Performance Detractors: Assesing the Low Carbohydrate Diet and Its Effect Upon Athletic Performance
The low carbohydrate diet plan has found increased popularity among U.S. adults due to its claims of rapid weight loss. It should be of no surprise that low-carb diets such as the Atikins’, South Beach and The Zone diet have resulted in top spots among The New York Times’ best-sellers. However, these plans may pose some risks for individuals that exercise regularly or participate in competitive athletic events.
A high carbohydrate intake is necessary to restore muscle and liver glycogen stores to ensure sufficient glucose availability for skeletal muscle contraction for training on successive days. The typical American diet contains between 40 and 50% of total calories as carbohydrate . The current daily intake of carbohydrates is 8 to 10g/kg body mass for athletes that train more than 2 hours a day Studies have shown that glycogen depleted muscles could be normalized within 24 hours when fed iso-caloric diets (3000 kcal) containing 525 to 648g of carbohydrate. Athletes engaging in activities such as middle distance running, swimming and cycling can expend between 3100-5900 kcal/day 3. However, initial phases of some low-carb diets call for extremely low carbohydrate intake in the first two weeks. For example, the initial phase of the Atkins’ Diet calls for no more than 20g of daily carbohydrate intake!
If the carbohydrate intake does not meet the body’s requirement for glucose, the body will start to burn energy that has been stored as fat. The fat catabilized for energy is not burned completely but results in the build up of ketone bodies in the blood, a condition known as ketosis. While not immediately harmful, the build up of ketosis has two side effects. First, ketone bodies undergo urinary excretion with a cat-ion to maintain electrical neutrality, resulting in the additional loss of cat-ions, including calcium, magnesium and potassium . Secondly, ketone bodies are acids so there is a natural increase in acid in the blood, also known as acidosis. Acidosis has been reported to impair red cell deformability in addition to increasing blood viscosity by up to 9% . Despite the changes, the body eventually compensates to maintain pH balance through the use of plasma sodium carbonate.
The low-carb diet is effective in short-term weight loss due to the increase in fat metabolism. However, a study of the safety and efficacy of low carbohydrate diets found that the greatest weight loss was achieved by participants who took in the fewest calories, regardless of carbohydrate content . While carb-counting and the consumption of low glycemic-index foods (typical of the low-carb diet plans) will help maintain adequate carbohydrate consumption and minimize fat storage of excess glucose, it might not be the only key to weight loss. Over the years, dieters have been told to watch their calories (Low Cal!), watch their fat intake (Low fat!) and now start their carb-counting (Low Carb!). Maybe the next New York Times best-seller will find a way to incorporate all of these plans and it would be called: Exercise and Low Intake.
References
McArdle, W.D., Katch, F. Katch, V.; (2001) Exercise Physiology. Fifth Edition. Baltimore: Lippincott Williams & Wilkers.
Bishop, N, Blannin, A., Walsh, N., Robson, P., Gleeson, M.; Nutriional Aspects of Immunosuppression in Athletes. Sports Medicine. Sep 1999, Vol 28 (3), p 153.
Cheuvront, S.N. The Zone Diet and Athletic Performance. Sports Medicine, April 1999, Vol 27 (4), p 213.
Atkins, R.; (2002) Dr. Atkins’ New Diet Revolution, New York: Evans and Co.
Stevens, A.; Robinson, D.; Turpin, P.; Groshong, T.; Tobias, Dr. J. Sudden Cardiac Death of Teen While Dieting, Souther Medical Journal, Sep 2002, Vol 95 (9), p. 1047.
Peyreigne, C., Bouix, D., Hemorheologic Effects of a Short-term Ketogenic Diet. Clinical Hemorheology and Microcirculation. 1999, Vol 21, pp147-153.
Consumer Reports; The New, Healthier fast food. Consumer Reports. April 2004, p 8.