It is clear, therefore, from what we have seen that the tissues proper, i.e. flesh and the various organs, can only be built out of proteins, mineral matter, and water, and that these alone have the power to manufacture and repair tissue.

On the other hand, energy or work, and its by-product heat, can be obtained from any of the food principles, and as adipose tissue wherever it may be deposited is simply a reserve store of energy, it may also be built up from any food substance. It is manifest that fat may form fat. We know from feeding pigs on a diet composed solely of potatoes, that carbohydrates can form fat, and we have seen that proteins have a non-nitrogenous molecule which may be used for the same purpose. The big meat-eater is almost always obese, and as he grows older has a tendency to be diabetic, or at any rate to pass sugar in his urine. Fat, of course, is not flesh, i.e. tissue, but it is body substance. So it is interesting to note that the body can grow in bulk either by the increase of its fleshy substance or by the deposit of fatty substance. When, however, a human being reaches full growth, which is attained at the latest by the twenty-fifth year of life, there is no further possibility - unless during exceptional circumstances, some of which I have already mentioned - of adding to his flesh.

Hence after this period of life, food is simply required (1) for repairing the daily wastes of the tissue, and this is effected by proteins in conjunction with mineral matter and water; and (2) for supplying energy necessary for the daily work, and this may come from carbohydrates, fats, or nitrogenous substances such as proteins, or relations of theirs called albuminoids, of which gelatine is a typical example. Doubtless also water and mineral matters play an important part in the process.

Any food in excess of this quantity is either rapidly burned up and excreted from the body without being used, and this is what happens to the surplus nitrogenous material in every case and in many than people to all the surplus food; or else manufactured into fat, which is deposited under the skin in the abdomen, back, and flanks, face and neck, and this is the fate not only of the fats and carbohydrates but even of the non-nitrogenous portion of the proteins.