My training, racing, things that go through my head, memorable moments, and such.

Tuesday, November 15, 2011

Fat burning and Performance

I wrote this brief paper for a presentation I did in Exercise Physiology. I found the research super interesting. I read a lot of papers, but I couldn't fit all the info in my time limit. ANyways, thought I would share it seeing as I think it is cool, plus I haven't written on my blog for a long time and don't want everyone to forget about me. Ps. the below cartoon makes me laugh every time I look at it. Ha ha. Ha

Role of Diet on Fuel Utilization during Endurance Exercise
“Hitting the wall” in endurance sport is a common phenomena, in which there is a definitive point where performance declines. This can generally be attributed to a depletion of endogenous and exogenous glycogen stores, and not a lack of stored energy. One way to prevent this is to increase carbohydrate consumption during extended periods of exercise (McCafferty, 2011). Another way, I hypothesized, would be to increase fat oxidation for as long as possible, thus saving some of the glycogen stores for the end of the race or training session. This paper is aimed at discovering whether there is such thing as “fat burners” and “carb burners”, if someone can change from being a carb burner to a fat burner in order to limit glycogen depletion, how diets can affect fat oxidation and carb burning, and diet’s impact on performance.
In the literature, it appears as though people do tend to be better at either burning fat or burning carbohydrates, making them either “fat burners” or “carb burners”. One study looked at 28 Pima Indian families on controlled diets, and found that family membership explained 28% of variance in 24 hour respiratory quotients (24h RER). This points to a genetic factor influencing substrate utilization (Zurlo, Lillioja, Esposito-Del Puente, et al., 1990). However, more recent research indicates that preceding diet highly influences macronutrient oxidation, and because families tend to share similar eating habits, the 24h RER could have been influenced by the diets normally eaten (Toubro, Sorensen, Hindsberger, Christensen, & Astrup, 1998). Toubro et al. preformed a study with 71 siblings from different families, and found that some factors towards RER variance could be explained by age, gender, energy balance, current dietary intake, fasting plasma insulin and free fatty acid concentrations, however this also had a strong familial resemblance. Another factor which was not considered was physical fitness, which is known to affect RER and which may also be influenced by familial habits (Toubro et al., 1998). Finally, a study done on 61 trained cyclists showed varying determinants to affect RER depending on intensity, but one important conclusion was that training, dietary intake and consequent muscle glycogen content and circulating substrates consistently predicted RER at rest and during exercise, demonstrating that being a carb burner or fat burner may be mostly controlled by diet and training (Goedecke, Gibson, Grobler, Collins, Noakes, & Lambert, 2000).
Diet can be used to modify RER and to increase fat oxidation. Both acute and chronic nutrition choices will play a part. An increase in carbohydrate will rapidly suppress fat metabolism as insulin is released to lower blood sugar, decreasing the release of glucagon (Jeukendrup, 2003). Both acute and chronic high carbohydrate consumption will therefore cause an increase in RER. A high fat diet will increase fat oxidation so long as there is minimal carbohydrates involved, although several days of adaptation are needed for optimal fat oxidation to come into effect (Jeukendrup, 2003). A diet high in protein will cause insulin to be released as well, which also inhibits fat utilization to a certain extent, although not as much as carbohydrates. Using carbohydrate supplements during training is not ideal, as training without glucose allows for greater GLUT 4 response and number, as well as increased basal muscle glycogen levels (Nybo, Pedersen, Christensen, Aagard, Brandt, & Kiens, 2009).
High fat diets and performance are an interesting and much debated topic. Firstly, from looking at various sources, performance in endurance activities of high intensities will generally be more successful if glycogen stores are full (Goedecke, & Havemann, 2008). Therefore, although a diet consisting almost purely of fat will be completely sufficient in submaximal conditions, performance will be hampered at high intensities if resting glycogen stores are not full, and doubly hampered if the individual has not adapted to the high fat diet (Phinney, 2004). Carbohydrate loading, long seen as crucial to performance, has now been shown to be unnecessary, as an increase in carbohydrates one day prior to an event can super-compensate muscle glycogen stores adequately (Goedecke, & Havemann, 2008). However, fat loading is now seen as a new approach, as a diet high in fat for 5 to 10 days, followed by a day of high carbohydrate intake seems to be associated with higher rates of fat oxidation, as well as the ability to draw on glucose stores as intensity rises. This is described as the train low, race high theory. A number of studies have been done on high performance athletes and high fat diets. One study looked at high fat diets versus habitual diets prior to carbohydrate loading on cycling time trial performances. It was found that the high fat diet trial increased total fat oxidation, and reduced carbohydrate oxidation but did not alter plasma glucose concentrations during exercise. Also, muscle glycogen and lactate oxidation were lower in the high fat trial. The high fat diet was also associated with improved time trial performances (Lambert, Goedecke, van Zyl, Murphy, Hawley, Dennis, & Noakes, 2001). Another study using trained cyclists on either high fat or high carbohydrate diets demonstrated that there was enhanced endurance during moderate intensity exercise as well as lowered RER values and decreased carbohydrate oxidation in the high fat diet group. Also, there was unimpaired performance at high intensities, and the rates of muscle glycogen utilization were similar despite lower initial muscle glycogen stores (Lambert, Speechly, Dennis, Noakes, 1994).
All in all it appears as though a high (in healthy) fat diet along with training can help to decrease RER and aid in fat oxidation during exercise, thus diminishing the chances of “hitting the wall”.


Goedecke, J. H., Gibson, A. S. C., Grobler, L., Collins, M., Noakes, T. D., & Lambert, E. V. (2000). Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes. American Journal of Physiology-Endocrinology and Metabolism, 279, E1325-E1334.
Goedecke, J. H., & Havemann, L. (2008). Fat as fuel during exercise. Continuing Medical Education Journal, 26(7), 347-349.
Jeukendrup, A. E. (2003). Modulation of carbohydrate and fat utilization by diet, exercise and environment. Biochemical Society Transactions, 31(6), 1270-1273.
Lambert, E. V., Goedecke, J. H., van Zyle, C., Murphy, K., Hawley, J. A., Dennis, S. C., & Noakes, T. D. (2001). High fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance. International Journal of Sport Nutrition Exercise and Metabolism, 11(2), 209-225.
Lambert, E. V., Speechly, D. P., Dennis, S. C., & Noakes, T. D. (1994). Enhanced endurance in trained cyclists during moderate intensity exercise following 2 weeks adaptation to a high fat diet. European Journal of Applied Physiology and Occupational Physiology, 69, 287-293.
McCafferty, S. P. (2011). Methods to optimize substrate utilization during endurance performance. Glasgow Theses Services.
Nybo, L., Pederson, K., Christensen, B., Aagaard, P., Brandt, N., & Kiens, B. (2009). Acta Physiologica, 197(2), 117-127.
Toubro, S., Sorenson, T. I. A., Hindsberger, C., Christensen, N. J., & Astrup, A. (1998). Twenty-four-hour respiratory quotient : The role of diet and familial resemblance. Journal of Clinical Endocrinology and Metabolism, 83(8), 2758-2764.
Zurlo, F., Lillioja, S., Esposito-Del Puente, A., Nyomba, B. L., Raz, I., Saad, M. F., Swindburg, B. A., Knowler, W. C., Bogardus, C., & Ravussin, E. (1990). Low ratio of fat to carbohydrate oxidation as predictor of weight gain: Study of 24-h RQ. American Journal of Physiology-Endocrinology and Metabolism, 259(5), E650-E657.

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