High blood pressure is linked to bone loss.
Researchers tracked 3,676 white women over age 65 at four clinics in or near Baltimore, Pittsburgh, Minneapolis, and Portland, Ore. Precise measurements of the thigh bone revealed that women with an initial systolic blood pressure reading of 148 or more had an average bone-density loss equal to roughly 0.6 percent of their bone mass per year. In contrast, women with a systolic reading of less than 124 suffered bone loss at about half that rate, says study coauthor Joseph M. Zmuda, an epidemiologist at the University of Pittsburgh. Declines in bone density, which can lead to osteoporosis, are due to calcium loss.
The researchers collected the data between 1988 and 1994, measuring bone density for each woman on two occasions 3 1/2 years apart. They report their findings in the Sept. 18 LANCET.
"We all lose bone as we age," Zmuda says. "Women with high blood pressure experience it faster."
The mechanism behind this loss remains unclear, says study coauthor Francesco P. Cappuccio, an epidemiologist and cardiovascular physician at St. George's Hospital Medical School in London. About 97 percent of the body's calcium is stored in bones. Small amounts of calcium also circulate in the blood, where they play a vital metabolic role. The body keeps its calcium in balance by excreting the excess in urine.
Earlier studies in animals and people indicated that as blood pressure rises, so does calcium in the urine. Although women taking drugs to lower blood pressure were excluded from the study at the start, 623 women began taking such medication during the 3 1/2 years that they were followed. The drugs did not change their bone-loss rate.
High blood pressure is probably not the cause of bone loss but a marker of another problem, Cappuccio says. Excluding these women from the final analysis didn't change the overall findings.
The 15 percent of women in the current study on hormone-replacement therapy had about half as much bone loss, on average, as others did.
The study shows "an interesting association" between high blood pressure and bone loss, says Jeri W. Nieves, an epidemiologist at Helen Hayes Hospital in West Haverstraw, N.Y. Further research that measures salt intake, calcium lost through the urine, or even the level of exercise might help clarify the mechanism at work, she says.
The researchers didn't measure salt intake among the women but suggest that it might play a part. Salt gradually siphons calcium out of the body, and high-salt diets often accompany high blood pressure. "It's pretty unlikely that this massive [bone loss] is due to constant high salt intake," Cappuccio says. "But it may be a contributing factor."
Meanwhile, Cappuccio says, lowering high blood pressure remains a good idea to lessen other health risks.