Elderly women should take in more vitamin D than previously recommended during the winter months, according to a new study.
Osteoporosis is one of the chief reasons why the elderly often suffer broken bones from relatively minor injuries. Postmenopausal women in particular experience a relatively rapid loss in bone mass due to a reduced concentration of estrogen, which is responsible for strong bone growth during youth.
Maintaining bone mass requires physical exercise and vitamin D, which is mainly produced in the skin with the help of UVB radiation. This is why, especially in the wintertime, many doctors prescribe vitamin D supplements to elderly women to maintain their bone mass.
“Experts are divided as to the ideal daily dose of vitamin D for maintaining bone mass,” says study leader Michael B. Zimmermann, professor for human nutrition at ETH Zurich. One camp believes that sunlight alone is enough to provide the body with sufficient vitamin D, and therefore only small quantities of supplements are necessary. The other asserts that it takes high doses of vitamin D supplements to prevent or slow down bone degeneration in elderly women.
A group of researchers took a closer look at this question. The team of scientists wanted to find out how much vitamin D there needs to be in the bloodstream to maintain bone strength.
The results of their study have come out clearly in favor of higher supplement doses. Particularly in the wintertime, much higher dosages of vitamin D are necessary than previously assumed in order to maintain bone health. In the study, the researchers come to the conclusion that a vitamin D concentration of 40 micrograms per liter of serum in the bloodstream is ideal for slowing or preventing bone degeneration in postmenopausal women.
Calcium in the skeleton
During the study, test participants were first given a single dose of calcium-41. This disperses like normal calcium throughout the body and into the bones and, given enough time, will mark the entire skeletal system evenly.
“It’s after about six months that things get interesting, because from that point on we can trace the absorption and depletion of calcium in the bones,” says Zimmerman. However, highly sensitive measuring equipment is required to detect the minute quantities of calcium-41 present.
Researchers took urine samples from test participants at regular intervals and then used highly sensitive accelerator mass spectrometry equipment to measure the quantities of calcium-41 and calcium-40 and determine the ratio between them. To put it simply, a very low ratio means more calcium is being added to the bones than released; a high one means the bones are releasing more calcium than they are taking up.
Over a period of nine months beginning half a year after the calcium-41 marking of their bones, the women were given daily vitamin D supplements. The first dose was administered in early spring, when the vitamin D concentration in the blood is expected to be at its lowest, and the dosage was increased in step increments every three months. In addition, the scientists modeled the paths the calcium took through the various segments of the body in order to calculate an ideal vitamin D quantity.
From winter to spring
At the beginning of the experiment, participants showed a concentration of 16 micrograms per liter of serum, which is to say they already had a deficiency. By the end of study, the average vitamin D concentration in their serum had risen to over 46 micrograms per liter thanks to the vitamin D supplementation—and to the sunshine, which increased over the course of the study to promote the body’s natural vitamin D production.
At the same time, the researchers noted that the ratio of calcium-41 to calcium-40 decreased abruptly following the start of the supplementation regimen—a sure sign that bone degeneration had been reduced.
For healthy postmenopausal women with sufficient calcium absorption and physical activity, a serum concentration of around 40 micrograms of vitamin D per liter of serum has the optimum effect on bone calcium absorption. “That the figure was so high was surprising,” says Zimmermann, “as previously I had tended to believe that a low dose of vitamin D was sufficient.”
The findings appear in the Journal of Nutrition.
Source: ETH Zurich
