Modulation of carbohydrate and fat utilization by diet, exercise and environment.

At rest and during exercise carbohydrate and fat are the predominant substrates. They are oxidized simultaneously but the relative contribution of these two substrates is dependent on a variety of factors including the exercise intensity and duration, diet, environmental conditions and training status. Changes in carbohydrate metabolism during the transition from rest to exercise and from low- to high-intensity exercise are mainly due to allosteric regulation. The factors that up-regulate fat metabolism in the transition to moderate-intensity exercise and the factors that result in a down-regulation of fat metabolism at higher intensities are incompletely understood. Substrate use is further modulated by the endocrine milieu (e.g. catecholamines, insulin, cortisol) and possibly cytokines (e.g. interleukin-6). With increasing duration of exercise there are marked increases in fat metabolism and decreases in carbohydrate metabolism and this has been ascribed mainly to substrate availability. Both acute food intake and chronic diets also have profound effects on substrate utilization. An increase in carbohydrate intake will rapidly suppress fat metabolism and increase carbohydrate metabolism whereas such an adaptation to a high-fat diet may take several days. The environmental conditions can also alter substrate use; high ambient temperatures can increase glycogen breakdown as a result of increased body core temperature and increased circulation catecholamines. Low temperatures can also increase carbohydrate metabolism, especially when shivering. In addition to these factors adaptation to training, in particular endurance training, will reduce the reliance on carbohydrate metabolism and increase fat oxidation, especially from intramuscular triacylglycerol stores.