A new study suggests that postmenopausal women taking vitamin D supplementation might not experience any of the expected benefits, such as improvements in bone mineral density, muscle mass or muscle function.
The results of the randomized clinical trial comparing the effects of low-dose vitamin D supplementation, high-dose vitamin D supplementation and placebo are published in JAMA Internal Medicine.
Vitamin D plays a key role in the regulation of calcium and phosphorus absorption and the maintenance of healthy bones and teeth. Individuals that do not get enough vitamin D are susceptible to osteoporosis due to reduced calcium absorption.
Studies have shown that nearly half of postmenopausal women sustain an osteoporotic fracture, with falling estrogen levels also a factor in osteoporosis development. The prevalence of this condition suggests that vitamin D supplementation is particularly important to this group.
Vitamin D insufficiency is also estimated to affect around 75% of postmenopausal women in the US, according to the authors of the study.
The optimum level of vitamin D for skeletal health is still up for debate, however. While the Institute of Medicine (IOM) recommend levels of 20 ng/mL or greater, others believe that vitamin D levels should be at least 30 ng/mL.
To investigate, Dr. Karen E. Hansen, of the University of Wisconsin School of Medicine and Public Health in Madison, and colleagues recruited a total of 230 postmenopausal women with vitamin D insufficiency - defined as a vitamin D level of 14-27 ng/mL.
Participants were randomly assigned into one of three groups. One group received a high dose of cholecalciferol - a form of vitamin D - that achieved and maintained vitamin D levels at 30 ng/mL and above. The other groups received low-dose cholecalciferol and placebo, respectively.
High-dose supplementation did not decrease total number of falls
For 1 year, the researchers recorded changes in calcium absorption, bone mineral density, muscle mass and sit-to-stand tests among the participants.
Although the researchers observed a 1% increase in calcium absorption in the high-dose group compared with 2% and 1.3% decreases in the low-dose and placebo groups, respectively, the high-dose was not considered to offer an overall benefit as no differences were found between the three groups in changes to bone density, muscle mass or sit-to-stand tests.
Even without improvements in these areas, the authors write that high-dose vitamin D supplementation could be justified if it reduced the numbers of falls, as these typically precede osteoporotic fractures.
However, no differences were found between the three groups in the number of falls that occurred among participants, the amount of physical activity they carried out or in functional status.
"Although we found no significant increase in bone resorption or decreases in [bone mineral density] associated with high-dose cholecalciferol, the benefits of high-dose cholecalciferol were too small to justify its routine use," the authors conclude.
The authors note that their findings are limited by the number of people that took part in the trial. Few African-Americans took part and all participants were aged 75 or younger. The findings may not, therefore, be generalizable to people inadequately represented by the participants.
In an accompanying editor's Note, Dr. Deborah Grady, deputy editor of JAMA Internal Medicine, states:
"It is possible that treatment beyond 1 year would result in better outcomes, but these data provide no support for use of higher-dose cholecalciferol replacement therapy or indeed any dose of cholecalciferol compared with placebo."