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│ SAME AMOUNT OF │ │ │ │FOOD CONSTITUENTS │ FOOD IN TERMS OF │YIELDING ENERGY│ │ │REQUIRED ├─────────┬─────────┤ IN CALORIES. │ │ │(IN GRAMMES). │ CARBON. │NITROGEN.│ │ │ ├──────────────────┼─────────┼─────────┼───────────────┤ │_Proteid_ │ 125 │ 62 │ 20 │ 512·5 │ │_Fat_ │ 500 │ 200 │ ── │ 2050· │ │_Carbohydrate_│ 50 │ 38 │ ── │ 465· │ │ ├──────────────────┼─────────┼─────────┼───────────────┤ │ │ 675 │ 300 │ 20 │ 3027·5 │ └──────────────┴──────────────────┴─────────┴─────────┴───────────────┘
Three of the best known standard dietaries give the amounts in grammes of each food constituent as follows:──