochondria. J Appl Physiol
2. Binnert C, Koistinen HA, Martin G, et al. Fatty acid transport protein-1 mRNA expression in skeletal muscle and in adipose tissue in humans. Am J Physiol Endocrinol Metab
3. Blatchford FK, Knowlton RG, Schneider DA. Plasma FFA responses to prolonged walking in untrained men and women. Eur J Appl Physiol Occup Physiol
4. Braun B, Gerson L, Hagobian T, Grow D, Chipkin SR. No effect of short-term testosterone manipulation on exercise substrate metabolism in men. J Appl Physiol
5. Brouns F, Saris WH, Stroecken J, et al. Eating, drinking, and cycling. A controlled Tour de France simulation study, part II. Effect of diet manipulation. Int J Sports Med
. 1989;10(Suppl. 1):S41-8.
6. Calloway DH, Kurzer MS. Menstrual cycle and protein requirements of women. J Nutr
7. Campbell SE, Febbraio MA. Effect of ovarian hormones on mitochondrial enzyme activity in the fat oxidation pathway of skeletal muscle. Am J Physiol Endocrinol Metab
8. Carter S, McKenzie S, Mourtzakis M, Mahoney DJ, Tarnopolsky MA. Short-term 17beta-estradiol decreases glucose R(a) but not whole body metabolism during endurance exercise. J Appl Physiol
9. Carter SL, Rennie C, Tarnopolsky MA. Substrate utilization during endurance exercise in men and women after endurance training. Am J Physiol Endocrinol Metab
10. Carter SL, Rennie CD, Hamilton SJ, Tarnopolsky MA. Changes in skeletal muscle in males and females following endurance training. Can J Physiol Pharmacol
11. Casazza GA, Suh SH, Miller BF, Navazio FM, Brooks GA. Effects of oral contraceptives on peak exercise capacity. J Appl Physiol
12. Costill DL, Daniels J, Evans W, Fink W, Krahenbuhl G, Saltin B. Skeletal muscle enzymes and fiber composition in male and female track athletes. J Appl Physiol
13. D'Eon TM, Sharoff C, Chipkin SR, Grow D, Ruby BC, Braun B. Regulation of exercise carbohydrate metabolism by estrogen and progesterone in women. Am J Physiol Endocrinol Metab
14. Devries MC, Hamadeh MJ, Graham TE, Tarnopolsky MA. 17beta-estradiol supplementation decreases glucose rate of appearance and disappearance with no effect on glycogen utilization during moderate intensity exercise in men. J Clin Endocrinol Metab
15. Devries MC, Hamadeh MJ, Phillips SM, Tarnopolsky MA. Menstrual cycle phase and sex influence muscle glycogen utilization and glucose turnover during moderate intensity endurance exercise. Am J Physiol Regul Integr Comp Physiol
16. Sherman WM, Costill DL, Fink WJ,Miller JM. Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. Int J Sports Med
17. Ellis GS, Lanza-Jacoby S, Gow A, Kendrick ZV. Effects of estradiol on lipoprotein lipase activity and lipid availability in exercised male rats. J Appl Physiol
18. Friedlander AL, Casazza GA, Horning MA, et al. Training-induced alterations of carbohydrate metabolism in women: women respond differently from men. J Appl Physiol
19. Friedman JE, Lemon PW. Effect of chronic endurance exercise on retention of dietary protein. Int J Sports Med
20. Froberg K, Pedersen PK. Sex differences in endurance capacity and metabolic response to prolonged, heavy exercise. Eur J Appl Physiol Occup Physiol
21. Hamadeh MJ, Devries MC, Tarnopolsky MA. Estrogen supplementation reduces whole body leucine and carbohydrate oxidation and increases lipid oxidation in men during endurance exercise. J Clin Endocrinol Metab
22. Holloway GP, Bezaire V, Heigenhauser GJ, et al. Mitochondrial long chain fatty acid oxidation, fatty acid translocase/CD36 content and carnitine palmitoyltransferase I activity in human skeletal muscle during aerobic exercise. J Physiol
23. Horton TJ, Grunwald GK, Lavely J, Donahoo WT. Glucose kinetics differ between women and men, during and after exercise. J Appl Physiol
24. Horton TJ, Miller EK, Glueck D, Tench K. No effect of menstrual cycle phase on glucose kinetics and fuel oxidation during moderate-intensity exercise. Am J Physiol Endocrinol Metab
25. Horton TJ, Pagliassotti MJ, Hobbs K, Hill JO. Fuel metabolism in men and women during and after long-duration exercise. J Appl Physiol
26. Kendrick ZV, Ellis GS. Effect of estradiol on tissue glycogen metabolism and lipid availability in exercised male rats. J Appl Physiol
27. Kendrick ZV, Steffen CA, Rumsey WL, Goldberg DI. Effect of estradiol on tissue glycogen metabolism in exercised oophorectomized rats. J Appl Physiol
