The changes which occur in ketone and FFA utilization during starvation are different for short and long term starvation. Both are discussed below.
Fat and ketone use during short term starvation
Measurements of fuel use show that approximately 90% of the body’s total fuel requirements are being met by FFA and ketones by the third day (20). After three weeks of starvation, the body may derive 93% of its fuel from FFA (10, 21).
For an individual with a metabolic rate of 2700 calories per day, roughly 2400 calories of FFA (approximately 260 grams of fat) are used to fuel the body. Considering that one pound of fat contains 3,500 calories, this represents a loss of almost two-thirds of a pound of fat per day.
Smaller individuals with lower metabolic rates will use proportionally less fat. While this extreme rate of fat loss makes starvation attractive as a treatment for obesity, the problems associated with total fasting (especially body protein loss) make it unacceptable.
The main point is that the metabolic state of ketosis causes a large scale shift from glucose to fat metabolism resulting in a much larger oxidation of fat than is seen on a more ‘balanced’ diet. The ketogenic diet is an attempt to harness this shift to cause maximum fat loss and minimum muscle loss, as discussed in greater detail in the upcoming sections.
Fat and ketone use during long term starvation
Most tissues except the brain, stop using ketones for fuel after the third week of ketosis.
This is especially true for skeletal muscle. While muscle initially derives up to 50% of its energy requirements from ketones (22), this drops to 4-6% by the third week of ketosis. (22, 23). This is thought to occur for the following reason.
During the first few days of ketosis, the brain is incapable of using ketones for fuel. By using a large amount of ketones for fuel, skeletal muscle prevents a rapid increase in blood ketone levels, which might cause acidosis. As time passes and the brain adapts to using ketones for fuel, skeletal muscle must stop using ketones for fuel, to avoid depriving the brain of fuel. For all practical purposes, with long term starvation, the primary fuel of all tissues except the brain is FFA, not ketones.
10. Owen OE et. al. Protein, fat and carbohydrate requirements during starvation: anaplerosis and cataplerosis. Am J Clin Nutr (1998) 68: 12-34.
11. Wolfe RR et. al. Effect of short-term fasting on lipolytic responsiveness in normal and obese human subjects. Am J Physiol (1987) 252: E189-E196
20. Elia M. et. al. Ketone body metabolism in lean male adults during short-term starvation, with particular reference to forearm muscle metabolism. Clinical Science (1990) 78: 579-584.
21. Felig P. et. al. Utilization of metabolic fuels in obese subjects. Am J Clin Nutr (1968) 21: 1429-1433.
22. Owen OE and Reichard GA Human forearm metabolism during progressive starvation. J Clin Invest (1976) 50: 1536-1545.