Americans aren't getting enough vitamin D, reports the Chicago Sun-Times today. Researchers from Harvard and the University of Colorado, Denver, say the number of people who get adequate amounts of vitamin D fell by almost half in the years 2001 to 2004 from 1988 to 1994, and that more time indoors and the use of lots of high-protection sunscreen may be to blame.

Sunscreen complicates matters because it blocks out rays that help the skin make this vitamin, even though wearing sunscreen is important to protect against skin cancer. Staying in the sun for a short while without sunscreen is likely to be safe, but it can be difficult to get the balance right.

To get the vitamin D you need, we recommend as little as 10 minutes outdoors three or four times a week. If you're older than age 60, have darker skin, are very overweight, or live in northern areas (where the sunlight is less intense), you need more. Talk to you doctor about how to get the vitamin D you need. And if you rarely get out in the sun, our experts recommend consuming 800 to 1,000 international units (IU) of vitamin D daily from supplements or food. A 3-ounce serving of fatty fish provides between 200 and 350 IU, and a cup of fortified milk, 100 IU.

Read more on how vitamin D can help you live longer, and how to get more of it from the foods you eat.