"Very low vitamin D levels are mainly associated with higher age and lower physical activity," which could lead to less exposure to the sun, Schoettker noted.
On the other hand, "the reasons for very high vitamin D levels are unknown," he added.
The researchers pointed out that vitamin D may not change levels of risk for health problems and earlier deaths. It's possible that levels of vitamin D reflect overall health. Low levels of vitamin D may just be a sign of poor health rather than a cause of it, according to the study.
It's not clear how vitamin D could help people live longer, but it might have something to do with the way it acts like a hormone, said Susan Mayne, chair of the department of chronic disease epidemiology at Yale School of Public Health. Still, she said, the science linking the vitamin to heart disease and cancer is in its early stages.
For now, both Mayne and Schoettker said people should follow the recommendations of the Institute of Medicine regarding vitamin D. Its 2010 report says most Americans and Canadians already get enough vitamin D. It says nothing about whether people with heart disease or cancer should take supplements, however.