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Current RDA for vitamin D may be higher than necessary, study shows

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The amount of vitamin D needed for bone health is lower than the current recommendation, according to a study presented at the recent annual scientific meeting of the Endocrine Society.

After re-measuring vitamin D using improved technology, the Recommended Daily Allowance (RDA) for vitamin D intake drops from 800 to 400 International Units (IU) per day.

“In estimating the RDA for vitamin D intake, the laboratory method used for measuring serum 25-hydroxyvitamin D — 25(OH)D — can affect the results,” explained principal investigator Dr J. Christopher Gallagher, professor and director of the Bone Metabolism Unit in the Division of Endocrinology of Creighton University School of Medicine in Omaha, Nebraska. “The estimated RDA based on the older immunoassay (DiaSorin S.p.A., Salugia, Italy) system was 800 IU daily, whereas the newer liquid chromatography tandem-mass spectrometry (LC-MS/MS) technique estimated that 400 IU daily would meet the RDA.”

The lower level is easily achievable with a supplement of 400 IU in winter, when vitamin D levels are lowest, Dr Gallagher said.

In an earlier double-blind dose-response clinical trial in the winter and spring of 2007 to 2008, Dr Gallagher and colleagues enrolled 163 healthy postmenopausal Caucasian women aged between 57 and 90 years, with vitamin D insufficiency, and followed them for one year.

The participants were randomised to one of seven vitamin D3 doses: 400, 800, 1600, 2400, 3200, 4000 or 4800 IU/day, or placebo, for one year, and all the women were given calcium supplements to maintain a total calcium intake.

After analysing the samples and estimating the RDA using the older immunoassay, the study authors reported that 800 IU daily would meet the vitamin D intake requirement for 97.5% of the population.

However, now that LC-MS/MS has become the “gold standard” for measuring 25(OH)D, the researchers have reanalysed the original samples using this new technology. By being able to determine a more precise dose-response curve, they have calculated the RDA for vitamin D to be 400 IU daily.

“Remember, this RDA is for bone health only,” Dr Gallagher noted. “It may be different for other diseases. Although trials looking into cancer, diabetes and other diseases are ongoing, we do not have information about this yet.”