28. Kiens B, Roepstorff C, Glatz JF, et al. Lipid-binding proteins and lipoprotein lipase activity in human skeletal muscle: influence of physical activity and gender. J Appl Physiol
29. Kim JS, Cross JM, Bamman MM. Impact of resistance loading on myostatin expression and cell cycle regulation in young and older men and women. Am J Physiol Endocrinol Metab
30. Lamont LS, McCullough AJ, Kalhan SC. Gender differences in leucine, but not lysine, kinetics. J Appl Physiol
31. Lamont LS, McCullough AJ, Kalhan SC. Gender differences in the regulation of amino acid metabolism. J Appl Physiol
32. McKenzie S, Phillips SM, Carter SL, Lowther S, Gibala MJ, Tarnopolsky MA. Endurance exercise training attenuates leucine oxidation and BCOAD activation during exercise in humans. Am J Physiol Endocrinol Metab
33. Meredith CN, Zackin MJ, Frontera WR, Evans WJ. Dietary protein requirements and body protein metabolism in endurance-trained men. J Appl Physiol
34. Otis CL, Drinkwater B, Johnson M, Loucks A, Wilmore J. American College of Sports Medicine position stand. The female athlete triad. Med Sci Sports Exerc
35. Phillips SM, Atkinson SA, Tarnopolsky MA, MacDougall JD. Gender differences in leucine kinetics and nitrogen balance in endurance athletes. J Appl Physiol
36. Riddell MC, Partington SL, Stupka N, Armstrong D, Rennie C, Tarnopolsky MA. Substrate utilization during exercise performed with and without glucose ingestion in female and male endurance trained athletes. Int J Sport Nutr Exerc Metab
37. Roepstorff C, Schjerling P, Vistisen B, et al. Regulation of oxidative enzyme activity and eukaryotic elongation factor 2 in human skeletal muscle: influence of gender and exercise. Acta Physiol Scand
38. Roepstorff C, Steffensen CH, Madsen M, et al. Gender differences in substrate utilization during submaximal exercise in endurance-trained subjects. Am J Physiol Endocrinol Metab
39. Roepstorff C, Thiele M, Hillig T, et al. Higher skeletal muscle alpha2AMPK activation and lower energy charge and fat oxidation in men than in women during submaximal exercise. J Physiol
40. Rooney TP, Kendrick ZV, Carlson J, et al. Effect of estradiol on the temporal pattern of exercise-induced tissue glycogen depletion in male rats. J Appl Physiol
41. Ruby BC, Robergs RA, Waters DL, Burge M, Mermier C, Stolarczyk L. Effects of estradiol on substrate turnover during exercise in amenorrheic females. Med Sci Sports Exerc
42. Speechly DP, Taylor SR, Rogers GG. Differences in ultra-endurance exercise in performance-matched male and female runners. Med Sci Sports Exerc
43. Steffensen CH, Roepstorff C, Madsen M, Kiens B. Myocellular triacylglycerol breakdown in females but not in males during exercise. Am J Physiol Endocrinol Metab
44. Tarnopolsky LJ, MacDougall JD, Atkinson SA, Tarnopolsky MA, Sutton JR. Gender differences in substrate for endurance exercise. J Appl Physiol
45. Tarnopolsky MA, Atkinson SA, Phillips SM, MacDougall JD. Carbohydrate loading and metabolism during exercise in men and women. J Appl Physiol
46. Tarnopolsky MA, Bosman M, Macdonald JR, Vandeputte D, Martin J, Roy BD. Postexercise protein-carbohydrate and carbohydrate supplements increase muscle glycogen in men and women. J Appl Physiol
47. Tarnopolsky MA, MacDougall JD, Atkinson SA. Influence of protein intake and training status on nitrogen balance and lean body mass. J Appl Physiol
48. Tarnopolsky MA, Rennie CD, Robertshaw HA, Fedak-Tarnopolsky SN, Devries MC, Hamadeh MJ. Influence of endurance exercise training and sex on intramyocellular lipid and mitochondrial ultrastructure, substrate use, and mitochondrial enzyme activity. Am J Physiol Regul Integr Comp Physiol
49. Tarnopolsky MA, Zawada C, Richmond LB, et al. Gender differences in carbohydrate loading are related to energy intake. J Appl Physiol
50. Todd KS, Butterfield GE, Calloway DH. Nitrogen balance in men with adequate and deficient energy intake at three levels of work. J Nutr
51. Venables MC, Achten J, Jeukendrup AE. Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol
52. Walker JL, Heigenhauser GJ, Hultman E, Spriet LL. Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women. J Appl Physiol
53. Wallis GA, Dawson R, Achten J, Webber J, Jeukendrup AE. Metabolic response to carbohydrate ingestion during exercise in males and females. Am J Physiol Endocrinol Metab