Monday, February 19, 2007

Insulin, Carbohydrates and Fat

Our body derives energy from 3 main food sources: Carbohydrate, protein and fat. In general, carbohydrates are our main fuel for exercise, though the lower the intensity or longer the duration of exercise, the more we will rely on fat; the predominant role for proteins is to provide the building blocks for muscle and other tissues, but during endurance exercise, up to 15% of energy may come from protein; fats provide a concentrated source of energy and are the second most important contributor to energy demands during exercise, behind carbohydrates.

During exercise, blood glucose levels increase, while blood insulin levels fall. This occurs due to the exercise-induced rise in catabolic hormones (hormones that induce the breaking-down of fuels for energy), which inhibit the release of insulin from the pancreas. In other words, during exercise, the body doesn’t need insulin for glucose mobilization, and therefore its’ secretion is suppressed. As a result, glucose is released from the liver to the bloodstream, and fat is oxidized for energy. The higher the intensity of your exercise, the more insulin is suppressed.

Endurance training also causes several major adaptations in the muscles to increase fat utilization. First, endurance training increases the number of capillaries in the trained muscles, so that the muscles receive more blood and oxygen. Second, endurance training increases the ability of the muscle to burn fat (activating specific enzymes). Third, endurance training increases tissue insulin sensitivity, which means the muscles need less insulin to allow glucose in, resulting in less insulin in the blood. In endurance athletes, the claim that a high carbohydrate diet promotes greater body fat storage through activiation of insulin is also, therefore, unfounded.

In conclusion, carbohydrate, and not fat, is the preferred energy source during exercise at or above 70% of VO2max (maximal oxygen consumption) – the intensity at which most endurance athletes train and compete. Fat supplies a secondary source of energy, but becomes more important the longer the duration of exercise. In fact, after several hours of endurance work, fat may supply up to 80% of calories. Even at this stage, however, fat still ‘burns in a carbohydrate flame’. This means that one must still be mindful of adequate carbohydrate ingestion even at the late stages of a prolonged endurance event when fat usage is maximal.

0 Comments:

Post a Comment

<< Home