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.
Section 2: Relationships in fuel use
Looking at table 1, it appears that there are least 4 distinct fuels which the body can use:
glucose, protein, free fatty acids (FFA), and ketones. However when we look at the relationships
between these four fuels, we see that only glucose and FFA need to be considered.
The difference in the proportion of each fuel used will depend on the metabolic state of the
body (i.e. aerobic exercise, weight training, normal diet, ketogenic diet/fasting). Exercise
metabolism is addressed in later chapters and we are only concerned here with the effects of
dietary changes on fuel utilization.
In general, tissues of the body will use a given fuel in proportion to its concentration in the
bloodstream. So if a given fuel (i.e. glucose) increases in the bloodstream, the body will utilize that
fuel in preference to others. By the same token, if the concentrations of a given fuel decrease in
the bloodstream, the body will use less of that fuel. By decreasing carbohydrate availability, the
ketogenic diet shifts the body to using fat as its primary fuel.
Glucose and protein use
When present in sufficient quantities, glucose is the preferred fuel for most tissues in the
body. The major exception to this is the heart, which uses a mix of glucose, FFA and ketones.
The major source of glucose in the body is from dietary carbohydrate. However, other
substances can be converted to glucose in the liver and kidney through a process called
gluconeogenesis (‘gluco’ = glucose, ‘neo’ = new, ‘genesis’ = the making). This includes certain
amino acids, especially alanine and glutamine.
With normal glucose availability, there is little gluconeogenesis from the body’s protein
stores. This has led many to state that carbohydrate has a ‘protein sparing’ effect in that it
prevents the breakdown of protein to make glucose. While it is true that a high carbohydrate
intake can be protein sparing, it is often ignored that this same high carbohydrate also decreases
the use of fat for fuel. Thus in addition to being ‘protein sparing’, carbohydrate is also ‘fat sparing’
(3).
If glucose requirements are high but glucose availability is low, as in the initial days of
fasting, the body will break down its own protein stores to produce glucose. This is probably the
origin of the concept that low carbohydrate diets are muscle wasting. As discussed in the next
chapter, an adequate protein intake during the first weeks of a ketogenic diet will prevent muscle
loss by supplying the amino acids for gluconeogenesis that would otherwise come from body
proteins.
By extension, under conditions of low glucose availability, if glucose requirements go down
due to increases in alternative fuels such as FFA and ketones, the need for gluconeogenesis from
protein will also decrease. The circumstances under which this occurs are discussed below.