Summary The minimum light threshold after complete dark adaptation was determined in 53 infants ranging in age from 1½ to 13 months. These infants were divided into four groups: Group 1 consisted of 26 infants receiving the average infant's diet; Group 2 consisted of 9 infants receiving a diet supplemented by large quantities of vitamin A (17,000 units) in the form of halibut-liver oil; Group 3 consisted of 4 infants, receiving a diet containing approximately one-fourth the vitamin A content of the average diet; and Group 4 consisted of 14 infants receiving about one-twelfth the vitamin A content of the average diet. These diets were given for periods varying from 3 to 10 months. The results of the dark adaptation tests were within normal limits in all four groups of infants. Thus, infants receiving only 135 to 200 vitamin A units daily had as low a threshold for vision in the dark as infants receiving an average diet supplemented by 17,000 vitamin A units. It should also be added that the infants in the low vitamin A group gained weight just as well as, and were no more susceptible to infections than, those in the high vitamin A group. These observations indicate that 135 to 200 units of vitamin A (approximately 25 units per kilogram of body weight) covered the minimum vitamin A requirements in these infants. Since the average diet contains approximately 12 times as many units of vitamin A as were contained in our low vitamin A diet, it is apparent that there is a large margin of safety in the infant's diet with respect to its vitamin A content. It would, therefore, seem unnecessary to supplement the average diet of infants with special preparations which contain vitamin A. Three infants, two of whom had eczema, received as a substitutefor milk a proprietary preparation containing soy bean, starch, and olive oil. This product is almost devoid of vitamin A. In 3 to 4 weeks these infants manifested poor dark adaptation. The addition of 120 units of vitamin A in the form of cod-liver oil resulted in normal dark adaptation in two of these infants. Computed on the basis of kilogram body weight this dosage (plus the vitamin A content of the diet) was equivalent to 18 units of vitamin A for one infant and 20 units of vitamin A for the other infant. When a single large dose of vitamin A, 30,000 to 50,000 units, was given to two infants who had developed high threshold values on this diet, a prompt response occurred; dark adaptation returned to normal in 40 minutes in one infant and in 65 minutes in the other infant. During the period that the three infants on the soy bean diet hadpoor dark adaptation, they gained well in height and in weight and were no more susceptible to infections than infants receiving large quantities of vitamin A. In view of the fact that these infants appeared normal in every way, we must conclude that they had a latent or subclinical form of vitamin A deficiency which became evident only as a result of the dark adaptation tests.
