The body uses nutrients to grow, restore body tissue, and provide energy to perform work. The body requires some nutrients in large quantities, called macronutrients, and other nutrients in smaller quantities, calledmicronutrients.
Macronutrients provide energy, whereas micronutrients do not yield any energy directly but they assist in the transfer of energy (e.g. B-vitamins).
The primary energy yielding nutrients used during exercise are:
Nutrients containing carbon are called organic (=alive)! Which nutrient predominantly provides the energy during exercise depends on:
When exercise intensity increases, the body will use more carbohydrates and less fat. The cross-over point,where more carbohydrates are being used than fat, occurs when exercise intensity increases steadily. At – and beyond the cross-over point, carbohydrates become the dominant source of fuel.
There are two factors, with respect to high exercise intensity, that cause the shift from fat to carbohydrates (fat -> carbs):
More and more fast-twitch fibers are being recruited when exercise intensity is high. Fast-twitch fibers are better equipped to metabolize carbohydrates than fat! Therefore, the more fast-twitch fibers an athlete has, the more carbohydrates are being used compared to fat.
With high intensity exercise, blood levels of epinephrine increase and this increase in epinephrine levels is responsible for higher glycogen breakdown, carbohydrate metabolism, and higher lactate production.
Lactate production is a byproduct of anaerobic metabolism, which increases with exercise intensity. High levels of lactate inhibits fat metabolism, which means less fat is being used during high-intensity exercise.
Therefore, the more lactate that is being produced, the less fat is metabolized, which leads to an increased usage of carbohydrates during high-intensity exercise.
Factors that control the rate of fat metabolism during prolonged exercise are the enzymes that control lipolysis (lipases). The lipases activity is stimulated by the hormones:
During prolonged low-to-medium intensity exercise, epinephrine levels in the blood rise, which leads to better lipase activity, which in turn promotes lipolysis and lipolysis promotes fat metabolism.
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