I think anyone in the endurance community has heard this theory before that females may be able to maintain that high intensity of exercise for a very long time...and perhaps even compete with, and beat some of the males. Especially when you get into ultra-distance events. More and more females are winning these things. I received a perfect score on this paper, so I'm hoping I'm on to something here :) Teacher said she enjoyed my topic and would love to see more research done in the area! Hope it helps anyone who reads it! Ok-here's the paper:
I found it very interesting while reading the chapter titled, “Macronutrient Metabolism in Exercise and Training,” from our book, “Sports and Exercise Nutrition,” to read that trained muscle has an improved ability to catabolize, or breakdown, carbohydrate aerobically for energy use (McArdle, Katch, & Katch, 2009). This is due to an increased oxidative capacity of the mitochondria and increased glycogen storage. Trained muscle uses more fat for energy during submaximal exercise, and relies less on muscle glycogen and blood glucose (McArdle, Katch, & Katch, 2009). Furthermore, when I read the section on gender differences in the training effects of substrate use, I was intrigued by the fact that at an equal percentage of an athlete’s VO2max, such as 80%, the female gender takes a smaller proportion of energy from carbohydrate oxidation than men during submaximal exercise. After cardiovascular exercise training, women show a greater shift towards fat catabolism, whereas men do not (McArdle, Katch, & Katch, 2009). This got me to thinking about women endurance athletes, especially female ultrarunners. Ultrarunning consists of running anything over a marathon distance, from a 50k (31 miles) to a 100 mile race or beyond. I have heard by many ultrarunner friends, as well as from a few articles over the past couple years about female ultrarunners and how some of them can outrun the men.
This phenomenon seems to be largely unexplained by science, however, and I was skeptical for the many reasons articles from ultrarunning sources online had given. I found an article from the magazine, Runners World, titled, “Why Women Rule,” by Lisa JHung. She states that, “In ultrarunning, not only do women compete on the same course and at the same time as the men, they really compete—and do well—against them” (JHung, 2010). The article then goes on to talk about some ultrarunning races where certain women have either won, or placed in the top three overall, against both genders. Lisa Jhung asks the question of what makes it possible, whether physiologically, psychologically, or both, for women to compete at these high levels with the men. Jason Coop, whom JHung interviewed, is an ultrarunning coach, and thinks that women have an edge in ultrarunning just because of their smaller body size. As the terrain in the race gets more challenging, the smaller body size of the female runner is a larger benefit. He says that the more difficult the course is the more of an advantage the female ultrarunner has. However, Coop states that on the uphills, it may not be a benefit to be a female because their hearts and lungs are smaller in size than the male. “But the descents—like the 36,000 feet of descending at Hardrock—take less of a toll on a smaller person's body. There's less eccentric work with the legs. Over the course of time, as descents add up that's a huge advantage” (JHung, 2010). Hardrock is a challenging 100-mile foot race in the mountains of Silverton, Colorado. Eccentric work is the part of the run cycle in which the controlled muscle lengthens under strain during downhill running; more muscle damage occurs during this phase of running (Eston, Mickleborough, & Baltzopoulos, 1995). Eccentric work is responsible for a lot of the soreness in runners after a hard workout or race (Eston, Mickleborough, & Baltzopoulos, 1995).
Coop thinks that women seem to excel the more challenging the race is, like in the Badwater ultramarathon, where temperatures can often reach 120-130 degrees in the desert of Death Valley, California. He then goes on to explain that there is also a psychological component in women that allows them to have an advantage over men. Coop states, "But I firmly believe that good women endurance athletes are also psychologically better than good male endurance athletes on the elite side" (JHung, 2010). A sports psychotherapist, Bruce Gottlieb, agrees with Coop, stating that the perseverance seen in these ultrarunning females may be due to the fact that they often think with greater willpower and persistence, whereas men are more inclined to think in terms of going harder, stronger, and faster (JHung, 2010). The article then goes on to mention an argument that is often brought up about these ultrarunning women. This is the thought that females are able to endure the pain of a challenging race for so long just because it is ingrained into their nature due to the ability to give birth. Whether some of these proposed physiological and psychological explanations are facts or not, I found an article from the Journal of Applied Physiology that may explain why some females can endure such long running events, in terms of substrate use during different exercise intensities.
In my opinion, Coop’s idea of women seeming to excel over the course of a race the more challenging it is, is due to more of a physiological explanation (although I do not personally disagree with the proposed psychological explanations). In accordance with our book, this study found that maximal fat oxidation was significantly greater in women than in men, with men showing lower rates of fat oxidation and a greater shift towards using carbohydrate as the primary fuel earlier in their workout. The primary fuels, carbohydrate and fat, oxidized by the muscle to produce energy when an individual exercises, is a widely known concept (Venables, Achten, & Jeukendrup, 2004). This can be influenced by the amount of glycogen in the muscle, one’s diet, intensity and duration of exercise, and fitness level. The article does say that there is still some debate on whether sex differences in fat oxidation exists. Many studies have shown that fat breakdown (oxidation) in the female sex plays a greater role in oxidative metabolism than men (Venables, Achten, & Jeukendrup, 2004).
