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Gender Differences in Amino Acid Use During Endurance Exercise

Posted on: Friday, 20 January 2006, 06:00 CST

By Lamont, Linda S

There are gender differences in the substrate used to fuel prolonged physical activity. Males rely on carbohydrate and amino acids (97%), while females use predominantly fat (62%) to fuel endurance exercise. This review looks at the mechanisms that regulate these gender differences, with a focus on amino acid metabolism. Gender-selective nutritional recommendations are indicated for physically active individuals and some cardiac patients who engage in intense aerobic activity.

Key words: dietary protein, sport nutrition, endurance exercise, gender-selective nutritional recommendations

2005 International Life Sciences Institute

doi: 10.1301/nr.2005.dec.419-422

In the beginning minutes of a prolonged exercise session, glycogen and muscle triglycerides are used to fuel energy needs. However, as time progresses, blood-borne free fatty acids, glucose, and (as will be discussed in this review) amino acids are needed to meet exercise energy demands. The branched-chain amino acids, in particular leucine, play a critical role in whole-body metabolism during prolonged exercise. These amino acids act as energy substrates and function as nitrogen donors for the formation of alanine, glutamine, and aspartate.1 One experimental technique used to study exercise amino acid kinetics is the isotope infusion procedure, with the stable isotope of leucine (1-^sup 13^C leucine) being the tracer of choice. Leucine can be completely degraded by skeletal muscle, and when the first carbon (the ^sup 13^C label) is removed, it is irreversibly committed to its oxidative pathway.1 The ^sup 13^C carbon label can then be detected in the expired CO2.

Research conducted over the past 25 years indicates that leucine is significantly oxidized during prolonged exercise.1 Recently, it has been determined that there are distinct gender patterns in the use of amino acids to fuel endurance exercise energy needs. These studies are detailed in Table 1.2-6 In all but one of the experiments summarized in Table 1, leucine oxidation was reported to be significantly greater in male than in female exercisers.4-8 Furthermore, that study had a small sample size (n = 4) and may not have met the requirements for adequate statistical power. These five gender comparisons indicate that the leucine oxidation rate was approximately double in exercising males than in equally trained females. It is worth noting that this gender difference in exercise leucine kinetics has not been reported for other amino acids such as lysine.5

Figure 1 contrasts amino acid use as an exercise fuel source with the contribution made by the other energy substrates, notably fat and carbohydrate.2,5 Figure 1 also shows that fat and carbohydrate provide the majority of exercise energy needs for both genders. Specifically, males get most of their energy from carbohydrate and amino acids (97%), while females depend primarily on fat (62%) to fuel their exercise needs. After being fully elucidated, these gender differences in amino acid use will be helpful when designing dietary recommendations for endurance athletes. The gender-specific need for macronutrients are computed from these fractional amounts (when using respiratory exchange ratio or RER), but the absolute daily energy expenditure must also be considered, and this is usually smaller in women. While emerging research indicates that the genders have different macronutrient needs, it also highlights the specific mechanisms that regulate these differences in whole-body fuel usage.

Some of the metabolic regulators that have been studied include sex steroid hormone activity, regulatory enzyme activity, substrate availability, and catecholamine responsiveness.6-9 Although it might appear intuitive that sex steroid hormones mediate exercise fuel use in each gender, the research has been extensively reviewed and has led to the opposite conclusion.7 A recent review concluded that the sex hormones may regulate gender differences in exercise metabolism in laboratory animals, but probably are not the major regulator in humans.7

Table 1. Summary of Recent Gender Comparisons of Leucine Oxidation

Another metabolic regulator of gender variability in exercise amino acid kinetics could be located within the metabolic pathway of the specific amino acid. Gender differences in the enzymatic activity of one or more critical enzymes within the oxidative pathway of the amino acid could account for whole-body differences in exercise amino acid metabolism between men and women. One human study failed to find a gender difference in the activity of a critical enzyme in leucine's oxidative pathway: branched-chain 2- oxoacid dehydrogenase.4 This study noted, however, that a differential activation of the hepatic form of the enzyme could not be ruled out. Also, the regulatory activity of branched-chain 2- oxoacid dehydrogenase has been found to vary in gender comparison studies using male and female laboratory animals.8

