AN EXPERIMENT IN HUMAN DIETARY NIGHT-BLINDNESS

Abstract

The dark adaptation of a human subject was measured regularly during a long control period and during a subsequent period on a diet containing only 50 to 200 U.S.P. units daily of vitamin A, but otherwise complete. A first effect of the deficient diet was noted within 24 hrs. Within 25 days the threshold of the dark adapted rods had risen about 50 times, that of the dark adapted cones about 4 times. Following temporary cure of the initial night-blindness with a single dose of vit. A, hemeralopia re-appeared with greatly increased rapidity, presumably due to depletion of vit. A reserves during the initial deficiency. The development of hemeralopia was repeatedly checked temporarily by oral administration of vit. A or carotene. Ingestion of either of these is followed by a "latent period" of about 30 mins. during which the hemeralopic threshold remains unchanged. Then the threshold descends to normal, about twice as rapidly after carotene as after vit. A administration, probably clue to greater efficiency in the absorption and transport of carotene. Following intramusc. inj. of colloidal carotene the hemeralopic threshold improved within 7 mins. and fell to normal within 20 mins. In all these changes the behavior of cones parallels that of rods. Vit. A appears to be the precursor of cone visual pigments as well as of the rhodopsin of the rods. Clinical measurements of hemeralopia omit preliminary light adaptation of the subject and accept the threshold of the completely dark adapted eye as the most satisfactory hemeralopic index.