Vitamin A from fish, beef, egg yolks and butter is known to promote good eyesight and healthy skin. But women who supplement their diets with the vitamin A compound, retinol, may do themselves harm, not good.
Research published recently in the Journal of the American Medical Association (JAMA) suggests too much retinol may increase the risk of hip fractures in postmenopausal women.
The findings indicate there may be a need to reassess the amounts of this vitamin that go into fortified foods and supplements.
Researchers at Brigham and Women's Hospital and the Harvard Medical School in Boston uncovered the problem of hip fractures after analyzing data on 72,337 women between 34 and 77 years of age, who were followed for 18 years as part of the ongoing Nurses' Health Study.
The Boston researchers assessed the relationship between high vitamin A intake from foods and supplements and the risk of hip fracture. From 1980 to 1998, they found 603 hip fractures in the women. Lead author Dr. Diane Feskanich said women who took 3,000 micrograms, or more, of vitamin A a day had a significantly increased (48 per cent) risk of hip fracture compared with women who took less than 1,250 micrograms a day.
The increased risk of fracture was mostly attributed to the retinol supplement. The same did not hold true for higher intakes of beta carotene, found in vegetables such as carrots, which is converted slowly to vitamin A in the body.
"Our findings provide further evidence that chronic intake of excessive vitamin A, particularly from retinol, may contribute to the development of osteoporotic hip fractures in women," said Feskanich.
Other studies have indicated the potential toxicity of vitamin A to bone, says Dr. Rich Adachi, a rheumatologist at St. Joseph's Healthcare in Hamilton, Ont. and an osteoporosis specialist.
Too much vitamin A is known to be toxic to the body and can even be fatal. Some of the symptoms of vitamin A toxicity are lasting headache, fatigue, muscle and joint pain, dry, cracking skin and lips, dry, irritated eyes, nausea or diarrhea, and hair loss.
In an accompanying editorial to the JAMA research paper, Dr. Margo A. Denke of the Center for Human Nutrition at the University of Texas, says: "This study serves as a reminder that vitamins are potent, essential nutrients which have effects that can precipitate harm as well as provide benefit. The optimal source is from the foods in our diets, not the dietary supplements often taken to simplify our complex world."