What vitamin A is: A fat-soluble vitamin, vitamin A is essential for the eyes, the skin, immune function, reproduction and bone growth. Preformed vitamin A, also called retinol, is found in animal products such as meats, fish, and dairy products. The body can also convert certain carotenoids (notably beta carotene) from plant foods into vitamin A, as needed.
Vitamin A is actually a family of related compounds called retinoids. You can get the vitamin two basic ways: by consuming preformed vitamin A, which comes only from animal products, supplements or fortified foods; or by consuming beta carotene (and a few other carotenoids) from fruits and vegetables, which is converted to vitamin A as needed in our bodies. Dietary supplements, fortified foods and some prescription drugs contain forms of retinol such as retinyl acetate, retinyl palmitate, tretinoin or retinoic acid. Vitamin A deficiency is rare in the U.S., in part because milk and some other foods are fortified with it, though it is common in many developing countries.
Claims, purported benefits: Supplements (as preformed vitamin A or carotenoids) are important for vision and can maintain immunity.
Vitamin A: the latest research
In 2002 the Nurses’ Health Study from Harvard linked high intakes of vitamin A to hip fractures in postmenopausal women. Specifically, those consuming at least 6,600 IU of preformed vitamin A a day from food and supplements had nearly a doubled risk of hip fracture. For many women, the largest source of vitamin A was multivitamins, which typically supplied 5,000 IU back then. In response to the study, most makers of multivitamins reduced the amount of preformed A in their formulas. The potential bone problems are posed only by preformed vitamin A, not beta carotene. You can’t get toxic levels of vitamin A by consuming beta carotene.
During the past decade, some studies have supported the findings of the Nurses’ Health Study, but others have found no decrease in bone density or no increase in risk of osteoporotic fracture.
An analysis of data from the Women’s Health Initiative Observational Study in 2009, for instance, found that high vitamin A intake was not linked with fracture risk, except modestly in women with low vitamin D intake. And in 2012 an Australian study in Osteoporosis International that tested very high doses of vitamin A (25,000 IU) as a possible cancer preventive in asbestos workers for anywhere from 1 to 16 years found no increase in fracture risk (it had earlier found no reduction in cancer). However, a Spanish study published in Archives of Osteoporosis in 2013 reported that the 36 percent of postmenopausal women with the highest blood levels of vitamin A (retinol) were up to eight times more likely to have osteoporosis than those with low levels. The risk was greatest in women who had low blood levels of vitamin D (less than 20 ng/mL) as well as high retinol levels.
Several review papers have concluded that research has been inconsistent and that more studies are needed.
There are plausible explanations for how excessive vitamin A could harm bones. While you need some vitamin A to form and maintain bones, high intakes of vitamin A can stimulate cells that are involved with bone breakdown (osteoclasts) and suppress cells involved with bone formation (osteoblasts). In addition, lab research suggests that high intakes can interfere with the action of vitamin D, which is essential for strong bones. That may explain why some studies have found that vitamin A harms bones only in people with low blood levels of vitamin D.