Vitamin D helps maintain normal blood levels of calcium and phosphorous, thereby helping to improve bone and muscle health. Recommended intake is 200 international units per day (IU/d) for young adults, 400 IU/d for those aged 51 to 70 years, and 600 IU/d for those over age 70 - although some scientists have argued in recent times that it should, in fact, be higher.
Dietary sources of vitamin D include milk, egg yolk, oily fish and liver, and it is also synthesized by the body on exposure to sunlight.
But because dark-skinned women synthesize less vitamin D from sunlight than do their fairer-skinned counterparts, the researchers hypothesized that supplementation could help prevent bone loss in middle age.
The three-year placebo-controlled, double-blind trial involved 280 healthy black postmenopausal women aged between 50 to 75 years, who were assigned to receive either 800 IU of vitamin D3, or a placebo every day for the first two years. In the third year, the vitamin D3 dose was increased to 2000 IU a day.
All the women also received calcium supplements to ensure that their daily intake was optimal - between 1200 and 1500mg.
The researchers measured the women's bone mineral density (BMD) at six-month intervals and other markers of bone turnover.
The researchers expected to find that supplementation decreases bone loss. However their findings, published in the Archives of Internal Medicine (vol 165, July 25), came as a surprise: no differences in bone loss or bone turnover markers were observed between the two groups of women.
While BMD of the total body, hip and radius increased in both groups at year one, over the two subsequent years it declined at these sites at a rate of between 0.26 and 0.55 percent per year.
In the discussion of the findings, the researchers note that some trials involving white women have indicated that supplementation prevents bone loss, but other have disputed the conclusion.
They note that African American women have an incidence of hip fracture only 40 percent that of white women - a statistic that may be attributable to higher bone mass, lower bone remodeling rates and an advantageous geometry.
What is more, African American adolescents have been observed to absorb calcium more efficiently and excrete less - an observation that has led to the estimation by some scientists that this racial group needs several hundred milligrams less calcium a day than white women.
"Although this may not be extrapolated to women of other ethnic groups, to elderly women, or to greater degrees of vitamin D insufficiency, it lends support to reexamination of optimal vitamin D nutrition for skeletal health in postmenopausal women of other ethnic groups," concluded the Winthop researchers.
They plan to conduct a similar trial to assess the effects of vitamin D3 supplementation on bone loss and bone turnover markers in white women.