DO get tested.
Your doctor is unlikely to order the test unless she suspects osteoporosis. But everyone should get tested, says Giovannucci, especially those with a family history of cancer or autoimmune disease.
DON'T assume you get enough from your diet.
D is found naturally in fatty fish, eggs, liver, and some cheeses, but none of these sources contain enough to meet your daily needs. (You'd have to eat 95 eggs to come close.) You can buy D-fortified foods, but Holick warns that labels can't always be trusted. His research found that only 30 percent of milk brands he tested contained the amount of D they claimed to. And 20 percent of nonfat milks had none.
DO get some unprotected sun exposure—safely.
The most common recommendation from physicians is ten to 15 minutes on your arms and legs (never your face) every day.
DO take a supplement.
If your levels are above 30 ng/ml (or you haven't been tested), Holick suggests 2,000 IU daily for adults and 1,000 IU for kids. If your levels are below 30 ng/ml: Take 5,000 to 6,000 IU daily for a couple of months, then 2,000 IU.
DON'T overdo it.
While it's hard to reach toxic levels (150 ng/ml), it's not impossible. Holick recommends no more than 10,000 IU daily, an amount proved to be free from side effects.
Knowing your vitamin D levels might save your life.