NUTRITIONAL REQUIREMENTS — ELVEHJEM 293 



Experiments with dogs indicate that when the Ca : P ratio is 2 : 1, 

 it is impossible to get normal calcification even with tremendous 

 doses of vitamin D. Sherman and coworkers (3) have recently re- 

 ported some improvement in the nutritional well-being of rats by 

 increasing the calcium intake from 0.2 percent to 0.35 percent or 

 even to 0.8 percent of the diet. The phosphorus requirement is 

 generally assumed to be a little higher than the calcium figures. 

 This may appear contradictory in the light of a calcium phosphorus 

 ratio of 1.2 to 1 ; however, the higher value may be advantageous 

 because some of the organic phosphorus may not be completely avail- 

 able. The phosphorus in phytic acid is now known (4) to be avail- 

 able, but it is less efficient for bone production than other sources 

 of phosphorus. 



The daily iron requirement is rather well established although 

 the exact figure has varied between 12 and 15 mg. per day. First 

 we must recognize that the iron is of no value in the body unless it 

 is accompanied by copper which functions in the conversion of iron 

 into hemoglobin. In addition the iron must be in an available form. 

 Extensive studies have been made on the available iron content of 

 foods, and the values obtained by chemical methods check fairly well 

 with those obtained through the feeding experiments with rats. 

 However, we still do not know whether the availability as measured 

 in rats is similar to that which we may find in the human being. 

 Much has been said about the relative value of ferric and ferrous 

 iron. This problem has recently been thoroughly studied by Tomp- 

 sett (5). Ferrous iron appears to be the only form to be absorbed 

 from the alimentary tract. The ferric iron consumed is reduced in 

 the stomach and this reduction is brought about by substances which 

 are common constituents of the diet. The diet may also contain 

 substances which tend to oxidize the ferrous iron to the ferric state. 

 The phosphatides of egg yolk appear to be one group of substances 

 which inhibit the absorption of iron through the autoxidation of the 

 ferrous iron. Tompsett states that it is difficult to give a definite 

 value for iron because the degree of iron absorption is dependent on 

 so many factors. The value of 12-15 mg. per day is probably liberal 

 enough to compensate for the iron present in the food in forms that 

 cannot be utilized. The daily copper requirement is probably about 

 2 mg. per day and practically all forms of copper show equal 

 availability. 



No requirements have been set up for the remaining so-called 

 trace elements and perhaps rightly so, since we have no specific 

 values available. But we must recognize that elements such as 

 manganese, zinc, and cobalt are important in the diet, and perhaps 

 values for these additional minerals may soon be available. 



