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Long-Term Experiments at or Near the Optimum Level of Intake of Vitamin A¹

Department of Chemistry, Columbia University, New York

In experiments with rats, an increase of vitamin A in the diet from 12 I.U. to 24 I.U. per gram did not measurably influence growth between the ages of 28 and 56 days. This was also true of later growth and adult size in the females. Males grew to slightly lower adult weights on the diet with 24 than on that with 12 I.U. per gram of food.

The ages of the females at the birth of their first young averaged the same for these two diets. The average weights of the young at 28 days of age were also the same.

In length of reproductive life, number of young borne, number of young reared, percentage reared, and total weight of young at 28 days, higher records were made by the females on diet 361 than by those on diet 362; that is, these values were more favorable with 12 I.U. than with 24 I.U. of vitamin A per gram of food. This was also true of length of life (appreciably for males, but only very slightly for females). Whether these small differences are more than accidental we do not know; their chief significance is in showing that nutritional response is essentially "plateaued" at this intake level.

As in all comparable work in this laboratory, the females lived longer than the corresponding males. By comparing the data of this with those of our previous papers, already noted, it is clearly shown that quadrupling an adequate (or minimum adequate) allowance of vitamin A may increase the normal length of life by 10 to 12%; the "useful" segment of the life cycle—from the attainment of maturity to the onset of senility—is increased also and in greater ratio. Thus it is shown that such an increase in intake of vitamin A, above the minimum adequate level, expedites development and defers old age in the same individuals. In these experiments the optimum concentration of vitamin A in the diet was at or somewhat above 12 I.U. per gram of air-dry food, or about 4 times the level usually accepted as minimum adequate.

Storage of vitamin A in the body of the rat increased with the intake level (from 3 I.U. up to 24 I.U. per gram of dry food) in both sexes and was greater at 700 than at 500 days of age, indicating that the ability to store this vitamin continues throughout middle age.

Manuscript received 17 December 1948.