glycogen

Unlike carbohydrates, which are stored in muscle and liver as glycogen, and fats, which are stored in adipose tissue, we have nowhere to store excess protein/amino acids.

"When flies are starving, their bodies break down storage molecules such as glycogen and fats. With a defect of the enzyme chameau, this process no longer works as effectively," says Imhof.

Once the glycogen stores are used up, the body shifts to metabolising fat to get the energy it needs to function.

During physical activity, the body primarily uses carbohydrate reserves, called glycogen, as its energy source.

Our blood sugar drops and we use the glycogen stored in our muscles.