Even though many foods are fortified with vitamin D, a growing number of medical experts say many people do not get enough.
Older men and women who fail to get enough vitamin D -- either from their diets or exposure to the sun -- are at heightened risk for muscle weakness and poor physical performance, a new study shows.
Vitamin D is vital to bone health because it allows the body to use calcium in a way that keeps bones healthy. People who don't have enough vitamin D get rickets, which is essentially damaged bones, though the condition is rare in the developed world.
The National Academy of Sciences says that for people older than 50, 200 international units (iu) of vitamin D per day is adequate. People between 51 and 70 need 400 iu and people older than 70 need 600 iu.
But many experts think that amount is too low, according to ABC News medical editor Dr. Tim Johnson.
"But there is probably no one who can tell you exactly how much do you need. Even the government calls these adequate amounts rather than daily recommended amounts," Johnson said.
Johnson recommends people get 1,000 iu of vitamin D a day. One of the best sources of Vitamin D is the sun.
"The sun's ultraviolet rays cause us to manufacture vitamin D," Johnson said, "but that's problematic because we are all told to stay out of the sun and to wear sunscreen, which means less vitamin D."
Dark-skinned people often don't get enough vitamin D, because the sun doesn't penetrate their skin. And the elderly often miss out on vitamin D in their diets, Johnson said.
Salmon and sardines are good sources of vitamin D. Many kinds of milk, orange juice and cereal are fortified with vitamin D, but Johnson says consumers should check the labels, because those products are often fortified with vitamin D2. Vitamin D3 is best used by the body.
Johnson said there were risks associated with getting too much vitamin D, including too much calcium buildup in the blood, which can lead to kidney and bone problems, buildup of calcium in muscles, and heart trouble.
Vitamin D is fat soluble, which means you can get excessive amounts of it that accumulate in your fat tissue, which can be dangerous.
"But it's hard to get too much vitamin D," Johnson said. "If you stick with the 1,000 iu a day, you don't need to worry."