Vitamin A supplements took a big hit in 2002 when the famous Nurses’ Health Study from Harvard linked high intakes to hip fractures in postmenopausal women. Specifically, in those consuming at least 6,600 IU of “preformed” vitamin A a day from food and supplements, the risk of hip fracture nearly doubled. 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 multivitamin makers reduced the preformed A in their formulas.
However, the case is not closed. Since 2002, research on the effect of vitamin A on bone density and fracture risk has produced unclear results. And some supplement companies are once again promoting formulas with high doses of vitamin A, claiming that the concerns were unfounded.
Beta carotene: better than A?
Vitamin A is essential for health, especially for growth, reproduction, vision, immune function and, yes, bone health. It is actually a family of related compounds called retinoids. You can get the vitamin two basic ways: by consuming preformed vitamin A (mostly retinol), which comes only from animal products, dietary supplements and fortified foods; or by consuming beta carotene (and a few other carotenoids) from fruits and vegetables. The beta carotene is converted to vitamin A as needed in our bodies. Supplements, fortified foods and some prescription drugs contain forms of retinol such as retinyl acetate, retinyl palmitate, tretinoin and retinoic acid.
The potential bone problems are posed only by preformed vitamin A, not beta carotene. High doses of vitamin A can also cause birth defects and, over the long term, serious damage to the liver, where the vitamin is stored. You can’t get toxic levels of vitamin A by consuming beta carotene.
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.
Vitamin A: inconsistent studies?
Some studies have supported the findings of the Nurses’ Health Study, but others have found no decrease in bone density or no increase in the risk of osteoporotic fractures.
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 published 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 one to 16 years found no increase in fracture risk (it had earlier found no reduction in cancer).
However, a Spanish study published in the Archives of Osteoporosis this year 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.
Not surprisingly, several review papers concluded that research has been inconsistent and that more studies are needed.
The inconsistencies may be due in part to the different methodologies of the studies. Some have looked at blood levels of vitamin A (which can be measured in different ways), others at estimated dietary intakes (which can be notoriously inaccurate). These were then correlated with bone mineral density (often at differing sites), various markers of bone metabolism, or fracture rate.
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 can stimulate cells involved with bone breakdown (osteoclasts) and suppress cells involved with bone formation (osteoblasts). In addition, lab research suggests that high intakes can interfere with vitamin D, which is essential for strong bones. That may be why some studies have found that vitamin A harms bones only in people with low blood levels of vitamin D.
What you should do
- Don’t consume more than 5,000 IU of preformed vitamin A a day on a regular basis.
- If you take a multivitamin, look for one with no more than 3,000 IU of preformed A. If the label says “Vitamin A, 5,000 IU (50% as beta carotene),” that means the preformed A content is only 2,500 IU.
- Check other supplement labels for vitamin A. The cold “remedy” Airborne, for instance, has 2,000 IU of preformed A.
- If you eat fortified foods, such as some breakfast cereals, check the labels for vitamin A content. Levels of preformed A have been reduced in recent years (often replaced by beta carotene), but they can add up. Milk is usually fortified with 500 IU (10 percent of the Daily Value) of vitamin A per cup.
- If you eat liver, do so sparingly and occasionally. It is a rich source of vitamin A—beef liver has about 24,000 IU in 3 ounces (cooked); chicken liver, 12,000 IU; and duck liver, more than 40,000 IU.
- Eat carotenoid-rich produce, such as leafy greens, broccoli, carrots and other colorful fruits and vegetables. There is no evidence that beta carotene or other carotenoids weaken bones. And these foods contain many vitamins, minerals, and other substances that help keep bones strong.
- Get enough vitamin D: We advise 800 to 1,000 IU a day—more if you have osteoporosis or your blood level is low.
- Don’t take cod liver oil, which is very high in vitamin A, unless the label says its vitamin A content has been reduced.
Published November 04, 2013