Animal cells under all conditions of life are constantly breaking down proteins into simpler substances which the body eliminates. Since this breaking down or "catabolism" of protein does not stop either in fasting or under the most liberal feeding with fats and carbohydrates, it follows that there is always a need for protein whatever the supply of other food.

Protein metabolism differs widely from energy metabolism in the conditions which determine its amount, for protein metabolism is governed mainly by the kind and amount of food, and to only a slight extent if at all by muscular exercise; whereas energy metabolism is governed mainly by the muscular exercise, and to only a relatively small extent by the food. By giving food rich in fats and carbohydrates but poor in protein, the protein metabolism of a healthy man can easily be brought to less than 50 grams per day, and then by changing to a diet rich in protein, it may be increased to 150 or even 200 grams per day; i.e. the rate of protein metabolism can be increased 200 to 300 per cent in a few days by a change in diet alone, all other conditions remaining the same.