Vitamin A and your bones

The message traveled fast: A high intake of vitamin A increases the risk of hip fractures in postmenopausal women.

Five years ago the Nurses’ Health Study, a famous Harvard study of 70,000 female nurses that has yielded a steady stream of information over the years, found that a daily consumption of 6,600 IU or more of vitamin A from food and supplements (not much more than the 5,000 IU found in most multivitamins) may actually weaken women’s bones. In response, some vitamin manufacturers lowered the amount of A in their formulas.

Vitamin A, while essential for good health, is toxic at high doses. It can cause liver damage; pregnant women are advised to avoid high doses because of the risk of birth defects. “Preformed” vitamin A comes only from animal products, fortified foods, and supplements. It is measured in International Units (IU) or sometimes in micrograms. The Recommended Dietary Allowance (RDA) is 2,333 IU daily for women, 3,000 IU for men.

Confusingly, the labels on foods use a different number for the “Daily Value” for A - 5,000 IU. The Institute of Medicine has set 10,000 IU as the safe upper limit, especially for pregnant women. Not all vitamin A comes preformed. Beta carotene (and some other carotenoids) found in plant foods, especially dark green and highly colored vegetables and fruits is converted to vitamin A as needed in our bodies. You can’t get dangerous levels of vitamin A by consuming beta carotene.

New theories, new research
Some research has supported the findings of the Nurses’ Health Study. Osteoporosis is under intensive study particularly in northern Europe, which has the highest rate of the disease in the world, as well as the highest intakes of vitamin A. An earlier Swedish study found that 5,000 IU of preformed A daily increased the risk of hip fractures in women. In a more recent Swedish study, men with the highest blood levels of vitamin A also had more fractures.

One explanation may be that vitamin A keeps the body from using vitamin D, which is essential for strong bones. Still, the case is not closed. A CDC study in 2001 found no link between blood levels of vitamin A and bone mineral density. A study in 2004 from the University of Minnesota found no link between hip fractures and vitamin A from foods, and only a slight increase in hip fractures (but not other fractures) from supplements.

In a 2005 study British researchers questioned the quality of the evidence showing that A harms bones. One problem: Vitamin A intake may not have been carefully measured in some studies.

What you should do
If you are a postmenopausal woman or an older man (men also get osteoporosis), there’s no reason to panic. But it’s a good idea to avoid consuming a lot of preformed vitamin A. Keep in mind:
• Liver 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. If you eat liver, do so sparingly and occasionally. It is also very high in cholesterol.
• There is no evidence that beta carotene weakens bones. Keep your intake of carotene-rich fruits and vegetables high. The body carefully regulates the conversion of beta carotene into vitamin A.
• If you take a multivitamin, check the label. In the Nurses’ Health Study, multivitamins were the largest source of A. Find a multi with no more than 3,000 IU of preformed vitamin A. If the label says “Vitamin A, 5,000 IU (50% as beta carotene),” that is okay, since that means the preformed A content is only 2,500 IU.
• If you eat highly fortified foods, such as some breakfast cereals, check the labels for vitamin A content. Milk is fortified with 300 to 500 IU of vitamin A per cup-an amount you need not worry about.
• Don’t take cod liver oil, which is very high in vitamin A. Fish oil supplements (and fish) are not.

UC Berkeley Wellness Letter, December 2007

Your Health and Mine