Fat Loading Not Good For Endurance Exercise
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Fat Loading Not Good For Endurance Exercise |
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Nutritional strategies to improve endurance usually focus on increasing muscle glycogen stores prior to exercise. However, several popularized studies have led some endurance athletes to try "fat loading" in place of carbohydrate loading (2,3).
There would be a greater total yield of ATP from fat than from carbohydrate if the muscle could oxidize fat at a sufficiently high rate during intense exercise. A 6-carbon glucose molecule produces 36 ATP, whereas an 18-carbon fatty acid (stearic acid) produces 147 ATP -- a 1.3 greater yield of ATP per carbon molecule. However, 6 molecules of oxygen are required to oxidize a glucose molecule, whereas 26 oxygen molecules are required to oxidize stearic acid. Thus, the oxygen requirement for glucose is 77% less than the oxygen requirement for stearic acid. During endurance exercise, the lower oxygen requirement for the oxidation of carbohydrate produces a lower cardiovascular stress than does the oxidation of fat (3).
A 1983 study evaluated how a high fat diet influenced cycling time to exhaustion and muscle glycogen utilization (3). The cyclists ate an average American diet (50% carbohydrate) for one week and then exercised to exhaustion at 63% VO2max. Then, they ate a high fat diet (85% of calories), which provided less than 20 gm of carbohydrate. After four weeks on this diet, they again exercised to exhaustion at 63% of VO2max.
The exercise time to exhaustion was not significantly different on the two diets (147 minutes and 152 minutes, respectively). After adapting to the high fat diet, muscle glycogen utilization dropped four-fold and glucose utilization dropped three-fold on the ride to exhaustion. Fat utilization rose to make up the difference.
The drawbacks of high fat diets outweigh any potential benefits. Such diets need medical supervision since they have been associated with sudden death and heart rhythm disturbances due to loss of protein from the heart and potassium depletion. The cyclists were constantly monitored for electrolyte losses and these were replaced throughout the study.
Although the cyclists trained heavily, their blood cholesterol levels increased while on the high fat diet. Since exercise by itself does not fully protect against cardiovascular disease, eating a high fat diet for a prolonged period may increase the athlete's risk of developing cardiovascular disease.
A high fat diet is also hard to digest. The high fat meals in the study consisted of butter, cheese, ground or marbled beef, and tuna with dollops of mayonnaise. Such a diet is not palatable and lacks the variety needed for optimum nutrient intake.
Adaptation to a high fat diet takes at least two weeks. Exercise during this time will be difficult and unpleasant, due to low muscle glycogen stores. Even when adaptation is complete, the ability to perform high intensity exercise (70% or more of V02Max) may be impaired.
This study may be criticized because it did not compare a high carbohydrate diet (70% of calories) to the high fat diet. A high carbohydrate diet would provide higher muscle glycogen stores and a longer cycling time to exhaustion than the 50% carbohydrate diet used in the study.
Furthermore, the cyclists rode to exhaustion at an exercise intensity low enough (63% of V02max) to be fueled by fat oxidation and not limited by muscle glycogen depletion. If the rides had been conducted at an exercise intensity known to be limited by muscle glycogen depletion (e.g. above 70% of V02Max), impaired endurance would be expected. Athletes usually train and compete at over 70% of VO2Max.
Unfortunately, the results of a 1994 study on dietary fat and performance continues to promote the fallacy that high fat diets enhance endurance (2). Diets were assigned for seven days in the order of normal diet (2,789 kcal, 61% carbohydrate, 24% fat), high fat diet (3,500 kcal, 50% carbohydrate, and 38% fat), and high carbohydrate diet (3,500 kcal, 73% carbohydrate, and 15% fat). The run time to exhaustion at 75-85% V02Max was significantly longer following the high fat diet (91 minutes), compared to the normal diet (69 minutes) and high carbohydrate diet (76 minutes).
Although these results suggest that a seven day, 38% fat diet improves endurance performance, the study design was flawed because the order of the diets was not random. Also, certain metabolic responses did not support the concept that the high fat diet produced the improvement in performance. Blood glycerol fell (it should have risen if more fat was burned) and the respiratory exchange ratio was unchanged (it should have decreased if more fat was burned). Thus, the results of this study are considerably weakened (1).
It's doubtful that adaptation to a high fat diet provides any endurance benefit. In any case, the potential adverse effects on health are too risky to warrant consuming a high fat diet during endurance training.
1. Phinney, S.D. et al. The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capacity with reduced carbohydrate oxidation. Metabolism 32:769-776, 1983.
2. Munio, D.M. et. al. Effect of dietary fat on metabolic adjustments to maximal V02 and endurance in runners. Med. Sci. Sports Exerc. 26:81-88, 1994.
3. Sherman, W.M. and N. Leenders. Fat loading: the next magic bullet? Int. J. Sports Nutr. Suppl. 5: S1-S12, 1995.
Ellen Coleman,
RD, MA, MPH
ellen@cruciblefitness.com
