Retinol, the animal form of vitamin A, is a yellow fat-soluble, antioxidant vitamin important in vision and bone growth. It belongs to the family of chemical compounds known as retinoids. Retinol is ingested in a precursor form; animal sources (milk and eggs) contain retinyl esters, whereas plants (carrots, spinach) contain pro-vitamin A carotenoids. Hydrolysis of retinyl esters results in retinol while pro-vitamin A carotenoids can be cleaved to produce retinal. Retinal, also known as retinaldehyde, can be reversibly reduced to produce retinol or it can be irreversibly oxidized to produce retinoic acid. The best described active retinoid metabolites are 11-cis-retinal and the all-trans and 9-cis-isomers of retinoic acid.

The study was led by the Cochrane Hepato-Biliary Group at Copenhagen University Hospital in Denmark. The researchers first analyzed 68 studies involving 232,606 people and found no significant effect on mortality — neither good nor bad — linked to taking antioxidants. When they eliminated the lower-quality studies and looked only at the most trustworthy ones, they actually found a higher risk of death for people taking vitamins: 4 percent for those taking vitamin E, 7 percent for beta-carotene and 16 percent for vitamin A. The actual cause of death in most studies was unknown, however. Those findings are based on an analysis of 47 studies involving 180,938 people who were randomly assigned to get real vitamins or dummy pills. Some involved superdoses far exceeding the recommended daily amount of the compounds; others involved normal doses. The study’s senior author, Dr. Christian Gluud of Copenhagen University Hospital, said, “The main message is that prevention by beta carotene, vitamin A and vitamin E cannot be recommended. These three antioxidant supplements may increase mortality.”

Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis.
JAMA. 2007 Feb 28;297(8):842-57. Review.
PMID: 17327526 [PubMed - indexed for MEDLINE]