Section 1 Bodily Fuel Stores

The body has three storage depots of fuel which it can tap during periods of caloric deficiency: protein, which can be converted to glucose in the liver and used for energy ; carbohydrate, which is stored primarily as glycogen in the muscle and liver ; and fat , which is stored primarily as body fat. A fourth potential fuel is ketones. Under normal dietary conditions, ketones play a non-existent role in energy production. In fasting or a ketogenic diet, ketones play a larger role in energy production, especially in the brain. A comparison of the various fuels available to the body appear in table 1.

Table 1: Comparison of bodily fuels in a 150 lb man with 22% bodyfat


Average weight (lbs)

Caloric worth (kcal)

Adipose tissue triglyceride



Muscle protein



Carbohydrate stores

Muscle glycogen (normal)



Liver glycogen



Blood glucose



Total carbohydrate stores



Source: "Textbook of Biochemistry with Clinical Correlations 4th ed." Ed. Thomas M. Devlin. Wiley-Liss, 1997.

The main point to take from this chart is that carbohydrate stores are minimal in comparison to protein and fat, sufficient to sustain roughly one day's worth of energy. Although stored protein could conceivably fuel the body for far longer than carbohydrate, excessive protein losses will eventually cause death. This leaves adipose tissue as the primary depot for long term energy storage (2). The average person has enough energy stored as bodyfat to exist for weeks or

energy storage (2). The average person has enough energy stored as bodyfat to exist for weeks or months without food intake and obese individuals have been fasted for periods of up to one year.

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