The food material as absorbed is carried by the blood, together with oxygen taken up from the lungs, to each cell in every part of the body. Utilization of this food material by the cells constitutes life and is not thoroughly known. The protoplasm of the cell itself plays an important part in this process. Some of the material is utilized by the cell for repair and new growth. The rest of the material goes through the process of combustion or oxidation which is caused by the activity of the cells. Starch, now in the form of sugar, sugars, and fats are burned to form carbonic acid and water. If these foods are eaten in excess of the body requirements they are stored in the body as fat. Hence a constant gain in weight of an adult is a sign of over-eating or under-exercise, and can be avoided with care of the diet. It is a sign of self-indulgence. In the burning of protein, on the other hand, the process is not so complete. Some of the hydrogen and of the carbon as well as the nitrogen is left in the waste products which are carried off in the urine in the form of urea and uric acid. Hence the heat produced from protein food is less in the body than if the same foods were burned in the furnace.