Avoiding the Wall : Why women do not need to carbohydrate load. Jamie Justice. Audience: Women s marathon running groups and charity marathon coaches

Transcription

1 1 Avoiding the Wall : Why women do not need to carbohydrate load Jamie Justice Audience: Women s marathon running groups and charity marathon coaches The marathon s dreaded mile 18 wall is enough to give any runner or coach nightmares. Coaches develop training plans and nutritional strategies so that novice and high level athletes alike can survive this devastating energy plummet. As any experienced marathon runner would tell you, the key to racing well in a long distance sporting event and burning through the wall is a carbohydrate-rich precompetition meal. As demonstrated in men, by eating a carbohydrate (CHO) rich meal the competitor maximizes his muscle glycogen, a form of stored carbohydrate. Having more glycogen means his body has greater glucose stores, the easiest fuel for muscle to use during exercise (1,2). However, one great flaw exists with this pre-race carbohydrate loading ritual: it may not benefit women. Only a few researchers have investigated carbohydrate loading in women, and not one has found that women prolong time to fatigue by eating more carbohydrates before running (3,4,5). For instance, researchers recently determined that women could carbohydrate load if they increased not only percentage of carbohydrates but total calorie intake by 34% as well (4,5). Yet these same researchers did not test performance. Not testing physical performance limits the scope of these studies because even though women can carbohydrate load, there is still no evidence that greater muscle glycogen content translates into less fatigue during race conditions in women. In fact, researchers suggest that even though women improve their glycogen content like men, this fuel source may go unused during the course of a marathon (7, 8, 9).

2 2 As new research accumulates regarding the distinct differences between how men and women use carbohydrates and fats to fuel exercise, these new findings suggest that women may not need to carbohydrate load to avoid that glycogen-depletion crash. For instance, researchers now know that women burn more fat and less carbohydrate than men during exercise (6, 10, 11,12). By not burning as much carbohydrate in exercise, women s glycogen stores are not being used to the same extent as men. This means that much of the extra glycogen obtained by carbohydrate loading may remain unused during a long race. Therefore women that carbohydrate load are not eating a diet specific to their unique physiology during exercise; these women are eating as if they are men. Yet marathon coaches and runners still advise women to carbohydrate load like men because a gap in knowledge exists between the research findings in which women burn more fat while sparing more glycogen and the real world in which women actually use these fats and carbohydrates to complete a race. For instance, some researchers claim that the magnitude of the difference between how men and women use carbohydrates versus fats during exercise is relatively small (11, 12, 13). If the difference in carbohydrate use is small between the genders, performance may not be affected. With a small enough difference in the rate of glycogen depletion, both genders may still need to carbohydrate load to prolong time to glycogen depletion and subsequent fatigue during the marathon. However, as I will show, these relatively small differences discovered through research in minute exercise bouts add up to become very large differences in an endurance feat such as the marathon. In fact, women burn fats so efficiently that carbohydrate loading may be unnecessary. I will argue that carbohydrate loading will not benefit women s marathon performance by (a) presenting studies that highlight women s ability to optimally rely on their fat stores to fuel exercise and (b) demonstrating how this difference in fat versus carbohydrate burning delays glycogen depletion, and eliminates the need for women to carbohydrate load.

3 3 Women Use More Fats for Fuel than Men during Submaximal Intensity Exercise Women may not need to carbohydrate load because they use more fat and less carbohydrate to fuel exercise at any given sub-maximal intensity compared with men. For instance, Tarnopolsky et al. (6) calculated the relative rates of carbohydrate and fat use during a 90 minute exercise bout. As shown in Table 1, the men burned approximately 242 kcal fat while women burned 428 kcal fat during the exercise bout. This clearly indicates that even though the women had less muscle mass and body weight, they still used a greater total amount of fats for fuel versus the men. In fact the women not only burned nearly twice the fat calories, but 43% of the women s total caloric expenditure consisted of fat compared to only 19% of total caloric expenditure from fats for the men. Additionally, men burned a significantly greater amount of carbohydrate; they oxidized 959 kcal accounting for 78% of total energy expenditure. Women, on the other hand, used only 549 kcal carbohydrate accounting for 54% of total energy expenditure. This indicates that because women burn a greater amount of fat during exercise they also spare muscle glycogen. Sparing glycogen means that women may not deplete their glycogen stores to the same extent that men do during exercise. Because women deplete glycogen at a slower rate, their need to enhance levels of glycogen through carbohydrate loading may not be necessary. Table 1. Differences in Carbohydrate Vs Fat Use between Men and Women (6) Men Women %VO2 max 62% 63% Treadmill Exercise Duration 93 minutes 98 minutes Total Energy Expenditure (kcal) 1236 kcal 1012 kcal Total Fats Used (grams) 26.9 g 47.6 g Total Fats Used (kcal) 242 kcal 428 kcal Total Carbohydrates Used (grams) 239 g 137 g Total Carbohydrates Used (kcal) 959 kcal 549 kcal The researchers controlled for both similar VO2 max and for similar training status. Controlling for similar VO2max may limit the results because of two very important facts about VO2max: (a) VO2

