NUTRITION 159 



materials, or feces, are carried to the exterior either through a 

 terminal cavity, the cloaca, into which also open the urogenital 

 ducts, or directly out through the anus as in most Mammals. 



E. Food Use 



Digestion, absorption, and distribution completed, the actual 

 food supplying matter and energy is at the disposal of the various 

 cells for carrying on the essential metabolic processes. Since the 

 daily energy output of the average man not carrying on heavy 

 physical labor is about 3000 calories, and the energy yield per gram 

 for proteins and carbohydrates is about 4.1 calories, and for fats 

 about 9.3 calories, it is not difficult to determine from these fuel 

 values how much of each foodstuff, or mixture of them, is re- 

 quired to supply the necessary energy. But, of course, provision 

 must also be made for tissue maintenance and this draws upon the 

 proteins with their nitrogen constituent. In general it may be 

 said that, since carbohydrates and fats adequately supply the 

 energy demands, protein consumption need not greatly exceed the 

 nitrogen required for tissue maintenance. 



Moreover the body also requires small amounts of certain so- 

 called accessory food substances, or vitamins, which are usually 

 present in sufficient quantity in a normal mixed diet. Vitamins 

 are organic substances, not related chemically to one another, that 

 do not supply energy or structural material, but are necessary for 

 cell metabolism. The exact chemical constitution of most vitamins 

 is unknown. 



Vitamin A has an effect on many of the membranes of the body, 

 a deficiency resulting in glandular disturbances and in lowered 

 resistance to infection. Rich sources are cod liver oil, carrots, and 

 butter. Vitamin Bi prevents an inflammation of the peripheral 

 nerves and paralysis known as beri-beri, as well as disturbances of 

 the functions of the intestine and kidneys. Excellent sources are 

 whole grain cereals, milk, and liver. Vitamin B 2 is usually asso- 

 ciated with Bi. Inadequate amounts give rise to the disease called 

 pellagra, of which brown pigmentation of the skin, general weak- 

 ness, digestive disturbances, and paralysis are symptoms. It is 

 readily available in egg-white, milk, lean meat, and green vege- 

 tables. Vitamin C prevents scurvy, a disease marked by loosening 

 of the teeth and hemorrhages in the joints. It has proved to be a 

 hexuronic acid that is present in most citrus fruits and green 



