METABOLISM, NUTRITION, AND DIET. 439 



excess are almost certain to give rise to dyspepsia, with acidity and flat- 

 ulence. Excess of starch or of sngar in the food may, however, be got 

 rid of by the urine in the form of sugar. There is evidently a limit to 

 the absorption of fat as well as of starch, since if in excessive amount 

 they may appear in the faeces. 



That salts are necessary as food is proved by the presence of scurvy 

 when they are not present, and we know that there is a consant excre- 

 tion of chlorides, phosphates and sulphates in the urine, so that in order 

 to balance the income and output, these salts in combination with 

 sodium, potassium, calcium, etc., must be taken in. 



The necessity for the taking in of water, in order to balance the ex 

 cretion, is sufficiently obvious. 



To summarize what has been said : 



Proteid. i. If the nitrogen of the income is less than that of the 

 output, the animal loses flesh and starves, gradually or quickly, accord- 

 ing to the extent of the deficiency. 



ii. If the nitrogen of the income be evenly balanced, the proteid 

 being only just sufficient, the animal does not lose flesh, but may increase 

 or diminish in weight (fat). 



iii. If the nitrogen of the ingesta exceed that of the egesta, the ex- 

 cess is mainly retained in the form of flesh. 



iv. If the proteid be in great excess, although there be a condition 

 of nitrogenous equilibrium, there may be increase in weight, but also a 

 likelihood of gout and similar affections. 



Fatty and Carbohydrate Foods are of no use either together or sepa- 

 rately without the addition of the other food-stuffs. In moderation, 

 either may diminish the amount of proteid necessary to produce nitro- 

 genous equilibrium. If the quantity of either be increased beyond a 

 certain amount, it is retained in the body in form of fat (and, in the 

 case of the carbohydrate, as glycogen). If in great excess, disorders of 

 digestion occur. Fats have more potential energy than carbohydrates, 

 but are less digestible. Fatty foods need more oxygen than carbohy- 

 drates when they are used up in the body. 



Gelatin will not entirely, but will partly replace the proteid in a diet. 



Salts of sodium, potassium, calcium, etc., are necessary in food, the 

 chlorides, phosphates and sulphates, and possibly the citrates, being the 

 most important of those required. 



Water is absolutely essential to life an animal will not survive 

 deprivation for longer than a few days. 



Effects of Deprivation of Food. The animal body deprived of all food 

 in the course of a variable time dies from starvation. The length of 

 time that any given animal will live in such a condition depends upon 

 many circumstances ; the chief may be supposed to be the nature and 

 activity of the metabolism of its tissues. 



