The role of vitamin D supplementation in preventing these diseases is also not well understood.
Exposure to sunlight is an efficient way to raise blood levels of vitamin D hormone, and food sources of the nutrient include oily fish like salmon, fortified milk, and other fortified foods.
But most people would have a hard time getting the vitamin D they need from food, and the increased use of sunscreen has reduced sun exposures.
By one recent estimate, as many as half of adults and children in the U.S. were deficient in the vitamin.
Current recommended daily vitamin D intake is 200 IU (international units) for those up to age 50; 400 IU for people 51 to70; and 600 IU for those over 70. Most experts say that these doses are too low.
Many experts, including Ramagopalan, say 2,000 IU of the vitamin may be optimal for preventing disease.
Blood levels of the vitamin are measured as 25-hydroxyvitamin D. Levels below 20 nanograms per milliliter are generally considered deficient.
Harvard School of Public Health nutrition researcher Edward Giovannucci, MD, says blood 25-hydroxyvitamin D levels of between 30 and 40 nanograms per milliliter may be about right for reducing the risk of autoimmune diseases and certain cancers.
While he says some people can reach these levels without supplementation, many others would need to take 1,000 to 2,000 IU of the vitamin a day.
"Based on what we know, I think it is reasonable to recommend that people maintain blood levels of around 30 nanograms per milliliter," he says.