Retinol, or vitamin A was first identified in 1907 by comparing rats fed protein and lard or olive oil for fat with rats fed a diet that added egg yolk or butterfat. The rats who ate the foods with a vitamin A deficient diet failed to grow, but recovered with the supplemental foods. Only animal fats contain vitamin A. Good sources are cod liver oil, egg yolks, butter, raw whole milk, and liver. Animals must have carotene or vitamin A sources in their diets in order to produce vitamin A and pass it on to humans. There are no plant sources of vitamin A. Betacarotene found in vegetables and fruits can be converted to vitamin A by the body in a ratio of 12:1. That is it takes 12 units of beta-carotene to produce one unit of vitamin A. Infants and people with diabetes or poor thyroid function cannot make the conversion at all. Children convert betacarotene to vitamin A very poorly. Therefore animal fat sources of vitamin A are essential for most of the population.
Vitamin A is needed for proper mucous membrane function. It is essential for the growth and repair of body tissues, and for efficient digestion of protein. Vitamin A promotes good eyesight, strong bones and teeth, and a vital immune system. White blood cells, T-lymphocytes, and every cell in the important mucosal barriers of the respiratory, digestive, and urinary tracts require vitamin A.
A high fat diet will help ensure adequate vitamin A intake. Whole milk products, butter, and free range eggs will help maintain necessary levels of this important nutrient. For those who may not be getting enough vitamin A, a supplement is essential.
The recommended daily amount (RDA) of vitamin A is 3,000 IU per day for adults (reduced from 5,000 IU) and 1,000-2,000 IU for children, depending on their age (1,000 at one year of age, 2,000 by age nine). Primitive diets probably maintained 10 times that amount. One egg contains 300 IU, one cup of whole milk or whole milk yogurt contains about 225-250 IU of vitamin A. One tablespoon of butter contains 350 IU of A. The amount of vitamin A may vary by the season and the feed of the animals.
People eating a vegan diet are at a significant risk of vitamin A deficiency. It would take six cups of raw carrots or 20 cups of broccoli to obtain the recommended daily requirement of vitamin A per day.
Most everyone would benefit from a vitamin A supplement derived from fish oil. One tablespoon of cod liver oil contains at least 3,000 IU of A. Proper dosage is one teaspoon per 50 pounds of body weight. For adults with hypothyroidism or immune system problems (allergies, recurrent infections, autoimmune disease) a capsule supplement of 20,000 IU of vitamin A from fish oil may be appropriate. During an acute illness an adult could take twice that amount.
The toxicity of vitamin A during pregnancy or at any other time applies primarily to synthetic rather than natural forms of vitamin A (fish oil). Vitamin D in fish oil protects the body from toxicity. A study of people taking 300,000 IU of vitamin A per day for over a year revealed no adverse effects. However, the toxicity of vitamin D is very real, and anyone who takes a supplement containing vitamin D should have vitamin D levels checked with a blood test for 25-hydroxyvitamin D. Note that the normal values are 45-55 ng/ml (115-140 nmol/l). Laboratory reference ranges are often too low.
There has been an association between vitamin A intake of 5,000 IU per day and an increased risk of osteoporosis, It is assumed that this increased risk is due to interference with the ability of vitamin D to maintain calcium balance. Taking vitamin D and other cofactors should mitigate this effect. Monitoring by a knowledgeable health care provider is important because of the complex interactions of nutrients including A, C, D, calcium, magnesium, and other minerals on bone health.
For a more thorough look at vitamin A see: