628 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 Play- 

 fair, subsists, or did subsist forty 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, 1 1 grammes fat, and 469 grammes carbo-hydrates ; but manual 

 labour is a part of tiie 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. Nor is there 

 any difference in the basal metabolism (p. 686) of vegetarians and 

 persons living on an ordinary diet. 



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

 indicate ; the expenditure of organisms of different size in the same 

 physiological condition is proportional not to the mass, but more 

 nearly to the surface area. 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 \J^, 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 this would indicate that he should have about half 

 as much food as the man. This is not all, the child is not in the same 

 physiological condition as the adult. It is growing. Its income 

 must exceed its expenditure, and by a far greater amount than the 

 actual gain in weight, for growth is a physiologically expensive 

 process. It needs many pounds of food to put on one pound of 

 flesh. And in accordance with this it has been found that the basal 

 metabolism of a child per unit of surface is decidedly greater than 

 that of an adult (p. 697). 



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- 



