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Chapter 3:

The basics of fuel utilization

Although this chapter does not discuss the ketogenic diet in great detail, the information
presented is helpful in understanding the following chapters. There are four primary fuels which
can be used in the human body: glucose, protein, free fatty acids, and ketones. These fuels are
stored in varying proportions in the body. Overall, the primary form of stored fuel is triglyceride,
stored in adipose tissue. Glucose and protein make up secondary sources. These fuels are used in
varying proportions depending on the metabolic state of the body.

The primary determinant of fuel utilization in humans is carbohydrate availability, which
affects hormone levels. Additional factors affecting fuel utilization are the status of liver glycogen
(full or empty) as well as the levels of certain enzymes.

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

Tissue Average weight (lbs) Caloric worth (kcal)
Adipose tissue triglyceride 33 135,
Muscle protein 13 24,
Carbohydrate stores
Muscle glycogen (normal) 00.25 480
Liver glycogen 00.5 280
Blood glucose 00.04 80
Total carbohydrate stores 00.8 840

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