A drug that has a similar effect to caffeine is heparin. The main action of heparin, a substance first isolated from snake venom, is to prevent blood clotting. However, like caffeine, heparin also stimulates the release of free fatty acids from adipose tissue (see Exercises 3.2), causing blood free-fatty-acid levels to rise rapidly, reaching peak values within 60 minutes (Costill et al, 1977). Like caffeine, heparin may reduce the rate of muscle glycogen utilization during subsequent exercise (Costill et al, 1977).
Blood free-fatty-acid levels rise after a fatty meal. A high-carbohydrate meal causes blood insulin levels to rise and to be elevated for up to 60 to 90 minutes thereafter.
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Insulin is the “antiexercise hormone,” because it inhibits fatty acid mobilization from fat cells and also inhibits the breakdown of glycogen in the liver. The result is that during exercise after a high-carbohydrate meal, there will be increased carbohydrate oxidation (G. Ahlborg & Bjorkman, 1987; Coyle et al, 1985a). If exercise starts when blood insulin levels are high, the rate of muscle glycogen utilization may increase; there is also a risk that blood glucose levels will fall precipitously, causing hypoglycemia, although this is not always found (Devlin et al, 1986).
Nicotinic acid is a vitamin that when present in the blood in high concentration has an insulinlike effect and inhibits fat mobilization (Bergstrom et al, 1969; Carlson et al, 1963). Nicotinic acid will therefore have the same effect as insulin on metabolism during exercise.
Fasting increases blood free-fatty-acid levels so that fat metabolism is increased during exercise after fasting. However, performance in both submaximal (Loy et al, 1986) and maximal (Gleeson et al, 1987) exercise is impaired by fasting; the former because of the premature onset of hypoglycemia, and the latter because of changes in acid-base status, with the buffering capacities of muscle and blood being reduced during fasting. Interestingly, dogs show no such impairment of running performance after fasting.