4 4 max increases with increased fitness and (b) women naturally have a lower VO2 max than men (1). Based on these two facts, the researcher s choice to control for VO2max may be confounding because in order for the women s VO2maxs to be equal with men the women may have achieved a greater level of fitness versus the men. A more highly trained athlete burns more fats and fewer carbohydrates for a given percent of VO2 max in comparison to someone not as well trained (1). Thus the women may have been better trained than the men and would have burned more fat due to differing fitness levels versus a true gender difference. However, Tarnopolsky et al. (6) also controlled for training status as evaluated by training logs and not just VO2 max, which negates the potential difference in fitness between men and women. Evaluating the training logs between the subjects ensures that the subjects were similarly trained. For instance, if two potential subjects were selected because they ran the same number of miles at the same intensity for the same amount of time, then they obviously have similar fitness levels. Therefore, because the researchers compared and controlled for the days spent training, number of miles and intensity of workouts between the genders, a true gender difference, not just a difference in training level, accounts for the study s result that women burn more fat and less carbohydrate than. Additionally, as seen in Figure 1 below, many other researchers confirm Tarnopolsky et al. s (6) conclusion that women burn more fat and less carbohydrate than men during exercise (Figure 1). These researchers confirm Tarnopolsky et al. s calculations and also explore the possible mechanisms accounting for the observed differences. Together these other researchers provide a clear picture of exactly how women rely on fats for fuel while sparing carbohydrates in exercise. Additionally, this mechanistic background further supports Tarnopolsky et al. s conclusion and confirms that their findings were not the result of a methodological mistake.

5 5 Figure 1. Multiple Studies Confirm the Existence of a Gender Difference during Exercise Fat in muscle use for fuel (13) Fat burning responses (15, 16, 17) Glycogen use for fuel (6, 10, 11, 18) Fat stores in muscle (14, 15) Use More Fat + Spare More CHO** (6, 10, 11, 18) CHO burning hormones (16, 17) No Need to CHO Load for Women* *all of the above research findings (in blue) factor into the more broad finding that women use more fat and while sparing carbohydrate during exercise (yellow). These facts ground my claim that because women burn fats more efficiently and do not use as much carbohydrate, the the time until glycogen stores are depleted is prolonged. Together this indicates that women may not need to carbohydrate load. **CHO = carbohdyrate (14, 15): Levels of intramuscular triglycerides (IMTG) are increased in women s muscle versus men. This means that women have more fat in their muscle that is readily available for use during exercise. (13): Women not only have a greater supply of IMTG s but also use this fat source to a much greater extent than men during exercise. (16,17): Women have an increase in the lypolytic responses which are responsible for improving the ability to burn and transport fats for fuel during exercise. Women also have less epinephrine present during exercise than men, which is a hormone that increase the level of glycogen breakdown and use, and may account for the increased burning of carbohydrates by men. (6,10,11,18): By using an indirect calorimetry technique to calculate the amount of carbohydrate versus fat being used by men and women (repiratory exchange ratio or RER) these researchers find that women burn more fats during sub-maximal exercise while sparing carbohydrates. Even Small Differences Add Up Over a Marathon Course As a counter to my claim, in much of the research demonstrating a gender difference in fat and carbohydrate use during exercise, the researchers report relatively small differences. For example, in a review article examining past research on fat burning in women versus men, Blaak (12) claims that the magnitude of gender difference in fat use during exercise is quite small. Blaak bases this claim on a study

