Vitamin A is involved in the immune system, vision, reproduction, and cellular communication. Perhaps its most important role is in helping with our vision - it is an essential component of rhodopsin, a protein that absorbs light in our eyes.
There are two forms of vitamin A that can be obtained from our diet.
1. Preformed vitamin A (retinol) found in food from animal sources, including dairy products, fish and meat. Levels are particularly high in animal livers, hence the popularity of cod liver oil supplements.
2. Provitamin A (carotenoids), the most important being beta-carotene, are plant pigments which the body can convert into active vitamin A compounds. Food sources of these include leafy green vegetables, orange and yellow vegetables, tomato products and fruit.
The Recommended Dietary Allowance (RDA) is around 700mcg for adult females, slightly more for males at around 900mcg and slightly less for children at around 400mcg.
Vitamin A deficiency is rare in developed countries but common in developing countries, and health consequences from deficiency most often become apparent during periods of high nutritional demand ie. during infancy, childhood, pregnancy, and lactation. The main health consequences are a greater severity of infections when they occur and an increased risk of blindness.
Everyone knows that carrots are important for vision as they contain a lot of vitamin A in the form of beta-carotene. Vitamin A supplements can reverse poor vision in people with a vitamin A deficiency. The widespread belief that carrots are good for our vision and particularly our night vision can be traced back to British propaganda from World War II. RAF night fighter John Cunningham, nicknamed “Cat’s Eyes”, racked up 20 kills, 19 of which were at night and the British put out information that it was due to all the carrots he ate, to mislead the Germans.
Vitamin A has been shown to be effective at reducing the risk of developing advanced age-related macular degeneration (AMD) when it is taken as part of a cocktail of supplements called AREDS which contains vitamin C, vitamin E and beta-carotene.
In a large study, the results were that the combination of the AREDS formula with zinc and copper reduced the risk of developing advanced AMD by 28%. Taking just zinc and copper reduced it by 25% and AREDS alone reduced the risk by 20%. Quite high doses were used in the study (vitamin C 500mg; vitamin E 400IU; beta-carotene 15mg; zinc oxide 80mg; and cupric oxide 2mg), higher than are typically found in a standard multivitamin product or even in a good diet.
Because vitamin A is fat soluble, the body stores excess amounts, mostly in the liver, and these levels can accumulate and so there is potential for harm from taking too much. This is called hypervitaminosis A and chronic intakes of excess vitamin A can lead to increased intracranial pressure, dizziness, nausea, headaches, skin irritation, pain in the joints and bones, coma and even death. This is rare of course and is usually due to consuming too much of the preformed vitamin A, (not from the plant based beta-carotene form). With chronic excess intake of vitamin A, the resulting liver damage may not be fully reversible.
The symptoms of hypervitaminosis A can also occur acutely if there is a sudden very large intake of vitamin A from animal sources. An example of this was published as a case report in a 1942 journal article. A group of Arctic explorers ate polar bear liver - some of the group ate small amounts without any problems, but others who ate more suffered from stupor and headaches, and then their skin peeled off. The indigenous people of the Arctic know not to eat polar bear liver for this reason.