We received numerous questions from readers directly following our March 2002 newsletter. First, let me give you a little background. Vitamin A is fat-soluble vitamin that can be stored in the body. It is needed for vision. In fact, vitamin A deficiency is the leading cause of blindness in developing countries. However, vitamin A deficiency is extremely rare in our country.
Vitamin A is also needed for the immune system and is important in cell growth and function. The daily value (DV) for vitamin A is 1,500 mcg (or 5000 IU - see below), which is what many multivitamins contain. However, the DRI (dietary reference intake) for women is 700 mcg/day. Most people get around this level in their diet. The DV is the amount that most people in the population are advised to consume, but adult women need not get this much - the DRI is more than adequate to promote health and prevent deficiency. The upper limit for vitamin A is 3,000 mcg/day. The upper limit is the level of dietary intake over which there is evidence of harm.
In foods such as fruits and vegetables, we convert beta-carotene into vitamin A. We also get pre-formed vitamin A in the form of retinol from some fats, liver, eggs, and milk. Most of the vitamin A that is in vitamin supplements comes from retinol.
If your vitamin gives a percentage of how much of the vitamin A is from beta-carotene, you can assume that the rest if from retinol. Therefore, if the vitamin says 20% is from beta-carotene, then 80% is from retinol. In some cases, 100% is from beta-carotene, with none from retinol. I recommend you opt for single nutrient supplements instead - vitamins B6, B12, C, D, E, folate, and calcium. Right now - to the best of my knowledge - there are no multivitamin supplements out there that do not contain vitamin A; hopefully, the supplement industry will quickly respond to the need for a multivitamin that does not contain vitamin A.
In terms of figuring out how International Units (IU) are related to micrograms (mcg), the conversion is 1 microgram of retinol = 3.33 IU. The DRI for vitamin A is 700 mcg, which is equivalent to approximately 2,331 IU (700 * 3.33). Alternatively, if your vitamin says that it contain 5,000 IU, that would mean that it contains approximately 1,500 mcg (5000 / 3.33).
In the research study that I spoke about in March, levels above 2,250 mcg of vitamin A intake were associated with an increased risk of hip fractures. Since we can convert plenty of beta-carotene into vitamin A in the body and we also get appreciable amounts of retinol in our diets, I recommend that you do not get any vitamin A in the form of retinol in a supplement unless specifically recommended by your doctor.