108 PRINCIPLES OF ANIMAL BIOLOGY 



the muscles, and the mesenteries of the intestine and other organs, are 

 particularly devoted to this function. 



Storage of indiffusible substances such as glycogen or fats necessitates 

 redigestion of them when they are to be used; consequently enzymes for 

 carbohydrate and fat digestion must be produced or producible in all 

 cells which store these products. 



Proteins are not stored in animals, as carbohydrates and fats are 

 stored. The supply of protein foods must therefore be rather steady; 

 that is, they should be included in the diet almost daily. Amino acids 

 enter the blood after the digestion of protein foods and are taken up 

 by the cells which require them. When the diet is deficient in proteins, 

 requirements of amino acids in vital situations are supplied only by 

 breaking down body proteins elsewhere, as happens in starvation. 



Energy Requirements. — Any balanced diet must provide two things, 

 energy and materials. Energy is measured in the units known as 

 calories, one calorie being the amount of heat necessaiy to raise the 

 temperature of a kilogram of water 1°C. Each gram of a carbohydrate 

 or protein food utilized in metabolism yields about 4 calories, a gram 

 of fat about 9 calories. A relaxed, fasting human body of average size 

 and shape, in prone position, requires about 1600 calories daily. More 

 than half of this energy goes to maintaining the body temperature. 

 The rest is expended by the vital organs such as the heart and the 

 muscles performing breathing movements. If food is taken, so that 

 muscles of the digestive tract are also active, the daily energy require- 

 ment is about 1800 calories. For sedentary workers leading normal lives 

 it rises to about 2400 calories, while manual laborers need 3000 to 5000 

 calories, depending on how hard and long they work. If an average 

 person consumes much more energy than is proper to his mode of life 

 and occupation, he may have an overactive thyroid gland or a fever. 

 If the energy consumption is much less than normal, the cause may be 

 a deficient thyroid or pituitary or adrenal gland, or low nutrition. 



So far as mere quantity of energy is concerned, it may be obtained 

 from any of the types of food. Pligh protein diet requires more work 

 of the kidneys because of the increased nitrogenous wastes, but the 

 kidneys are capable of much more than an average load if they are 

 healthy. An excess of fat is objecticmable chiefly because fats do not 

 oxidize very completely unless carbohydrates are being oxidized at the 

 same time. To some extent the human body can alter the proportion 

 of the different kinds of compounds derived from its food, for amino acids 

 can be converted to glucose, and carbohydrates to fat; but there is little 

 conversion of fat to carbohydrate, and only the simpler amino acids can 

 be made from nonprotein foods. 



If the food currently taken does not provide the required energy. 



