Low levels are linked to...

Heart disease
People with insufficient D levels have an 80 percent greater risk of narrowing of the arteries, according to a long-term study at Johns Hopkins. This might have to do with D's role in regulating more than 200 genes and controlling inflammation, and its possible involvement in modulating blood pressure.

Diabetes
Since D stimulates insulin production, it's no surprise that too little is associated with diabetes. Research has also shown that kids who are deficient in D have a 200 percent greater chance of developing type 1.

Chronic pain
A 2008 study showed that more than 25 percent of chronic pain patients have low D levels, which could be because D helps control neuromuscular function. And a 2010 study correlated low levels of the vitamin with migraines and headaches. A dearth of D may prevent blood vessels from constricting and dilating properly, which can lead to throbbing pain.

Depression
D may help stimulate serotonin production, which could explain why people who don't get enough are more susceptible to the blues.

Higher risk of death
After analyzing D levels of more than 13,000 people, researchers at Johns Hopkins found that those with the lowest levels had a 26 percent greater chance of dying—from any cause.

Next: 5 things that increase your risk of vitamin D deficiency