6 6 conducted by Horton et al. (11) which found that women burn slightly more fat and men burn slightly more carbohydrate during exercise at 40% VO2max. You may reason that if these differences are small enough, then the effect of these gender differences on performance may be insignificant. However, most research exercise bouts are less than 90 minutes to 2 hours. While it may be true that the reported differences in fuel use may not significantly impact performance for exercise durations of less than 2 hours, 2hours is a meager training run for the marathon. This means even the small differences will certainly add up just like the miles in the race. For example, Horton et al. (11) found a slight gender difference in which women derived 50.9% of total energy from fat while men derived 43.7% of total energy from fat when exercising at 40% VO2max. While a difference of 7.9% may not seem large, over the course of a long race or competition like the marathon, even this little difference of greater fat burning will add up to create large differences in time to glycogen depletion and fatigue. For example, a 135 pound woman expends about 85 kcal/mile. If she runs an 8-minute-mile she completes 7.5 miles/hour, and burns 638 kcal/hour. Over the course of 26.2 miles in 3.5 hours (just below the new Boston qualifying times for women), her total energy expenditure is 2231 kcal (19). As shown in Table 2, over the course of a marathon she burns a total of 1137 kcal fat and 1019 kcal carbohydrates. A similar sized man running the same pace burns only 967 kcal of fat but 1185 kcal of carbohydrate. This is a difference of 170 kcal. An important limitation of the Horton et al. study is that this study was conducted at 40%VO2max, or approximately the percent VO2max used for walking. In my scenario in which the man and woman are running an 8-minute-mile, the correct percent of VO2max is approximately 60-70% VO2max (19). The percent of VO2max is significant because at a lower percent of VO2max the athlete uses more fat and less carbohydrate for fuel. Therefore the results from Tarnopolsky et al. s (6) study may be more consistent. As shown in Table 2, our marathon woman uses 43% fat expending 960kcal fat

7 7 while our man uses 19% fat and only 424kcal from fat. In this case the marathon woman expends 536 more fat calories than the man while sparing 536kcal of glycogen. Table 2. Results from Horton et al. and Tarnopolsky et al. Applied to Marathon % Total EE* fat % Total EE CHO** Total Marathon EE (kcal) Total fat (kcal) Total CHO (kcal) CHOin normal muscle Horton Men 43.7% 53.1% No Women 50.9% 45.7% No Tarnopolsky Men 19% 78% Yes Women 43% 54% No *EE = energy expenditure ** CHO = carbohydrate Will CHO Deplete? Yet, the most interesting thing about how the calculated differences between the genders add over the course of the marathon is that only the men running at 60-70%VO2max actually deplete their glycogen stores. In a normal resting muscle, both men and women have approximately 300 grams or 1350kcal glycogen stored. According to the calculated estimates for fuel use from the Horton et al. study, neither the women nor the men need to worry about carbohydrate loading because they never exceed the 1350kcal normal reservoir for glycogen. Even at 60-70% of VO2max, as calculated from Tarnopolsky et al. s results, the women would not exceed this 1350kcal supply and would not need to carbohydrate load. Also, from these same calculations, only the men at running at 60-70% VO2max exceed the 1350 kcal normal reservoir of stored glycogen. Therefore, only these men would benefit from carbohydrate loading to boost their levels of muscle glycogen for marathon competition. In fact, at 60-70% VO2max men burn out of their natural carbohydrate stores at 2 hours and 42 minutes. As shown in Figure 2, if the marathoner runs at an 8 minute mile, this corresponds to the 20.2 mile mark which leaves the man 6 miles to complete the marathon with no carbohydrate gas left in the

8 8 fuel tank. This necessitates carbohydrate loading in men for endurance feats such as the marathon. In the woman however, at 60-70% VO2max glycogen depletion will not occur until 3 hours and 55 minutes of steady exercise. At an 8-minute-mile this corresponds to mile 29.4, which means the woman can complete the marathon then continue to run another 5k before she hits the glycogen depletion wall! Therefore, because women burn greater amount of fat, and spare more glycogen than men, there is no need for women running at 60-70% of VO2max to carbohydrate load for the marathon. Figure 2. Women will NOT Deplete Glycogen Stores during Marathon Women Men Running on glycogen stores Mile Number Marathon finish Glycogen stores depleted *the women (top bar) will run out of glycogen stores at mile 29.4 (after the Marathon finish as indicated on the horizontal number line). The men (bottom bar) will run out of glycogen stores at mile The men will run glycogen depleted (red zone) for 6 more miles until Marathon finish.