More than half the U.S. population—including about 70 percent of the elderly and 90 percent of minorities—is vitamin-D deficient, according to Dr. James E. Dowd, author of “The Vitamin D Cure.” The nutrient helps maintain normal levels of calcium and phosphorus in the blood. It also helps the body absorb calcium and keeps bones strong. Vitamin D may also protect against osteoporosis, hypertension, cancer and other diseases, according to the Mayo Clinic.

Vitamin D is mostly produced in the skin after UV exposure from the sun, but it can also be derived from milk, fish, egg yolks and vitamin supplements. It’s harder for the obese and people with more melanin in their skin to absorb vitamin D. It doesn’t help that we’ve become a nation of sunscreen wearers who eat low-vitamin processed foods and work mostly indoors; that all leads to D deficiency, which can cause susceptibility to seasonal affective disorder, fatigue, headaches and a variety of immune-related diseases, according to Dowd.

Luckily, the cure is hardly painful: the National Institutes of Health prescribes a balanced diet and weekly sun exposure in order to produce the body’s natural require-ment of vitamin D. Other easy sources of the nutrient include cod-liver oil, salmon and milk. Raw fish is even better. Some doctors recommend an additional vitamin supplement, available at health stores, but go easy on those. Overzealous pill-popping can result in vitamin-D toxicity, for which the side effects include nausea, vomiting, constipation and an increase of calcium in the blood. That, in turn, can lead to mental confusion. Check online at dietary-supplements.info.nih.gov for NIH’s recommended daily intake levels; the older you are, the more you require.

Doctors say keeping track of your vitamin-D intake is especially important for people at risk of deficiency—those with dark skin, the elderly, the overweight, those who suffer from diseases that interfere with the body’s absorption of fat, and infants who were exclusively breast-fed.

Surprisingly, people who wear sunscreen whenever they go out are at risk, too. Although sunscreen protects your skin from UV damage, SPF numbers as low as 8 can reduce vitamin-D absorption by 95 percent, according to the Linus Pauling Institute at Oregon State University.

Although most doctors balk at suggesting that Americans cut back significantly on sunscreen usage, many recommend 10 to 15 minutes of sunshine, three times a week— without the lotion.