Certain foods do not directly serve either as tissue or energy producers, but are useful in aiding the assimilation of food. Such are the various condiments which may be classed as adjuncts to food. Salt is so necessary to the assimilation of food and to the composition of the various tissues, that it may be ranked as an important food. Water, again, though already oxidised, and so not an immediate source of energy, is absolutely necessary to the assimilation of food, to the interchange between the various tissues and the blood, and to the elimination of effete products.
CLASSIFICATION OF FOODS.—Inasmuch as milk supplies all the food necessary for health and growth during the first year of life, it may reasonably be expected to afford some guidance as to the necessary constituents of a diet for the adult; although the conditions of life being altered in the latter, we can hardly expect the same proportions of the different materials to hold good. In the infant rapid growth and building up of new tissues and organs are going on, involving the necessity for a larger proportional amount of nitrogenous food than in the adult.
The following is the average composition of 100 parts of
┌────────────────┬─────────────┬─────────────┐ │ │ HUMAN MILK. │ COW’S MILK. │ ├────────────────┼─────────────┼─────────────┤ │_Casein_ │ 2.4 │ 4.0 │ │_Albumin_ │ .6 │ .9 │ │_Fat_ │ 2.9 │ 3.5 │ │_Sugar_ │ 5.9 │ 4.0 │ │_Salts_ │ .16 │ .7 │ │ │ ──── │ ──── │ │ _Total Solids_ │ 11.96 │ 13.1 │ │ _Water_ │ 88.04 │ 86.9 │ └────────────────┴─────────────┴─────────────┘
It is evident from this analysis of milk that our food must contain (at least) representatives of all the above divisions. We have, therefore:—
1. =Nitrogenous Foods.= 2. =Hydrocarbons or Fats.= 3. =Carbohydrates or Amyloids.= 4. =Salts.= 5. =Water.=
Condiments and stimulants (tea, coffee, alcohol) are not foods in the strict sense of the word, and will be discussed in a later chapter.
=Nitrogenous Foods= include albumin, casein, gluten, legumen, fibrin, and gelatin. They all agree in consisting of a complex molecule containing many atoms of carbon, hydrogen, oxygen, and nitrogen, with the addition of smaller quantities of sulphur, and in some cases phosphorus. The nitrogenous substances used as food may be divided into two groups, (_a_) those containing gelatin, and (_b_) numerous bodies which receive the common name of proteids or albuminoids.