Physical exercise as a modulator of adaptation to low and high carbohydrate and low and high fat intakes

Abstract

Quanti®cation of the metabolic response aids in ascertaining the nature and extent of the energy requirements imposed by exercise. During high intensity exercise, virtually all of the energy is supplied by the net oxidation of glycogen while fat oxidation plays a more prominent role during lower intensity exercise. Therefore, the lower limit of carbohydrate required above resting needs is equal to the portion of the total energy cost derived from carbohydrate sources. There is no upper limit of additional carbohydrate intake that could be eaten to satisfy the extra caloric requirement since carbohydrate intake will restore any endogenous energy stores that were used during exercise, regardless of the intensity of exercise. The recommendation of a high carbohydrate intake to provide caloric balance in exercising individuals is supported by the observation that exercise performance at high intensity is improved by a high carbohydrate diet, and exercise performance at low intensity is relatively insensitive to the source of the caloric intake. Limited dietary studies are consistent with predictions based on the metabolic response. At exercise intensities below 65% VO 2 max, the percent fat and carbohydrate in the diet makes little difference on exercise performance, provided adequate time is allowed to adapt to a high-fat diet. On the other hand, exercise ability during high-intensity exercise is signi®cantly limited by a high-fat diet. A consideration of importance beyond the aspect of energy balance is the anabolic effect of insulin on muscle protein synthesis after exercise. Provision of carbohydrate after exercise is likely to stimulate muscle protein synthesis to a greater extent than a corresponding amount of fat. Dietary fats may offer practical advantages to the athlete but if fats are consumed at the expense of carbohydrate intake, many established bene®ts of high carbohydrate intake in terms of performance may be sacri®ed.

Effects of a Low Carbohydrate Weight Loss Diet on Exercise Capacity and Tolerance in Obese Subjects

“…It has previously been shown that when obese subjects consumed an LC diet for 6 weeks and performed their usual activities (but not strenuous exercise), muscle glycogen content was reduced, albeit only moderately (by ∼30%) (11). Although the present study was not designed to analyze the precise mechanisms driving the changes in fat oxidation following the LC diet, proposed metabolic and hormonal adaptations have been previously described (37,40). Additionally, it is likely that the shift from glucose to ketone body oxidation by the brain, as occurs during fasting (41), assists in preserving glycogen stores.…”

Energy Turnover at the Race Across AMerica (RAAM) - a Case Report

“…Now the question remains, whether the deficiency is either from loss of fat of subcutaneous adipose tissue or from protein of skeletal muscle. In the human body energy is stored in subcutaneous adipose tissue (80 543 kcal), in intramyocellular lipids (3597 kcal), in glycogen in the muscles (1434 kcal), in glycogen in the liver (360 kcal), in glucose in the plasma (76 kcal) and in protein (2103) [21]. It has been shown in several studies, that during extreme endurance running over hundreds and thousands of kilometers, muscle mass [23] and subcutaneous adipose tissue [24] are reduced.…”

The Influence of Cyclical Ketogenic Reduction Diet vs. Nutritionally Balanced Reduction Diet on Body Composition, Strength, and Endurance Performance in Healthy Young Males: A Randomized Controlled Trial

“…The cyclical ketogenic diet (CKD) alternates periods of ketogenic dieting with periods of high-carbohydrate consumption [ 8 ]. The period of high-carbohydrate eating is supposed to refill muscle glycogen to sustain exercise performance [ 9 ].…”