Vitamin D plays an important role in building healthy bones by helping the body absorb calcium.

It also appears to protect against these 11 diseases: diabetes, gum disease, multiple sclerosis, muscle weakness, mental decline in the elderly, osteoporosis and possibly stroke, heart disease, and cancer of the breast, colon and prostate.

In 1933, the U.S. implemented a Vitamin D milk fortification program to combat rickets, then a major public health problem. It virtually eliminated the disorder at that time.

Adults need 2 to 4 glasses of fortified milk each day to get minimum recommended levels of vitamin D — two glasses for people ages 19 to 50 and four for people 51 to 70. According to a recent study, 77 percent of teens and adults are deficient in vitamin D.

Sun exposure is one way to get vitamin D: 5 to 30 minutes twice a week produces sufficient amounts.

But a better way to obtain vitamin D, according to Dr. Christine Gerbstadt, spokeswoman for the American Dietetic Association, is through fortified foods, including cow's milk and soy milk, orange juice, and cereals. "As a physician, I recommend SPF 30 or greater every time a person steps outside," she adds.

Not sure if you're getting enough vitamin D or other essential nutrients? A dietitian or nutrition consultant can assess the gaps, as well as help with meal planning and weight-loss goals. Check Angie's List for highly rated professionals.