184 BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



contained in the bones and teeth. About half the remainder is combined 

 with the organic constituents of the muscles, while the rest is distributed 

 throughout the blood and other tissues of the body. Mention was made 

 above of the buffer action of the alkali phosphates contained in the blood, 

 and it will be recalled that phosphorus is also a constituent of various 

 organic compounds previously studied, such as lecithins, certain proteins, 

 and nucleic acids, as well as a whole series of substances involved in 

 intermediary carbohydrate and fat metabolism and in muscle contrac- 

 tion (Chaps. 10 and 16). AVithout a supply of phosphorus none of these 

 essential materials could be formed by the living cell, so it is easy to 

 understand why this element is vital to normal health and development. 



The amounts of calcium and phosphorus needed daily by human beings 

 depend on many factors, one of which is the Ca : P ratio in the food 

 eaten. When either element is consumed in a large excess, the excretion 

 of the other is increased, so it is desirable that this ratio be about 0.7-1.0. 

 In other words, the food eaten should contain about equal quantities of 

 calcium and phosphorus, or slightly more of the phosphorus. Further- 

 more, unless enough vitamin D is furnished (see p. 211) absorption of 

 calcium through the intestinal wall is greatly reduced. 



A third factor affecting calcium requirements is the presence in the 

 food eaten of oxalic acid, or soluble salts of oxalic acid, which produce 

 insoluble calcium oxalate. This compound is not utilized as a source 

 of calcium in the body. The concentration of soluble oxalates in some 

 plants reaches toxic levels (see p. 161). 



For these and other reasons there is a tremendous variation among 

 individuals in their ability to make use of dietary calcium, some ab- 

 sorbing as little as 5 per cent of the amount eaten. There is evidence, 

 however, that the efficiency of utilization increases when the calcium 

 content of the food is low and the needs of the body are acute. 



The amounts of various dietary essentials that should be supplied 

 daily by a good diet have been carefully studied by the Food and 

 Nutrition Board of the National Research Council. Their recommended 

 daily allowances, which are designed to provide an excess over the bare 

 minimum requirements for life, were announced in 1941 and revised in 

 1948. For calcium the recommended amounts are: adults 1.0 g., preg- 

 nancy (latter half) 1.5 g., lactation 2.0 g., children up to 10 years 1.0 g., 

 adolescents 1.0 to 1.4 g. The phosphorus intake should be at least 

 equal to that of calcium for children, adolescents, and women during 

 pregnancy and lactation, and for other adults about 1.5 times the calcium 

 intake. 



Calcium is the one essential mineral element which is most likely to 

 be supplied in inadequate amounts by the average American diet. The 

 reasons for this situation are the facts that two of the principal types 

 of foodstuffs, meats and cereals, are notably deficient in this element and 



