Cells that have not properly differentiated are more likely to undergo precancerous changes. Vitamin A, by maintaining healthy cell membranes, helps prevent invasion by disease-causing microorganisms. Vitamin A also stimulates immunity and is needed for formation of bone, protein, and growth hormone. The earliest deficiency sign is poor night vision.
Severe deficiencies causing blindness are extremely rare in Western societies.
Whether the average person would benefit from vitamin A supplementation remains unclear. Three major malformations occurred in this study, but all could have happened in the absence of vitamin A supplementation. Surprisingly, no congenital malformations happened in any of the 120 infants exposed to maternal intakes of vitamin A that exceeded 50,000 IU per day. The authors note that previous trials based the link to birth defects on very few cases, didn't measure vitamin A intake, or found no link to birth defects whatsoever. A supplementation is not known, though it may well be higher than 10,000 IU per day.