The age of the individual not only modifies the absolute amount of food required, but also the relative quantity in proportion to body-weight. The rapid growth of children in early life necessitates a relatively larger consumption of food than at any other period of life; more especially it calls for a large supply of building material, i.e. proteins.

The standard of increase usually adopted is as follows: -

A child under 2 requires 0.3

the food of a man

doing

moderate work.

A child of 3 to 5 requires .4

"

"

"

A child of 6 to 9 requires .5

"

"

"

A child of 10 to 13 requires .6

"

"

"

A girl of 14 to 16 requires 7

"

"

"

A boy of 14 to 16 requires .8

"

"

"

In old age the processes of assimilation are less active and the bodily activities are restricted, hence less food is required.

Over-feeding is a fruitful cause of serious trouble in elderly subjects.

Under similar conditions women require less food than men, for the reason that they are usually smaller, and do less muscular work. Given precisely similar conditions as to weight of subject and amount of physical work performed, there is probably no difference in the amount of food consumed. The diet in pregnancy should be liberal but not excessive. During lactation the amount of food must be increased, more especially in protein and fat, in order to make good the nutritive elements lost in the milk.

Rest And Exercise

Much less food is required during rest than during exercise. The following is the generally accepted statement of the number of calories which must be supplied for work of different degrees of severity (Rubner).

Calories.

Rest, e.g. a child at a desk ...

2500

Moderate muscular work, e.g. house-painter ...

3121

Severe muscular work, e.g. shoemaker ...

3659

Hard labour, e.g. blacksmith ...

5213

The researches of Chittenden, however, already alluded to, throw great doubt on the correctness of this teaching. Chittenden found that the average daily nitrogen excretion of eight athletes on a reduced protein diet was 8.8 grammes, this being obtained from a protein intake of 55 grammes, the caloric value of the total food being from 2500 to 2000 calories. Similar results were obtained in the case of the soldiers on the prescribed diet. The meals were selected from the following: -

Breakfast

Coffee, rolls and butter, bananas, fruit, hominy with sugar and cream, farina, Indian meal, baked potato, boiled rice or oatmeal.

Lunch

Coffee, bread and butter, sphagetti, potato, stewed tomato, boiled onions, string beans, fruit, hominy and syrup, oysters, cold tongue, baked apple.

Dinner

Soup, pea, cream of celery, bean, tomato; fish, bacon, sausage, chicken, lamb chop, steak, fried or boiled potatoes, spinach, lettuce, celery, apple salad, Lima beans, cream puffs.

In the present state of our knowledge it is not possible to be too dogmatic on the subject, but there is in the writer's view little doubt that, just as the hitherto accepted standard of 3000calories for a man engaged in light work has been proved by Chittenden to be excessive, so the above increase of over 2000 calorics is materially greater than is necessary or advisable for subjects engaged in hard physical labour. It is probable that an increase of food by one-fourth or one-third of the normal is all that is necessary for subjects engaged in severe muscular work.

The question arises as to whether the increase of food undoubtedly necessary for subjects engaged in hard physical work should affect mainly the proteins, fats, or carbohydrates. There are no data by which this can be precisely determined. By those engaged in training for athletics, proteins are largely relied on to supply the extra energy; but for ordinary purposes it is certainly advisable that the increase should be equally distributed among the proteins, fats, and carbohydrates of a mixed dietary, sugar being specially advantageous.

Climate And Seasons

Climate and temperature influence the quantity and quality of food required. In tropical countries the natives live largely on vegetable foods and fruits, and consume less animal food and fat; in the Arctic regions the inhabitants consume enormous quantities of flesh and fat. The explanation of this is to some extent to be found in the fact that the natives of any country eat what they can most easily obtain. At the same time, the necessity for supplying the body with a large proportion of combustible food, such as carbohydrates and fats, when the external temperature is very low is obvious; and it is also clear that less food of this kind is required when the external temperature is high. In this country common experience has shown the advisability of modifying the diet in the heat of summer, less animal food being taken, vegetable substances and fruits entering more largely into the dietary.

Personal Idiosyncrasy

It is a matter of common observation that some people habitually eat much less than others, although living under the same general conditions. This is, in all probability, to some extent a result of habit and general training. At the same time there is no reason to doubt that some tissues work more economically than others, with corresponding advantage to the individual. Many special idiosyncrasies with regard to diet have to be considered. Some persons cannot take eggs in any form; others cannot take milk; some are made ill by certain kinds of fruit or meat. These may be the result of inherited tendencies, or may be the growth of habit.