Venables, Achten, & Jeukendrup (2004), gathered 300 volunteers (143 females and 157 males) to participate in their study to assess which substrate was being used during a maximal oxygen consumption test to exhaustion. The test consisted of participants jogging or running on a treadmill, in which the percent grade and speed were increased every few minutes. During this time, substrate (carbohydrate and fat) use was measured through indirect calorimetry, which uses a metabolic cart to measure oxygen consumption and carbon dioxide breathed out. For each participant, the maximal fat oxidation and corresponding intensity at which maximal fat oxidation appeared was measured. On average, maximal fat oxidation was seen at 48+/- 1% of VO2max, which corresponded to exercising at 62+/- 1% of maximal heart rate. At about 62% of an individual’s maximum heart rate is when the substrate use transitions over from fat use to mostly carbohydrate use to fuel the working muscles (Venables, Achten, & Jeukendrup, 2004). When exercise intensity was increased even more, a greater decrease in fat oxidation occurred, at about 84% of VO2max and 89% of maximal heart rate, and a RER (respiratory exchange ratio) value of 1 was reached, meaning the exercising individual was burning mainly carbohydrates (Venables, Achten, & Jeukendrup, 2004).
In contrast with the women, however, the men who participated in the study burned significantly less fat and notably more carbohydrates. This is due to their greater fat-free mass and lesser body fat mass as opposed to women (Venables, Achten, & Jeukendrup, 2004). What I found interesting was that the exercise intensity at which maximal fat oxidation was observed was considerably lower in the males than the females. “When the oxidation data were expressed as a percentage of total energy expenditure, the contribution of fat oxidation to total energy expenditure was greater in women than the men with a concomitant lower contribution of CHO oxidation to total. This effect was equal across all exercise intensities” (Venables, Achten, & Jeukendrup, 2004). What this means is that at any given exercise intensity, women actually use more energy fuel from fat than carbohydrates, whereas men use more carbohydrates for fuel rather than fat. However, females are still burning more carbohydrates at a high intensity of exercise than fat, but what this article and our book is showing is that they actually are able to burn a greater amount of fat than men even at higher intensities. This means that for a long endurance event, the exercising female is able to hold off on depleting their glycogen stores; they use more energy from fat metabolism for a longer period of time than males.
However, this study did not control for menstrual phase or diet, so it really is unknown how much of the discrepancy in fat oxidation contributes to these elements. Even though there are studies that found the menstrual phase and oral contraceptives both do not contribute to fat oxidation during moderate-intensity exercise, some studies have found conflicting evidence (Venables, Achten, & Jeukendrup, 2004). Nevertheless, this study has shown that even though those elements were not controlled for, during exercise, there are sex differences in the type of fuel being burned. “Also, it is highly likely that a degree of the variation found in fat oxidation could be accounted for by diet because it has been shown that altering the diet, either to a high fat/low-CHO or low-fat/high-CHO diet, can increase or decrease fat oxidation, respectively” (Venables, Achten, & Jeukendrup, 2004). This also means that as the more trained the individual is to exercise at these moderate and higher intensities for a long duration, the more efficient they are going to be at using fat or carbohydrates for fuel during a certain exercise intensity, whether male or female. However, the female gender is able to hold off carbohydrate oxidation for a longer period of time. From reading and learning all of this, I am going to assume that some of these female ultrarunners who can run for so long and beat the men, or come close to it, eat a higher fat diet so they can spare their muscle glycogen for later use when they have burned through all of their fat stores for energy use. Women have also benefited from the training factor, in which trained muscle uses more fat from energy during submaximal exercise (such as running a very long run). Men can do this, as well, however not to the same extent as women, as shown by this study and previous literature. Perhaps this is why some female ultra-distance runners have an advantage over the men.
In conclusion, this study says that when maximal fat oxidation was corrected for fat-free mass in both genders (since some women have less fat relative to their body weight than their higher fat counterparts), a fraction of the inconsistency across individuals may be due to training status and sex, but not percent body fat. “Although gender accounts for only a small fraction of the observed total variability in fat oxidation, it is apparent from this study that a gender dimorphism does exist in that women have higher maximal rates of fat oxidation and that lipid remains the dominant fuel at higher exercise intensities than in the men” (Venables, Achten, & Jeukendrup, 2004). Lastly, the article states that in this study, the explanation for many of the differences across individuals in substrate use at a given exercise intensity cannot be rationalized with the variables that were tested (Venables, Achten, & Jeukendrup, 2004). One other article I found stated this, “A recent study, however, once again suggests that women may in fact have some way, not yet understood, to burn fatty acids better than men do. If that’s the case, and you combine that ability with women’s greater body fat reserves, the implications are obvious. If some glycogen-sparing goes on along the way, women might be able to get more out of that premium fuel than men do” (Donaldson, 2011). More research needs to be done in order to figure out what factors across both genders contribute to an increase in fat oxidation, which could be due to hormones or body fat percentage. We do know this, however: women can use more energy from fat metabolism for a greater period of time, sparing muscle glycogen across a long endurance race, which may give them an advantage.
Donaldson, Jamie. (2011). The secret feminism of “Born to Run” pt. 1: women and
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Eston, R. G., Mickleborough, J., & Baltzopoulos, V. (1995). Eccentric activation and muscle
damage: biomechanical and physiological considerations during downhill running.
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JHung, Lisa. (2010). Why women rule: In the sport of ultrarunning, women compete shoulder-
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McArdle, W. D., Katch, F. I., & Katch, V. L. (2009). Sports and exercise nutrition. (3rd ed.).
Batimore, MD: Lippincott Williams & Wilkins.