Proteid foods are pre-eminently important, as they construct and keep in repair the tissues of the body. They are not used solely for this purpose. A large share of the energy of the body is derived from the metabolism of proteids. The amount required for these purposes will be discussed on page 32. Meanwhile, it may be said that it is not found to be compatible with efficient health simply to supply an amount of proteid food which will suffice to replace the wear and tear of the tissues, leaving fats and carbohydrates to supply the energy of the body. Deficiency of proteid food always leads to ill-health; and it would appear that in all cases proteid food determines, to a large extent, the metabolism of non-nitrogenous food, and so is favourable to all vital action. The action of nitrogenous food in thus increasing metabolism may make it, when in _relative excess_, a tissue waster. Banting’s cure for corpulence is founded on this principle, lean meat alone being taken, all starchy and saccharine foods being carefully avoided. By _metabolism_ is meant the changes undergone by food before it reaches the state in which it is finally eliminated from the body. It is commonly spoken of as _oxidation_, but this word less exactly represents the facts. The complexity of the changes undergone by food in the body may be better appreciated by a glance at the following schematic statement, which only gives an approximation to the truth:- HYPOTHETICAL TYPICAL FAT ALBUMINOID. (STEARIN). STARCH. GRAPE-SUGAR. C₇₂H₁₁₂N₁₈SO₂₂. C₃H₅(C₁₈H₃₅O₂)₃. _x_(C₆H₁₀O₅). (C₆H₁₂O₆). ———————————————————————————————————-┬——————————————————————————————— │ Various intermediate products, which are finally broken down into and eliminated as │ ┌————————————————————┼————————————————————┐ │ │ │ Urea, CH₄N₂O. Carbonic acid, CO₂. Water, H₂O. =Hydrocarbons=, or fats, consist of three elements, carbon, hydrogen, and oxygen, the amount of oxygen present not being sufficient to oxidise completely either the hydrogen or the carbon. Thus the molecule of stearin, which may be taken as a typical fat, has the formula C₃H₅ (C_{18}H_{35}O₂)₈. In respect to their comparatively unoxidised condition fats compare favourably with starch and sugar, C₆H_{10}O₅ and C₆H_{12}O₆ respectively. It is evident that in starch the H_{10}O₅ = 5H₂O, and that in sugar H_{12}O₆ = 6H₂O, so that in both cases only carbon remains uncombined with oxygen. Dried fats produce by their oxidation 2¼ times as much heat as a corresponding amount of sugar or starch; but the relative advantage of fat is not quite so great as would appear from this comparison, inasmuch as metabolism within the body is not identical with oxidation.