Hundreds of studies have shown that D affects various illnesses, but most of the links are associations, not proven cause and effect. “We need more research to determine if there’s a real connection,” McManus says. Some answers should emerge in the next few years. The newly launched VITAL study at Harvard Medical School and Brigham and Women’s Hospital will follow 20,000 people to see whether vitamin D, fish oil or both reduce the risk of developing cancer, heart disease and stroke. In the meantime, here’s an overview of what we now know.
Bone health Older people who took D and calcium had a decreased risk of fractures and bone loss in a review by the University of Ottawa Evidence-Based Practice Center. The Ottawa EPC also found that menopausal women who were getting up to 800 IU per day with calcium experienced small increases in bone mineral density of the spine, total body, femoral neck and hip.
Colon cancer In May, Italian researchers reported that people with higher blood levels of D had a lower risk of colorectal cancer but not breast cancer.
Immune system Massachusetts General Hospital researchers looked at data from nearly 20,000 people and found that those with the lowest levels of D had the highest risk of upper-respiratory infections.
Heart disease risk factors According to the Michigan Bone Health and Metabolism Study, D may help keep blood pressure low, and other research has shown that it may reduce diabetes risk.
Mental health D may protect you from depression, but promising preliminary studies need to be confirmed with randomized, controlled trials. “We know there are D receptors in the brain, but research in this area is just beginning,” Norman says.
Originally published in the September 2010 issue of More.