Vitamin D deficiency is a common problem which, left untreated, can lead to bone disorders such as rickets in children and bone fractures in adults. In an article published in The American Journal of Clinical Nutrition, Wortsman et al. present research results on the bioavailability of vitamin D through different routes of administration in obese and lean individuals. Study subjects were first exposed to simulated sunlight to stimulate production of vitamin D in the skin, and one month later were administered oral doses of vitamin D. In the obese subjects oral vitamin D was more bioavailable than vitamin D from sunlight exposure, suggesting that the obese individuals did not process vitamin D from sunlight as well as the nonobese. The authors propose that vitamin D is being sequestered in body fat in obese persons, giving rise to a relative deficiency which could be corrected with oral administration of extra vitamin D.
The study was conducted during the winter months using two groups of 19 healthy white subjects, one lean and one obese. All were exposed to whole-body irradiation in a phototherapy unit (tanning bed), and follow-up blood samples were obtained to ascertain serum vitamin D levels. One month later the subjects were challenged with an oral dose of 50 000 IU of vitamin D, and follow-up blood samples again obtained. Obese individuals showed an attenuated response to simulated sunlight, with serum vitamin D concentrations less than half those of the lean individuals after irradiation. Both groups had the same initial rise in levels of the steroid precursor vitamin D3 in the skin, but in the obese subjects less vitamin D3 was released from the skin into the circulation. Oral administration of vitamin D2 –the synthetic form of the vitamin used for supplementation -- resulted in serum concentrations that were not significantly different between the two groups. The authors hypothesized that oral vitamin D2 was more bioavailable than vitamin D3 synthesized in the skin in the obese. Additional in vitro studies on vitamin D production in skin cells procured during surgery from obese and nonobese subjects were performed. Both, when exposed to sunlight, were equally efficient in the production of the precursor vitamin D3.
In their conclusion the authors state that most humans derive their vitamin D requirement from casual exposure to sunlight, and that obese individuals appear to have > 50% lower bioavailability of vitamin D from cutaneously synthesized vitamin D3. Thus, the more accessible oral form should be relied upon to correct vitamin D deficiency associated with obesity.
Wortsman, Jacobo et al. Decreased bioavailability of vitamin D in obesity. Am J Clin Nut 2000;72:690-693.
For more information please contact: Dr. M F Holick at mfholick@bu.edu
This media release is provided by The American Society for Clinical Nutrition, to provide current information on nutrition-related research. This information should not be construed as medical advice. If you have a medical concern, consult your doctor.
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Journal
American Journal of Clinical Nutrition