Vitamin D supplements may not always make a difference in a child's bone density, a new review finds.

Children with normal vitamin D levels didn't show improvements in their bone density after taking vitamin D supplements, according to a review of six studies. The studies involved nearly 900 participants, ages 1 month to 19 years, who took a vitamin D supplement or a placebo for three months.

More research is needed to confirm that children with a vitamin D deficiency might benefit from the supplements, said researchers from the Menzies Research Institute at the University of Tasmania in Australia.

About 70 percent of U.S. children aren't getting enough vitamin D, according to a 2009 article in the journal Pediatrics.

Why they're deficient

Vitamin D — found naturally in milk and in fatty fish like salmon, tuna and trout, but in few other foods — helps the body absorb calcium, and therefore increases bone density, which helps to shield against broken bones and osteoporosis later in life.

Supplements are still a good idea if you're not eating enough vitamin-D-rich foods, said Connie Weaver, head of the Department of Foods and Nutrition at Purdue University, who was not involved with the study.

"There's hardly any place you can get it," she said.

Children and teens should consume 400 international units of vitamin D a day, according to recommendations from the American Academy of Pediatrics. To put that in perspective, a glass of milk has about 100 international units of vitamin D, Weaver said.

Fatty fish also have vitamin D, as does fortified orange juice, she said.

Weaver said the new research provided a good summary of what is known about vitamin D and its effects on children, but that variations in the studies reviewed mean the results shouldn't be taken at face value.

The wide age range of children in the studies could hide any benefits vitamin D supplements might have in specific age groups, such as pre-pubertal adolescents, Weaver said.

And because supplement dosages varied across the studies, there's no way to determine whether a particular dose of extra vitamin D might be beneficial, she said.

The effects of race

Studies should also be done to determine whether vitamin D supplementation is more effective in certain races, Weaver said. For example, Caucasians are more prone to osteoporosis, she said, but African-Americans are more likely to be deficient in vitamin D.

Aside from its role in bone density, vitamin D supplementation may benefit health in other ways. The nutrient has a role in controlling infection and lowering the risk of colon cancer, according to the Harvard School of Public Health. And it has also been shown to lower the risk of Parkinson's disease, according to a study earlier this year in the journal Archives of Neurology.

The study was published today (Oct. 5) in the Cochrane Systematic Review.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.