148 THE ABDOMEN. 



only in small amount by proteins. So no one food can 

 form the whole diet but there must be. variety. Carbo- 

 hydrates and fats are not sufficient for life, some protein 

 is necessary. Carbohydrates are more digestible than 

 fats but have less potential energy. Gelatine saves waste 

 of nitrogen, though it does not increase the supply. 

 Water and salts are not nutritive but they aid the body 

 processes, the water helping to dilute and dissolve 

 substances for digestion. 



The end-product of the consumption of protein is 

 urea, which is eliminated by the kidneys. Just where it 

 is formed is unknown, but many think in the liver. A 

 trifling amount of urea is also eliminated in the sweat and 

 in the breath as well as in the feces. Proteins increase 

 nitrogenous metabolism and also the metabolism of 

 other foods, but the amount of nitrogen eliminated is just 

 equal to that taken in. Probably some comes from the 

 tissues themselves and not from the food. The oxida- 

 tion of carbohydrates and fats is measured by the amount 

 of carbon excreted. At first as much is given off as is 

 taken in, but after a while the carbohydrate is stored up 

 as glycogen in the liver and the fats are stored as fat. 



The amount of food needed varies with the person's 

 size and occupation, less being needed for a child than for 

 an adult and more for a hard-working man than for one 

 who is doing less work. In general, 100 to 130 grams of 

 protein, 40 to 80 grams of fat, 450 to 550 grams of carbo- 

 hydrates, 30 grams of salts, and 28,000 grams of water 

 is a fair amount. 



Foods are cooked to make them more digestible and to 

 develop their flavor, so that they will taste better. Cook- 

 ing also kills germs and parasites that might be harmful. 

 Meats should be cooked rapidly on the outside to coagu- 

 late the surface albumen and keep in the juices. The 

 heat, besides coagulating the albumen, turns the tough 

 parts to gelatine. In cereals the tough envelope of 

 cellulose is broken up and in vegetables the tough fibrous 

 parts are softened and made more digestible. 



