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Some Observations of Dark Adaptation in Man and Their Bearing on the Problem of Human Requirement for Vitamin A¹

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Department of Home Economics, Rhode Island State College, Kingston, Rhede Island

Four young adults were fed on a diet deficient in vitamin A but adequate in all other known nutrients. Their dark adaptation was measured by means of the Rhodometer, previously described. One subject showed slight initial rise in visual threshold, but after the eighty-fifth day, showed a gradual improvement in dark adaptation up to the one hundred and eightieth day when the study was discontinued. The others showed a rise in visual threshold after 65 to 100 days. Certain aspects of the rate of change in visual threshold have been brought out. When the rise amounted to about 0.5-log unit, one subject was fed 2,000 I. U. of vitamin A daily for 27 days. Since the threshold continued to rise, the dose was increased to 4,000 I. U. This resulted in maintenance of a constant 30-minute threshold for about 45 days. Two subjects were treated in a similar manner except that the visual threshold was allowed to rise 1.0-log unit before vitamin A was fed. In both instances, 4,000 I. U. were inadequate for maintenance, but 5,000 I. U. served to maintain constant but subnormal dark adaptation for about 1 month. Individual differences in response to large doses of vitamin A and in return to normal visual sensitivity were observed.

In three different subjects, 74 I. U. vitamin A per kilogram body weight (5,000 I. U. per day) sufficed only to maintain a threshold 1.5-log units above normal; 84 I. U. per kilogram (4,000 per day) maintained a threshold 0.5-log units above normal; and 81 I. U. per kilogram (5,000 I. U. per day) maintained an approximately normal threshold.

² Now of the Bureau of Human Nutrition and Home Economics, United States Department of Agriculture.

³ Now of the Subsistence Branch, Office of The Quartermaster General, War Department.

Manuscript received 22 November 1943.