Energy Utilization in a Human Body

There are four substances which humans can derive calories from: carbohydrate, protein, fats, and alcohol.

The body will tend to utilize a given fuel for energy in relation to its availability and concentration in the bloodstream. In general, the body can increase or decrease its use of glucose in direct proportion to the amount of dietary carbohydrate being consumed. This is an attempt to maintain body glycogen stores at a certain level. If carbohydrate consumption increases, carbohydrate use will go up and vice versa.

Protein is slightly less regulated. When protein intake goes up, protein oxidation will also go up to some degree. By the same token, if protein intake drops, the body will use less protein for fuel. This is an attempt to maintain body protein stores at constant levels.

In contrast, the amount of dietary fat being eaten does not significantly increase the amount of fat used for fuel by the body. Rather fat oxidation is determined indirectly: by alcohol and carbohydrate consumption. The consumption of alcohol will almost completely impair the body's use of fat for fuel.

Similarly the consumption of carbohydrate affects the amount of fat used by the body for fuel. A high carbohydrate diet decreases the use of fat for fuel and vice versa. Thus, the greatest rates of fat oxidation will occur under conditions when carbohydrates are restricted. As well, the level of muscle glycogen regulates how much fat is used by the muscle. Using exercise and/or carbohydrate restriction to lower muscle and liver glycogen levels increases fat utilization.