The percentage composition of gelatin is:—
┌───────────┬───────────┬───────────┬───────────┐ │ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │ ├───────────┼───────────┼───────────┼───────────┤ │ 50.0 │ 6.6 │ 18.3 │ 25.1 │ └───────────┴───────────┴───────────┴───────────┘
The percentage composition of all proteids lies within the following limits:-
┌───────────┬───────────┬───────────┬───────────┬───────────┐ │ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │ SULPHUR. │ ├───────────┼───────────┼───────────┼───────────┼───────────┤ │ 52.7 to │ 6.9 to │ 15.4 to │ 20.9 to │ 0.8 to │ │ 54.5 │ 7.3 │ 16.5 │ 23.5 │ 1.6 │ └───────────┴───────────┴───────────┴───────────┴───────────┘
Proteids also contain a small amount of phosphorus, chiefly as phosphate of lime, but also in minute quantity in their essential structure. Various proteids are used in food, _e.g._ serum-albumin in the blood and tissues of animals; egg-albumin in the white of eggs; myosin in flesh; casein in milk; legumin, or plant-casein, in the seeds of leguminous plants; gluten in wheat-flour, etc.
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:-