Science Briefs

Too Much Vitamin A Poses Hip Fracture Risk

Swedish women who eat diets high in vitamin A have reduced bone density and an increased risk of hip fracture, according to a study conducted by Hakan Melhus, M.D., of University Hospital in Uppsala. Hip fracture rates are so high among Swedes that even the men break their hips more often than most European women do.

Swedes also have six times the vitamin A intake of Southern Europeans—a difference due mainly to their taste for dairy products, fortified margarine and cod liver oil. To determine if a link exists between hip fracture and vitamin A intake, Melhus studied 175 women ages 28 to 74 from two Swedish counties. After measuring the women's bone density and comparing it to their dietary vitamin A intake, he concluded that women getting less than 0.5 mg/day of vitamin A had 10 to 14 percent higher bone density than women who took in more than 1.5 mg/day. The average intake was 0.8 mg/day.

Next, 66,651 women ages 40 to 74 in the two counties completed a food questionnaire. During the next five years, 247 of the women had hip fractures. For each of these cases, Melhus designated two controls from the group and compared their vitamin A intake. Women consuming more than 1.5 mg/day of vitamin A had twice the risk of hip fracture.

Excess vitamin A inhibits bone-forming cells (osteoblasts), stimulates bone-destroying cells (osteoclasts) and ultimately causes bone loss. Although carotenes are converted to vitamin A in the body, they do not result in excess vitamin A, even when eaten in large amounts. Nor do they increase the risk of fracture or osteoporosis. Researchers think the conversion is determined by the body's vitamin status—the less it needs, the less it converts.

American women who drink five cups of milk a day to meet the RDA for calcium and ward off osteoporosis are also getting approximately 0.5 mg of vitamin A in their fortified milk. That, coupled with a vitamin A supplement, may be delivering in excess of the 1-mg RDA for vitamin A and putting some women at greater risk for hip fracture.

—Annals of Internal Medicine 1998 Nov;129:770-8