VITAMIN AND GROWTH FACTOR RESEARCH 



Examples of this nature could be multiplied indefinitely, 

 and a more complete discussion has been presented elsewhere 

 (66,67). They have their basis in the fact that the vitamins are 

 required only to permit the functioning of enzymes that in turn 

 manufacture products essential for growth. Frequently, if 

 these products are supplied preformed, or tlie necessity for the 

 enzymic functioning is otherwise bypassed, need for the enzyme 

 (and hence for the vitamin) is reduced, and with some simple 

 organisms can be eliminated completely. One strongly suspects 

 that closer investigation would show a similar variability 

 (though less marked because of the greater organization and 

 range of metabolic reactions present) in higher animals. The 

 well-authenticated variability in the thiamin requirement of 

 animals with the proportion of the caloric intake supplied as 

 fat, and in the vitamin B12 requirement with the choline intake, 

 are straws indicating the direction research may take in these 

 matters. 



As the number of new nutrients available for study decreases, 

 perhaps animal nutritionists will turn more toward the study of 

 such interrelationships between nutrients and their significance 

 for practical nutrition. 



Some Future Areas for Nutrition Research 



The last few years have seen a decrease in the rate of dis- 

 covery of new vitamins and growth factors. Some experimental 

 animals can now be grown on essentially synthetic diets, and it 

 is becoming more and more difficult to detect entirely new 

 bacterial growth factors. Does this mean we are approaching 

 the time when no more dietary factors remain to be discovered? 

 Although this time must eventually arrive, it appears unlikely 

 that it is yet here. Some of the readily available (though experi- 

 mentally more difficult) experimental animals have not been 

 grown on defined diets as yet, and there are indications of un- 

 identified dietary requirements for some of these (e.g., monkeys 

 (78a)). Similarly, little is known of the nutritional requirements 



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