Carbohydrate Utilization During Exercise

Low carbohydrate levels result in exercise fatigue. Since carbohydrate stores (or, glycogen stores) are limited (~350 kilocalories of glycogen in the liver; ~1,400 kilocalories of glycogen in muscles), athletes should consider how to initiate exercise with glycogen stores full and should establish a routine that keeps glycogen stores from running low. Even if muscle glycogen stores are adequate, low liver glycogen stores will result in hypoglycemia and mental fatigue, and mental fatigue leads to muscle fatigue.

The higher the exercise intensity, the greater the reliance athletes have on carbohydrate as an energy substrate. However, even low-intensity (i.e., mainly aerobic) exercise that derives most of its fuel from fat still requires some level of carbohydrate for the complete combustion of fat and to maintain blood glucose. Therefore, all modes of physical activity have some degree of carbohydrate dependence.

Several factors influence the proportionate contribution of carbohydrate to total fuel requirements during exercise. Factors that increase the reliance on carbohydrate include the following:

• High-intensity activity
• Long-duration activity
• Exercise in hot and cold temperature extremes
• Exercise at high altitude
• Age (higher in young boys than in men)

Factors that decrease the relative energy expenditure from carbohydrate include the following:

• Endurance training
• Good conditioning
• Temperature adaptation
• Gender

There is a common misconception that low-intensity activity (up to ~65 percent.VO₂max) is the most efficient means of fat loss. In fact, many popular exercise programs have been organized around the idea that a greater amount of fat is most efficiently “Burned” with low-intensity aerobic exercise. However, the proportion of fat burned should not be confused with the volume of fat burned. While you're sitting there reading this sentence, you are very likely deriving the vast majority of your energy requirements from fat. However, the total volume of fat being burned is extremely low. (If this were not so, sitting in front of a TV would be a terrific means of initiating a fat-loss program.) When exercise intensity is increased, the proportion of energy derived from fat is decreased, and the proportion of energy derived from carbohydrate is increased, but some level of fat is always being burned. The total caloric requirement per unit of time is much greater in high-intensity activity than in low-intensity activity, and the volume of fat burned is greater in high-intensity activity (despite a lower proportion of fat meeting total energy requirements.) Therefore, athletes should work as intensely as possible within a given time frame to increase fat loss and optimize body composition.

As the intensity of exercise increases so does the need for carbohydrate intake, making the prerace high-carbohydrate meal a necessity.

Carbohydrate is a critical fuel for athletes because we can more efficiently create energy per unit of oxygen from carbohydrate than from any other fuel. One liter of oxygen can yield approximately 5 calories from carbohydrate but only 4.7 calories from fat. In addition, aerobic glycolysis can produce ATP for muscular work in a larger quantity and at a faster rate than the oxidation of fat can produce it. The increased energy efficiency of carbohydrate helps explain the muscular fatigue that quickly occurs during high-intensity activities when muscle glycogen is nearly depleted. We simply cannot supply sufficient ATP to working muscles to maintain the workload.