2. The Physiological Test.—Not only is a proper proportion of proteid, fat, and carbohydrates required, but these must be capable of digestion and absorption and of oxidation in the body. Cheese is a highly concentrated food, but its value is less than its percentage composition would indicate, because of the difficulty of digesting considerable quantities of it. Green vegetables consist largely of cellulose, which is only imperfectly capable of absorption into the blood, although it can experimentally be oxidised by combustion. The proportion between absorbed food and food rejected in the fæces can be ascertained by analysis. Many experiments made on these lines show that on a purely animal diet (meat, eggs, milk) but little nitrogen is lost, while with vegetable foods (carrots, potatoes, peas, etc.) the waste of nitrogen is considerable. Fats are very completely absorbed from the alimentary canal. The amount remaining unabsorbed is greatest with mutton fat (10 per cent.), least with butter (2½ per cent.). Experimentally it has been found that an amount up to 150 grammes (about 5½ oz.) of fat can be absorbed without appreciable loss. Carbohydrates are very completely absorbed, even starchy foods rarely escaping digestion. Completeness of absorption from the alimentary canal is not desirable for all foods; a certain amount of unabsorbed residue is required to stimulate peristalsis. With a purely vegetable diet this amount is excessive, and there is physiological waste of effort.

3. In practical dietetics the Economic test is important. Carbohydrate is by far the cheapest food, and generally vegetable are cheaper than animal foods. Thus a shilling’s-worth of bread yields 10,764 Calories, while the same sum spent on milk would only yield 1 ∕ 3, and on beef 1 ∕ 10 this number of heat units. Similarly a shilling’s-worth of peas contains 572 grammes of proteid, about double as much as the same money’s-worth of cheese; while to obtain the same amount of proteid from eggs would cost more than eight, and from beef more than five times as much as from peas (Hutchison). The market price of foods is no certain indication of their nutritive value. Thus haddock will supply as much nutriment as sole at a fourth of the cost; Dutch as much nutriment as Stilton cheese at less than half the cost. Similarly the most economical fats are margarine and dripping.

4. An Examination of Actual Dietaries under various conditions has strikingly confirmed the results obtained by other methods. It has been found that (a) the potential energy required by a healthy man weighing 11 stones, and doing a moderate amount of muscular work is 3,000 to 3,500 Calories (=310 grains); and that (b) about 20 grammes of nitrogen and 320 grammes (=4,960 grains) of carbon are excreted by such a man. (c) Expressing the 3,000 Calories required in terms of grammes of food, it is found that 125 grammes of proteid, 500 of carbohydrate and 50 of fat are necessary. These facts are expressed in the following table (Hutchison):—

STANDARD AMOUNT OF
FOOD CONSTITUENTS REQUIRED
(IN GRAMMES).		SAME AMOUNT OF FOOD FOOD IN TERMS OFYIELDING ENERGY IN Calories.
CARBON.NITROGEN.
Proteid1256220512·5
Fat500200───2050·
Carbohydrate5038───465·
675300203027·5

Three of the best known standard dietaries give the amounts in grammes of each food constituent as follows:

PLAYFAIR.MOLESCHOTT.ATWATER.AVERAGE.
Proteid119130125125
Fat514012572
Carbohydrate531550450510
Calories3140316035203273

Expressing the same facts in English ounces instead of grammes, 42 ∕ 5 oz. of proteid, 2½ oz. of fat, and 18 oz. of carbohydrate, would represent the ounces of each constituent required according to

(1)(2)
AVERAGE OF ABOVE
THREE DIETARIES.		HUTCHISON.
Proteid4 ∕ 254 ∕ 25
Fat2 1 ∕ 21 ∕ 45
Carbohydrate18 17 ∕ 35
Ounces of dry food24 ∕ 91023 ∕ 45

The chief point of divergence in the above standard dietaries is in the relative proportion of carbohydrate and fat. Probably the correct proportion between these is as 1 to 10; but it will vary according to climate and other circumstances. Detailed examination of a large number of dietaries shows that the amount of daily proteid should be about 125 grammes, or 4⅖ozs. This is contained in 20 eggs, or in 18 oz. i.e. about 4½ ordinary platesful of cooked meat.

It must be noted that the 23-24 oz. of food given above as the standard daily amount represents dry food. This represents 40 oz. or nearly 3 lbs. of ordinary food.