SOURCES OF ENERGY AND MATERIALS 109 



stored foods are consumed. The carbohydrates (glycogen of the Uver and 

 muscles) are used first. Fats are used simultaneously with the carbo- 

 hydrates but usually last until after the carbohydrates are exhausted. 

 Then the materials of the protoplasm itself are used, first those of the 

 less essential organs, then of the brain, spinal cord, and heart. Death 

 usually follows quickly upon such extreme starvation. 



Materials Required. — Besides furnishing energy, food must also pro- 

 vide materials with which to build protoplasm and such secreted products 

 as the hard parts of bone and teeth. One of the most urgently required 

 materials is water — 2000 cc. a day in an average person. Certain salts 

 must be regularly supplied, since about 30 grams are lost per day, mostly 

 in urine and sweat. Most ordinary foods contain about the right pro- 

 portion of the various salts, though vegetable foods are deficient in 

 sodium chloride (NaCl). This is the reason for the common use of 

 table salt. Any one sweating profusely because of heavy labor in hot 

 places must usually drink salt water to avoid muscular spasms. 



There are certain minerals which are necessary. The ones most 

 likely to be poorly represented in the diet are iron, calcium, and iodine. 

 The hemoglobin of red blood cells requires iron, and this is adequately 

 provided in liver, meats in general, eggs, and many vegetables and fruits. 

 Calcium is needed for bone and teeth, and is obtained from milk, cereals, 

 peas and beans. Iodine is necessary for the hormone of the thyroid 

 gland. It is abundant in sea foods; and in inland communities health 

 authorities often require that potassium iodide be introduced into table 

 salt. Other minerals, including copper, zinc, manganese, and cobalt, are 

 essential for the production of important enzymes, but the amounts 

 needed are exceedingly small and natural diets usually contain enough 

 of them. 



For construction of protoplasm proteins are steadily required — a 

 minimum of 50 grams a day for an average adult person. A variety 

 of amino acids is necessary, and since only a few of the simplest ones 

 can be synthesized from other substances, the others must be included 

 in the diet. Foods which supply all the necessary amino acids are the 

 proteins of eggs and lean meat, the glutenin of wheat, and the lactalbumin 

 of milk and cheese. Most other protein foods lack, or include too small 

 quantities of, certain amino acids. Some fat is also required; for though 

 most of the fatty acids can be synthesized from carbohydrates, the ones 

 which the human body can not synthesize are quite essential, and these 

 must be received ready-made. 



Vitamins. — One group of required specific substances deserves sepa- 

 rate treatment. It has long been known that a diet consisting of purified 

 proteins, carbohydrates, and fats leads to serious trouble. Natural foods 

 evidently contain something that does not occur in the purified foods. 



