NUTRITIONAL REQUIREMENTS — ELVEHJEM 295 



have sliown that 70 mg. per day will maintain the vitamin C content 

 of the blood at high levels. The important question in this case is 

 whether we should supply sufficient vitamin C to produce complete 

 saturation of the blood stream, or whether optimum health can be 

 maintained at lower levels of saturation. 



At the present time there appears to be greater interest in the vita- 

 min Bi requirement than any of the other vitamins. This is per- 

 haps due to the fact that it is practically impossible to study the 

 vitamin Bi content of the blood and we must resort largely to balance 

 studies. Many of the balance studies have been conducted for short 

 periods of time, and such studies may give results considerably higher 

 than those obtained over a long period of time. Another factor which 

 perhaps enters into this problem is the fact that the vitamin Bi re- 

 quirement is definitely reduced by a high fat diet. Recent studies 

 at the Banting Institute have suggested that high-fat diets may 

 eliminate all symptoms of Bi deficiency except those changes taking 

 place in the heart. If this is true, we should perhaps recommend 

 the high level of Bi intake regardless of what the fat content of the 

 diet is. A level of 1.5 to 2 mg. per day should supply an adequate 

 intake for most individuals. In studies conducted by Williams, Ma- 

 son, Wilder and Smith (7) it was found that about 1 mg. prevented 

 symptoms but higher levels gave a high degree of well-being. 



The very recent work on human beings (8) seems to establish rather 

 definitely that the riboflavin content is 3 mg. per day. These studies 

 were conducted on human beings receiving average diets, but again, 

 changing the fat and carbohydrate ratio may alter the riboflavin re- 

 quirement. We have recently shown in our laboratory that when the 

 fat content of the diet is increased we find a considerable increase in 

 the riboflavin requirement. This we interpret in a preliminary way 

 as being caused by a change in the bacterial flora. That is, mider nor- 

 mal conditions there may be a certain degree of riboflavin synthesis 

 in the intestinal tract and we are merely measuring the riboflavin re- 

 quired above this synthesis. If there are sudden changes in the in- 

 testinal flora', the entire riboflavin requirement may have to be supplied 

 through the diet. 



It would appear that the nicotinic acid requirement might be es- 

 tablished very easily since nicotinic acid is such a simple chemical 

 compound. Tremendous difficulties have been encountered in at- 

 tempting to determine the nicotinic acid content of foods or to meas- 

 ure the nicotinic acid excreted in the urine. Two recent develop- 

 ments, I believe, will be of considerable value in establishing the 

 nicotinic acid requirement. One is that we are now able to feed 

 dogs on a highly synthetic diet containing no nicotinic acid and from 

 these studies it would appear that the daily requirement for main- 

 tenance is about 0.25 mg. per kilo and the requirement for growth 



