In addition to a knowledge of food constituents, of the proportion of which these exist in our food, and of the use of food in the body, we need to know the amount of food necessary to supply our daily needs under different conditions. Many factors will influence not only the total amount of food that we need, but also the proportions in which we shall use the pro-teids, the carbohydrates and the fats. The flesh weight of the body is important in deciding the amount of proteid (that is, the muscle weight, not the total weight of the body) since the greater the flesh weight the greater the nitrogenous waste. The shape of the person, whether tall or thin, or short and plump, influences the amount of fuel food required, since the amount of surface exposed affects the loss of heat. The degree of activity has an important influence upon the amount of all the food principles. Variations in climate to a certain extent affect the amount of heat to be produced in the body, and occupation also has an important influence.
The age of the individual is, within certain limits, one of the greatest factors. The growing child needs a large amount of building material, while the old person needs distinctly to lessen the tissue building foods. The accompanying diagram gives an idea of the way in which these proportions vary with different ages. It will be seen that the proportion of proteid is much greater in comparison with other food materials in the case of the child than of the adult. The total amount of food is also greater in proportion to body weight in the child than in the adult. Although not shown in the table, mineral salts are needed in large proportion in the child's diet, while they may well be cut down in the diet of the old. The amount of food needed increases rapidly from birth to about four years of age, very slowly from four to about ten, with a rapid increase from this time to twenty-four. From ten to twenty-four the carbohydrates should increase in amount more rapidly than the other food principles.
Diagram Showing the Varying Amounts of Food Principles Required at Different Ages.
To put in terms of the nutrient ratio the difference between the diet of the child and that of the adult - in the adult diet the ratio is about 1:5.3; in the diet of the child, 1 4.3.
These statements are of course true only approximately, yet one familiar with children must recognize in them a fair generalization from the facts.
The proportions of the different food principles needed daily constitute the dietary, and dietary standards have been made up taking into account as far as possible these different conditions. These dietaries are sometimes called experimental, and sometimes statistical, according to the method used in formulating them. An experimental dietary is the result of careful observations of the effect of different proportions of food nutrients upon an individual under determined conditions. The statistical dietary is the outcome of the study of the actual ration of large numbers of people. Each of these has its drawbacks. In the first case it is difficult to decide how far the result is due to individual idiosyncrasy, and a large number of experiments must be tried before the personal factor can be eliminated. In the second case it is hard to determine whether some variation in the diet might not produce better results.