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"We found that daily intakes of vitamin D by adults in the range of 4,000-8,000 IU are needed to maintain blood levels of vitamin D metabolites in the range needed to reduce by about half the risk of several diseases -- breast cancer, colon cancer, multiple sclerosis and type 1 diabetes," study co-author Dr. Cedric Garland, a professor of family and preventive medicine at the University of California at San Diego, said in a university news release.
Garland admitted that he was surprised that the levels required were so much higher than the 400 IU a day needed to vanquish rickets in the 20th century.
Vitamin D supplements often come in pills or capsules containing 1,000 or 2,000 international units. But 4,000 to 8,000 IU a day is still much lower than the range considered safe by the National Academy of Science's Institute of Medicine, the researchers noted.
The study -- which also involved the Creighton University School of Medicine in Omaha -- was based on a survey of several thousand people who took supplements ranging from 1,000 to 10,000 IU per day. The volunteers also underwent blood tests to determine the levels of vitamin D metabolites circulating in their blood.
Some studies suggest that only 10% of people in the United States have the appropriate level of the vitamin D-related form in their blood to prevent disease linked to a deficiency of the vitamin. These people tend to work outdoors, where their vitamin D levels are boosted through sun exposure.
Last year, a National Academy of Sciences Institute of Medicine (IOM) committee announced that 4,000 IU a day of vitamin D appears safe for adults and kids aged 9 and up.
The IOM's recommended minimum daily level is 600 IU, however, and the Institute noted there were preliminary signals that there might be some harms associated with consuming high levels of vitamin D daily, even at amounts under the recommended upper safe limit.
Garland and his colleagues suggested that 4,000 IU a day is a safe level.
"Now that the results of this study are in, it will become common for almost every adult to take 4000 IU/day," Garland predicted in the news release. "This is comfortably under the 10,000 IU/day that the IOM Committee Report considers as the lower limit of risk, and the benefits are substantial."
The findings appear in the journal Anticancer Research.
-- Randy Dotinga
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