CHAP, v.] NUTRITION. 477 



very low level. Here too the factor of idiosyncrasy makes itself 

 exceedingly felt. 



In feeding for fattening purposes the comparatively cheap 

 carbohydrates are of course chiefly depended on. If the view 

 mentioned on p. 454 be correct, that the fat really stored up all 

 comes from proteid metabolism, an equivalent of this food-stuff 

 must always be given. If, as seems probable, this view is a too 

 hurried generalisation, there still remains the possibility that for 

 economical fattening, with the least waste, a certain proportion 

 between the nitrogenous and non-nitrogenous foods must always be 

 maintained. 



From what has been previously said it is evident that proteid 

 food is not the only food-stuff to be regarded in selecting a diet for 

 muscular labour. We should however equally err in the opposite 

 direction if we selected exclusively non-nitrogenous food on which 

 to do work, since, as we have seen, there is no evidence that the 

 fats or carbohydrates are the direct, though they may be in part 

 the ultimate source, of muscular energy. Considering how complex 

 a thing strength is, how much it depends on the vigour of parts of 

 the body other than the muscles, a normal diet, calculated to 

 develope equally all parts of the body, is probably the best diet for 

 active labour. It is possible however that an excess of proteid 

 food, by reason of the renewal of tissue caused by its metabolic 

 activity, may be, in such cases, of service. 



Lastly, the several saline matters, including the extractives of 

 animal and vegetable food, are no less essential elements of a diet 

 than proteids, fats, or carbohydrates. Of use, not for the energy 

 they themselves possess, but by reason of their regulating the 

 energy of the food-stuffs more strictly so called, they are necessary 

 to life: the body in their absence fails to carry out its usual 

 metabolism, and disease if not death follows. 



The dietetic superiority of fresh meat and vegetables depends 

 in part on their still retaining these various saline and extractive 

 matters. A diet from which phosphorus (or even possibly phos- 

 phates), or chlorides, or potash, or soda salts are absent, is, as soon 

 as the store of the substance in the body is exhausted, useless 

 for nutritive purposes. Calcium and magnesia may, to a certain 

 extent, be replaced by bases closely allied to them; but the 

 metabolic rdle of phosphorus or of sulphur cannot be taken up 

 by an analogous body ; and, as is illustrated by their distribution 

 in the body, the physiological functions of potash and soda are 

 widely different if not antagonistic, closely allied as are these two 

 alkalis when regarded from a chemical point of view. Like 

 medicines and poisons and indeed they are in a manner natural 

 medicines the action of these bodies depends in part on their 

 dose. Indispensable as are potash salts to the economy, a large 

 dose of them is injurious ; and a dog fed on nothing but Liebig's 



