How does the body use protein when there are not enough carbohydrates or fats in the diet?

Answer 1

This is similar to a starvation period. In fact, glucose is the main metabolite of both brain and muscle. The body stores less than a day's supply of carbohydrate, thus, the less blood sugar resulting from even an overnight fast results, through an increase in glucagon secretion and a decrease of insulin secreted into blood current. With this events, fatty acids are movilized from adipose tissue and glucose uptake by muscle tissue is inhibited by insulin levels. Therefore, muscles switch from glucose to fatty acid metabolism for energy production. However, the brain remains heavily dependent on glucose.

In animal metabolism, glucose cannot be sinthesized from fatty acids, because neither pyruvate nor oxalacetate, the precursors of glucose in gluconeogenesis pathway, can be sinthesized from acetyl-CoA . During starvation, glucose therefore must be sinthesized from the glycerol product of triacylglycerol breakdown and, more importantly, from the amino acids derived from the proteolytic degradation of proteins mainly from muscles. The continued breakdown of muscle proteins during long starvation periods would ensure that this process became irreversible since a large muscle mass is essential for an animal to move about in search of food.

Answer 2

Starvation ensues when the fat reserves are completely exhausted and protein is the only fuel source available to the body.
About 20–30 g of protein is broken down each day to make 10 g of glucose to keep the brain alive. However, this number may decrease the longer the fasting period is continued in order to conserve protein.


The human starvation response is unique among animals in that human brains do not require the ingestion of glucose to function.
Because the human brain can use ketone bodies as major fuel sources, the body is not forced to break down skeletal muscles at a high rate, thereby maintaining both thinking and mobility for several weeks.
This response is important in human evolution and allowed for humans to continue to find food even in the face of prolonged starvation