Question

Individuals who fast for a prolonged period of time to lose weight suffer from ketosis, high levels for circulating ketone bodies. Why does this occur?

Answer 1

There are two primary types of food-based fuel that our cells can use to produce energy: The first cellular fuel is glucose and the second type of cellular fuel are ketone bodies. When glucose levels are low, especially over time during fasting, most cells will switch to using ketone bodies for fuel. As glucose levels fall and fatty acid levels in the blood rise, the liver cells ramp up beta-oxidation which increases the amounts of a molecule called Acetyl-CoA. As the level of Acetyl-CoA rises, it is shunted to a process called ketogenesis. Ketogenesis generates a ketone body called acetoacetate first, and this ketone is then converted into the two other types of ketones: beta-hydroxybutyrate, and acetone. Meanwhile, the glycerol part of the fat molecule gets converted into glucose in a process called gluconeogenesis, which means "make new sugar".

For the first few days, weight loss averages around 1-2 pounds a day, both because you’re shedding water weight and because you’re using more protein. But consuming protein for fuel is risky, because it means breaking down muscles – not only your biceps, but also more essential muscles like your heart. This makes fat by far a preferential energy source.