March 23 (HealthDay News) -- Over the course of two decades, vitamin D levels have dramatically decreased among Americans, a new study finds.
Low levels of vitamin D have been associated with rickets in children and lower bone mineral density in adults. Recent research has also linked insufficient vitamin D to cancer, heart disease, infection and poorer health overall. Optimal levels range from 30 nanograms per milliliter to 40 nanograms per milliliter, the researchers said.
"We found a marked increase in vitamin D deficiency over the past two decades," said lead researcher Dr. Adit Ginde, an assistant professor of surgery at the University of Colorado Denver School of Medicine. "Over three out of every four Americans now have vitamin D levels below what we believe is necessary for optimal health. African-Americans and Hispanics are at particularly high risk -- nearly all have suboptimal levels."
The report was published in the March 23 issue of the Archives of Internal Medicine.
For the study, Ginde's group collected data on vitamin D levels in 18,883 people collected between 1988 and 1994, and 13,369 people collected between 2001 and 2004. All the data came from the Third National Health and Nutrition Examination Survey.
The researchers found that average vitamin D levels were 30 nanograms per milliliter from 1988 to 1994, but decreased to 24 nanograms per milliliter between 2001 and 2004. Moreover, vitamin D levels of less than 10 nanograms per milliliter increased from 2 percent to 6 percent over the study period. There were also fewer people with vitamin D levels of 30 nanograms per milliliter or higher (45 percent vs. 23 percent).
The greatest drops in vitamin D levels were seen among blacks, where levels of vitamin D of less than 10 nanograms per milliliter rose from 9 percent to 29 percent, and levels of more than 30 nanograms per milliliter or higher dropped from 12 percent to 3 percent, the researchers found.
"Increases in vitamin D deficiency in the population may have reduced the overall health of the population," Ginde said. "Since sunlight is the body's major source of vitamin D, increases in sunscreen, sun avoidance, and overall decreased outdoor activity, while successful in reducing skin cancers, has probably reduced vitamin D levels in the population."
Ten minutes of sunlight on exposed arms and legs two to three times per week would significantly improve vitamin D production, but must be weighed against the risk for skin cancer, Ginde noted. Vitamin D supplementation is another way to increase levels. However, current recommended doses of vitamin D supplements are outdated and inadequate, he added.
Right now, recommended levels of vitamin D supplements are 200 international units per day from birth to age 50, 400 international units (IU) per day from age 51 to 70, and 600 international units per day for adults aged 71 and older. These recommendations are primarily for improving bone health.
"Vitamin D is an important and underappreciated public health issue and may be responsible for some racial differences in health outcomes," Ginde said. "Most Americans could use more vitamin D. Higher doses of vitamin D supplementation than currently recommended, at least 1,000 to 2,000 IU daily, are likely needed to raise vitamin D levels for many people."
Another report in the same journal highlights the importance of vitamin D for bone health. In the study, Swiss researchers conclude that 400 IU of vitamin D supplements per day are associated with a reduced risk of fractures in older adults.
"Given the frequency, severity and cost of non-vertebral fractures, everyone age 65 and older should take vitamin D in a dose close to 800 IU per day," said lead researcher Heike A. Bischoff-Ferrari, an assistant professor at the University of Zurich.
To reach their conclusion, Bischoff-Ferrari and colleagues reviewed the findings of 12 clinical trials of looking at the benefits of vitamin D supplements in reducing fractures in adults aged 65 and older. In all, the trials involved 42,279 participants.
The researchers found that vitamin D supplements decreased the risk of non-vertebral fractures by 14 percent and of hip fractures by 9 percent. In trials where people were given doses of more than 400 IUs a day, fractures were reduced by 20 percent and hip fractures by 18 percent.
In addition, for people taking high doses of vitamin D, calcium supplements did not appear to have any additional protective effect against fractures, the researchers reported.
"At the higher dose, this benefit is not restricted to frail older individuals, but is also present in community-dwelling older individuals," Bischoff-Ferrari said. "In the subgroup of community-dwelling older individuals, vitamin D at the higher dose reduced non-vertebral fractures by 33 percent."
Dr. Michael F. Holick, director of the Vitamin D Laboratory at Boston University, noted that the recommended levels of vitamin D are under review and likely to be increased.
"An Institute of Medicine panel is planning to have new recommendations out by mid-2010," Holick said. "It's pretty clear that you need a minimum of 1,400 and up to 2,000 IU a day, and if you are obese, you probably need at least one and a half to two times as much, because the fat sequesters the vitamin D," he said.
Holick said people are drinking less milk and staying out of the sun, which are the main reasons for the decreasing vitamin D levels in the population.
One way to combat the problem is to increase vitamin D supplementation in foods, Holick said. New recommendations that increase vitamin D levels will let the food industry increase vitamin D levels in foods and add vitamin D to more foods, he said.
"We are in desperate need to have a marked increase in the adequate intake recommendation, and hopefully, that will be 1,000 to 2,000 IUs per day and raise the safe upper limit to at least 10,000 IUs a day," Holick said. "The plan would be to increase the amount per serving and increase the number of foods fortified with vitamin D."
SOURCES: Adit Ginde, M.D., M.P.H., assistant professor, surgery, University of Colorado Denver School of Medicine; Heike A. Bischoff-Ferrari, Dr.P.H., assistant professor, University of Zurich, University Hospital, Zurich, Switzerland; Michael F. Holick, M.D., Ph.D., director, Vitamin D Laboratory, Boston University; March 23, 2009, Archives of Internal Medicine