A third potential regulator of these gender differences in whole- body amino acid use during exercise are that there are variations in catecholamine responsiveness between men and women.6,10 Experimental support for this theory can be found in a recent study of exercise metabolism conducted during pharmacological blockade of the catecholamine receptors. In this study, a 7-day dose of propranalol (80 mg twice daily) was given to a group of men and women to block the β1- and β2-adrenergic receptors of the sympathetic nervous system.6 An hour of exercise was performed at half of the subject's maximal capacity prior to and after the chronic beta- blockade. Figures 2, 3, and 4 illustrate the results of the four experimental trials that were compared: placebo control in men, placebo control in women, beta-blockade in men, and beta-blockade in women.

Figure 1. Substrate contribution to whole-body energy needs during exercise in women (left) and men (right). Adapted from data in Lamont et al.5 and Phillips et al.2

The men and women were matched for important factors that would alter exercise metabolism, such as pre-exercise diets, age, and exercise training habits. Figure 2 shows that the women exercisers had an higher free fatty acid concentration than their male counterparts during the placebo trial. When women exercised in the presence of a beta-adrenergic blockade, their free fatty acid concentrations further increased above placebo control values. In contrast, the 7-day beta-blockade failed to alter free fatty acid circulation in the males (placebo trial vs. beta-blockade trial). These males did demonstrate a greater carbohydrate catabolism during the placebo control trial than did the females when determined with indirect calorimetry (RER) (Figure 3). When the male subjects exercised in the presence of the beta-blockade, their reliance on carbohydrate and amino acids (Figure 4) was further enhanced above their normal values (placebo control trial). Figure 4 highlights the dramatic increase in amino acid oxidation in the exercised, beta- blocked males, and underscores the fact that no such increase was found in the exercising women. Figures 2, 3, and 4 highlight the gender-selective nature of nutrient use during prolonged aerobic activity.

Figure 2. Gender differences in circulating free fatty acids (FFA) with a beta-adrenergic blockade and 60 minutes of exercise. Women had a significantly greater FFA compared with men during placebo exercise. The females exhibited further increased FFA concentrations when exercise was conducted during beta-blockade. Black circles = female placebo; gray circles = female beta- blockade; black triangles = male placebo; gray triangles = male beta- blockade; * = P < 0.05 between placebo and beta-blockade; ** = P < 0.001 between genders. (From Lament et al.6; reprinted with permission from the American Physiological Society.)

Males and females also differ in their catecholamine responsiveness.6,10 While there was an up-regulation of leucine oxidation in men who were beta-blocked, amino acid oxidation remained unchanged in beta-blocked women. On the other hand, women had an increased fat mobilization when their catecholamine receptors were blocked.

While research on gender-selective nutrient use continues, these findings have implications for professionals who provide athletes and active individuals with dietary advice. First, these gender comparisons indicate that dietary protein requirements will not be similarly impacted in habitually active males and females. The question of what is an optimal dietary protein intake for endurance athletes remains a controversial topic.11 Some argue that endurance exercise increases the need for dietary protein, while others suggest that an increased intake is not necessary.11-13 This gender- comparison research further clouds the issue, because it indicates that dietary protein recommendations must be gender specific. second, males have a dramatic increase in amino acid oxidation when exercising in the presence of a pharmacological dose of a common beta-blocking drug. These drugs are commonly used to treat heart disease and hypertension, and prolonged, aerobic exercise is indicated as a lifestyle modification for these diseases. Therefore, nutritionists may need to monitor the dietary protein and \specific amino acid intakes in their male patients who are being simultaneously treated with beta-blocking drugs and prolonged exercise as interventions for heart health.6-10 This nutrient monitoring might not be necessary for all cardiac patients, but would seem to be indicated in those who are engaged in prolonged aerobic exercise of moderate to high intensity. Third, because females demonstrate a preference for fat and not carbohydrate or amino acids as an exercise substrate, dietary interventions such as carbohydrate loading may not be equally beneficial for men and women.14,15

Figure 3. Gender differences in respiratory exchange ratio (RER) during 60 minutes of exercise. Males had a significantly greater RER compared with women during placebo exercise. During beta-blockade, the males further increased RER above resting values, while no such change was observed in women. A, Black circles = female placebo; open circles = female beta-blockade; ** P < 0.001 between genders. B, Black triangles = male placebo; open triangles = male beta- blockade; + = P < 0.05 rest vs. exercise; * = P < 0.04 placebo vs. blockade. (From Lament et al.6; reprinted with permission from the American Physiological Society).

