"I think the definitive study will just look at the effects of vitamin D," said lead author Rebecca Rossom, from HealthPartners Institute for Education and Research, a nonprofit arm of a health maintenance organization based in Minnesota.
But she added that the current study is still important because it "gets closer to how women take vitamin D now" to build bone density.
Rossom and her colleagues analyzed data on 4,100 women who were simultaneously enrolled in two trials, including the Women's Health Initiative (WHI) Calcium and Vitamin D trial that ended in 2005, and a WHI memory study.
All of the women, who averaged 71 years old at the outset of the studies, were also free of cognitive problems to start.
Half of the women were assigned to take the supplements, and the rest were given identical-looking dummy pills.
Ultimately, about 100 women, or five percent, in each group developed mild cognitive impairment a term that can include everything from memory trouble to the serious dementia found in Alzheimer's disease.
The researchers noted that since the study ended, guidelines on vitamin and mineral intakes have changed, Currently the U.S. Institute of Medicine suggests getting 600 IUs per day of vitamin D for men and women up to age 70, and 800 IUs for older people. Suggested calcium ranges from 700 mg to 1,300 mg per day, based on age, with an upper limit of 3,000 mg.
In both cases, intake recommendations cover both food and supplement sources. So, the authors say, their findings are specific only to the assigned amounts of vitamin D and calcium taken by women in the study - which are relatively low by today's standards.
"The sum of information does show conflicting evidence," said Katherine Tucker of Northeastern University, who was not involved in the study.
"Some recent studies suggest that too much calcium could have negative effects. The preponderance of evidence shows that vitamin D is protective, but some studies have shown no effect," she told Reuters Health.
Rossom's team acknowledges their study's limitations. In addition to the doses of supplements, the results are strictly limited to women, who were mostly white. Also, the study participants were relatively young.
"The next step is to test a higher level of vitamin D," said study coauthor JoAnn Manson of Harvard Medical School.