616 METABOLISM, NUTRITION AND DIETETICS 



animal fats ; the cheapest possible diet contains a minimum of animal 

 fat and proteins. 



Many poor persons live on a diet which would not maintain a strong 

 man, for an emaciated body has a smaller mass of flesh to keep up, and 

 therefore needs less protein ; it can do little work, and therefore needs 

 less food of all kinds. A London needlewoman, according to Playfair, 

 subsists, or did subsist thirty years ago, on 54 grammes protein, 

 29 grammes fat, and 292 grammes carbo-hydrates. But this is the 

 irreducible minimum of the deepest poverty, not so much in the protein 

 content, perhaps, as in the very low heat equivalent (1,600 calories); 

 and a woman, with a smaller mass of flesh and leading a less active life 

 than a man, requires less food of all sorts. Even the Trappist monk, 

 who has reduced asceticism to a science, and, instead of eating in order 

 to live, lives in order not to eat, consumes, according to Voit, 68 grammes 

 protein, n grammes fat, and 469 grammes carbo-hydrates; but manual 

 labour is a part of the discipline of the brotherhood, and this must be 

 still above the lowest subsistence diet. 



The question whether it is best to derive the proteins (and fats) of 

 the food mainly from plants or mainly from animals is one which is 

 never left to physiology alone to decide. But it has been definitely 

 proved that vegetable proteins and vegetable fats are (when properly 

 prepared) digested and absorbed as completely as those of animal origin, 

 and play the same part in the metabolism of the body. 



A growing child needs far more food than its weight alone would 

 indicate ; for, in the first place, its income must exceed its expendi- 

 ture so that it may grow ; and, in the second place, the expenditure 

 of an organism is pretty nearly proportional, not to its mass, but to 

 its surface. Now, speaking roughly, the cube of the surface of an 

 animal varies as the square of the mass; when the weight is doubled, 

 the surface only becomes ^4, or one and a half times as great. 

 The surface of a boy of six to nine years, with a body-weight of 

 18 to 24 kilos, is two-fifths to one-half that of a man of 70 kilos; 

 and he should have about half as much food as the man. A child 

 of four months, weighing 5-3 kilos, consumed per diem food con- 

 taining 0-6 gramme nitrogen per kilo of body- weight, or 3-18 

 grammes nitrogen altogether, as against a daily consumption of 

 only 0-275 gramme nitrogen per kilo in a man of 71 kilos (Voit). 



An infant for the first seven months should have nothing except 

 milk. Up to this age vegetable food is unsuited to it ; it is a purely 

 carnivorous animal. By careful observations on the amount of 

 carbon dioxide and nitrogen excreted by a child nine weeks old, fed 

 exclusively on its mother's milk, it has been shown that the ab- 

 sorption and assimilation of milk in the infant is very complete, 

 over 91 per cent, of the total energy being utilized ; while an adult, 

 taking as much milk as is necessary for the maintenance of nitrog- 

 enous equilibrium, does not utilize at most more than 84 per cent. 

 Human milk contains about 2 per cent, of protein (mainly caseino- 

 gen), 3 per cent, of fat, 5 or 6 per cent, of carbo-hydrate (lactose or 

 milk-sugar), and from 0-2 to 0-3 per cent, of salts. Cow's milk 

 contains about 4 per cent, of protein, 4 to 6 per cent, of fat, 4 per 

 cent of lactose* and 07 per cent, of salts. When given to infants it 