Figure 4. Gender differences in leucine oxidation with beta- adrenergic blockade. Males had a greater leucine oxidation rate compared with females (during rest and during 60 minutes of exercise) during placebo control. Males also had a significant increase in leucine oxidation when exercising in the presence of a beta-blockade. Black bars = female placebo; light gray right crosshatches = female beta-blockade; dark gray bars = male placebo; light gray left crosshatches = male beta-blockade; * = P < 0.05 between male placebo and beta-blockade trial (exercise); ** = P < 0.005 between genders (rest and exercise). From Lament et al.6; reprinted with permission from the American Physiological Society.

Athletes wishing to enhance their endurance performance with pre- exercise dietary manipulations may find the technique of carbohydrate loading of selective value to one gender and not the other.15 Nutritional supplementation with amino acid and/or protein formulations, although still very controversial, may not be expected to have similar effects in male and female endurance athletes.

REFERENCES

1. Hood DA, Terjung RL. Amino acid metabolism during exercise and following endurance training. Sports Med. 1990;9:23-35.

2. Phillips SM, Atkinson SA, Tarnopolsky MA, MacDougall JD. Gender differences in leucine kinetics and nitrogen balance in endurance athletes. J Appl Physiol. 1993;75:2134-2141.

3. Bowtell JL, Leese GP, Smith K, et al. Modulation of whole body protein metabolism, during and after exercise, by variation of dietary protein. J Appl Physiol. 1998;85:1744-1752.

4. 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. 2000;278:E580-E587.

5. Lamont LS, McCullough AJ, Kalhan SC. Gender differences in leucine, but not lysine, kinetics. J Appl Physiol. 2001;91:357-362.

6. Lamont LS, McCullough AJ, Kalhan SC. Gender differences in the regulation of amino acid metabolism. J Appl Physiol. 2003;95:1259- 1265.

7. Tarnopolsky MA. Gender differences in substrate metabolism during endurance exercise. Can J Appl Physiol. 2000;25:312-327.

8. Kobayashi R, Shimomura Y, Murakami T, et al. Gender difference in regulation of branched-chain amino acid catabolism. Biochem J. 1997;327(part 2):449-453.

9. Tate CA, Holtz RW. Gender and fat metabolism during exercise: a review. Can J Appl Physiol. 1998; 23:570-582.

10. Lamont LS, McCullough AJ, Kalhan SC. β-Adrenergic blockade heightens the exercise-induced increase in leucine oxidation. Am J Physiol. 1995; 268(5 part 1):E910-E916.

11. Rennie MJ, Tipton KD. Protein and amino acid metabolism during and after exercise and the effects of nutrition. Ann Rev Nutr. 2000;20:457-483.

12. Nutrition and athletic performance. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine. J Am Diet Assoc. 2000;100:1543-1556.

13. Tarnopolsky M. Protein requirements for endurance athletes. Nutrition. 2004;20:662-668.

14. Tarnopolsky MA, Zawada C, Richmond LB, et al. Gender differences in carbohydrate loading are related to energy intake. J Appl Physiol. 2001 ;91:225-230.

15. Andrews JL, Sedlock DA, Flynn MG, Navalta JW, Ji H. Carbohydrate loading and supplementation in endurance-trained women runner. J Appl Physiol. 2003;95:584-590.

Linda S. Lamont, PhD

Dr. Lament is with the Exercise Science Program, University of Rhode Island, Kingston, Rhode Island, USA.

Please address all correspondence to: Dr. Linda Lamont, 25 West Independence Way, Suite J, Room 105, Kingston, RI 02881; Phone: 401- 874-5449; Fax: 401-874-5630; E-mail: LLA4983U@ postoffice.uri.edu.

Copyright International Life Sciences Institute Dec 2005

Source: Nutrition Reviews

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