NUTRITION 



Carbohydrates. 



357 



Non - nitrogenous Food. ■ — The all - important feature in 

 connection with the non-protein substances in food is that 

 they are directly associated with muscular work. During 

 work the intake of oxygen and output of carbon dioxide is 

 immediately raised, and in the trained — that is, the ' fit ' — 

 animal, the increased intake is proportional to the work per- 

 formed. The nitrogen of the body, it will be remembered, is 

 unaffected, excepting during excessive work, and even then the 

 amount bears no proportion to the work performed. 



The whole of the carbohydrate matters found in food — viz., the 

 starch, sugar, gum, and cellulose — must, as we have seen (p. 288), 

 be first rendered soluble before they can enter the system. 

 Further, they can only enter as some form of sugar, and are then 

 stored up for future use as fat in certain depots, and as glycogen 

 in the muscles and liver ; while for present use they exist as 

 dextrose in the circulating blood. The supply of carbohydrates 

 is added to by the splitting up of proteins into a nitrogenous and 

 non-nitrogenous portion ; whether the non-nitrogenous portion 

 of protein can form fat is uncertain, but it is "undoubted that it 

 forms glycogen. Carbohydrates are readily oxidised, as we 

 have already seen. In this respect they are a great contrast 

 to the fats. In dealing with the question of the ' respiratory 

 quotient ' (p. 118), it was explained that this fraction repre- 

 sented the relative amounts of carbonic acid produced and 

 oxygen absorbed. The theoretical value of the respiratory 

 quotient on a carbohydrate diet is 1, but with fats the volume of 

 oxygen absorbed is greater than the volume of carbonic acid pro- 

 duced, and the respiratory quotient becomes 0707. For example : 



1 gramme (15I grains) of carbohydrate requires 0*832 litre 



(50*8 cubic inches) of oxygen, and produces 0*832 litre 



(50-8 cubic inches) of C0 2 . 

 1 gramme (15^ grains) of fat requires 2-8875 litres (176 cubic 



inches) of oxygen, and produces 1*434 litres (87*5 cubic 



inches) of C0 2 . 



In speaking of the respiratory exchange, attention was drawn 

 to the fact that all the oxygen absorbed by the lungs was not 

 returned as carbon dioxide ; there was a missing portion. This 

 Oxygen Deficit is greater when the amount of fat and protein in 

 the diet is considerable, whereas it is smaller the more the carbo- 

 hydrates preponderate in the food. The missing oxygen, in 

 other words, is being employed in the oxidation of the hydrogen 

 of fats, which, as we have seen, only possess in themselves one- 

 eighth of the total oxygen required for the oxidation of their 

 hydrogen to water. Similarly the proteins are short of oxygen 

 for the oxidation of their hydrogen. These two sources, together 



