Salmon, sardines, tuna and mackerel contain vitamin D. (CBC)

Taking high doses of vitamin D supplements may help older adults to reduce their risk of fractures, a Swiss review suggests.

"The anti-fracture benefits of vitamin D have been questioned by several recent trials, leading to uncertainty among patients and physicians regarding recommendations for vitamin D supplementation," Heike Bischoff-Ferrari of University Hospital in Zurich and colleagues wrote in the March 23 issue of the Archives of Internal Medicine.

The benefits of vitamin D for preventing broken bones could be obscured by poor adherence to the treatment, low dose or use of a less potent form called ergocalciferol or vitamin D2, the researchers said.

The team reviewed 12 previously published clinical trials on oral vitamin D supplements in more than 42,000 people with an average age of 78.

By combining the results of nine trials in which participants took doses of more than 400 international units per day, vitamin D supplements decreased the risk of non-vertebral fractures by 20 per cent and of hip fractures by 18 per cent, researchers said.

Doses of 400 IU or lower did not reduce the risk of either type of fracture.

It's "possible that greater benefits may be achieved with earlier initiation of vitamin D supplementation and longer duration of use," the researchers wrote.

Americans found lacking in vitamin

A second study appearing in the same issue concluded that Americans of all ages are not getting enough of the "sunshine vitamin."

Based on data from U.S. government health surveys, Dr. Adit Gindle of the University of Colorado Denver school of medicine said three out of four adult Americans had "insufficient" levels of vitamin D, compared with one out of two 20 years ago.

Evidence suggests that levels of 30 nanograms per millilitre to 40 nanograms per millilitre of blood may be needed for optimum health, the researchers said.

Current recommendations are based on improving bone health, but recent research suggests the vitamin may play other roles in the body, since other types of cells contain receptors for vitamin D.

But campaigns to reduce sun exposure to prevent skin cancer, and decreases in outdoor physical activity have contributed to vitamin D insufficiency, Ginde said.

The percentage of Americans with less than 10 nanograms per millilitre of blood tripled from two per cent in 1988-1994 to six per cent in 2001-2004, the researchers found.

"This drop was associated with an overall increase in vitamin D insufficiency to nearly three of every four adolescent and adult Americans," the study's authors wrote.

"Current recommendations for dosage of vitamin D supplements are inadequate to address this growing epidemic of vitamin D insufficiency," they concluded.

"Increased intake of vitamin D (1,000 international units per day or more) — particularly during the winter months and at higher latitudes — and judicious sun exposure would improve vitamin D status and likely improve the overall health of the U.S. population."

The Canadian Cancer Society recommends Canadians take in 1,000 IU of vitamin D every day. Health Canada warns that taking more than 2,000 IU a day could result in an overdose that can cause kidney stones as well as damage to the heart, lungs and blood vessels.

Large, randomized control trials of higher vitamin D supplementation are needed to evaluate the effect on general health and mortality, the researchers said.