346 - Multicellular Animals, Especially Man 



the loss of salt. Finally it was learned that 

 salt water should be provided, and during 

 hot heavy work, salt solutions can be drunk 

 without distaste. 



Certain specific minerals are regularly 

 needed by the body for the synthesis of its 

 essential components. The minerals most 

 likely to be deficient in the diet are iron, cal- 

 cium, and iodine. Without a source of iron, 

 from the organic and inorganic components 

 of our food, the body cannot continue to 

 synthesize hemoglobin, and anemia must re- 

 sult. However, most meats (especially liver), 

 eggs, and many fruits and vegetables provide 

 an adequate source of iron. Calcium com- 

 pounds are necessary for the formation of 

 bone and teeth: about 1.0 gram for children; 

 2.0 grams for women during pregnancy and 

 nursing; and 0.8 gram for other adults — com- 

 puted on a daily basis, in terms of the 

 calcium content. The best source of this min- 

 eral is milk, although cereals and such vege- 

 tables as beans and peas are also rich in 

 calcium. Most meats provide a good source 

 of phosphorus, which is essential for the 

 synthesis of ATP, nucleic acids, etc., as well 

 as for a proper formation of bones and teeth. 



Without iodine the body cannot synthesize 

 thyroxin, the essential hormone of the thy- 

 roid gland; and the resulting hypothyroidism 

 (p. 403) represents a very serious and, in 

 former times, fairly prevalent disease. Iodine 

 is especially abundant in the sea and in sea- 

 foods. Formerly, iodine deficiencies were en- 

 countered mainly in populations living in 

 inland regions that never were inundated 

 by the ocean during any geological period. 

 Nowadays, however, dietary hypothyroidism 

 (including cretinism) is seldom found, due 

 mainly to the fact that commercial salts are 

 artificially fortified by the addition of small 

 amounts of potassium iodide (KI). 



Recent investigations indicate that health- 

 maintaining diets must contain zinc, copper, 

 manganese, cobalt, and perhaps one or two 

 other metals. But only the merest traces of 

 these substances are sufficient. These trace 

 metals appear to be needed for the activa- 



tion of some of the metabolic enzymes, and 

 most natural diets contain more than ade- 

 quate amounts of the trace elements. 



Organic Requirements. In addition to the 

 fuel foods, 50 grams of protein, at the very 

 least, must be included in the diet of an 

 adult, and more protein is needed in growing 

 individuals. Such a quantity of protein pro- 

 vides only the minimal amount and variety 

 of the amino acids needed by the tissues for 

 the synthesis of essential proteins. As in ani- 

 mals generally, the cells of the human body 

 cannot synthesize all of the amino acids from 

 simpler substances. Consequently the body 

 depends upon a ready-made supply of these 

 nonsynthesizable (essential) amino acids. Not 

 all protein foods are equally valuable in con- 

 tributing these essential amino acids, as may 

 be seen in Table 18-2. 



Some natural fat must also be included in 

 a balanced diet. The body is able to synthe- 

 size all except a few of the fatty acids, but 

 these specific exceptions are essential. Also, 

 natural fats are our main source of some of 

 the vitamins, without which the body can- 

 not grow or function properly. 



The Vitamins. In 1912 it was definitely 

 proved that animals cannot survive on diets 

 in which the organic food is provided en- 

 tirely by purified proteins, carbohydrates, 

 and lipids. This result indicated that the 

 natural sources of organic food must contain 

 small amounts of then unknown essential 

 substances, which were named the vitamins. 

 Since 1912, many experiments have been per- 

 formed, using all kinds of controlled diets; 

 and these experiments have amply substanti- 

 ated the vitamin hypothesis. At the present 

 time, therefore, it is possible to define the 

 missing factors very precisely. Each vitamin 

 is a simple specific organic food compound, 

 regularly needed by an animal in very small 

 amounts for the maintenance of health, 

 growth, and even of life itself. Almost all the 

 vitamins have now been identified as essen- 

 tial coenzymes without which a number of 

 important steps in cellular metabolism can- 

 not proceed (see Table 18-3). Thus if any 



