How much vitamin D do you need?
It's a difficult question, offering a reminder that there are more mysteries than certainties when it comes to understanding the biology of our bodies.
Scientists have long understood that vitamin D promotes healthy bones. Now evidence is mounting that it may also prevent a host of chronic diseases, such as cancer, diabetes, heart disease, high blood pressure and several autoimmune diseases. Naturopathic doctors (NDs) and conventional physicians are increasingly prescribing vitamin D supplements because the potential benefits appear to outweigh the risks.
But scientists lack evidence on questions as basic as how the body absorbs vitamin D into the bloodstream from different sources, and on basic strategies to raise low vitamin-D levels. That's where a new study from Ryan Bradley, ND, MPH, a Bastyr clinical research assistant professor, comes in.
Our bodies absorb vitamin D from sunlight and foods such as fish, eggs, fortified milk and cod liver oil. It's also sold as a supplement in a variety of forms. Dr. Bradley selected three popular types — a capsule, an emulsified-oil drop and a chocolate-flavored chewable tablet — to compare how they affect vitamin D in blood levels.
"A lot of the vitamin-D research out there looks at the relationship between health conditions and vitamin levels, but that doesn't tell you if one is causing the other, or if there are other factors," says Dr. Bradley, a 2003 graduate of Bastyr's naturopathic medicine program. "Few studies have given people vitamin D in the form of dietary supplements to see how much vitamin D status can change, and to see which risk factors change, if any. That's what we wanted to do."
Full results aren't published yet, but the project has already produced two interesting findings.
First, low vitamin-D levels were surprisingly common. Dr. Bradley recruited test subjects from both the Seattle area and a Kona, Hawaii, practice run by co-investigator Michael Traub, ND. Of about 100 people screened in Seattle, only 60 had "sufficient" levels, according to widely used standards from The Endocrine Society, a hormone research group. The rest had levels considered "insufficient" (which is less extreme than "deficient"). Insufficient levels were common in the Hawaii group too.
"People don't get very much time outside anymore, even in Hawaii," says Dr. Bradley. "Even if the sun's out, if you're not outside, or if you're using sunscreen, you're not making vitamin D."
The second surprise was that supplements often didn't contain what their labels promised. The study used products marked as 2,000 IUs (International Units). But the actual content ranged widely and often exceeded the claim in the label.
"NDs (and all providers) want to have reliable products to give to patients," Dr. Bradley says. "Without clinical research, we don't have that. Product manufacturers should support the research necessary to accurately label their products."
The study, funded primarily by the Diabetes Action Research and Education Foundation, yielded encouraging results on safety. None of the 67 subjects experienced hypercalcemia (absorbing too much calcium), a suspected risk of vitamin D supplements, even at a daily dosage of 10,000 IUs.
Although the participants knew what they were taking, the study remains blinded to the investigators, who do not know which results correspond with which supplement form.
When the blind is removed, the results will also be useful to budget-conscious shoppers. "If you can get the same result from a $3 bottle of capsules and a $15 bottle of chewable tablets, people can feel free to take the capsule," Dr. Bradley says.
He expects to present and publish results later this year at the Integrative Medicine and Health international research conference in Portland, Oregon, from May 15-18.
The study also produced more data to be analyzed on links between vitamin D and chronic diseases. One such project is already under way by Traci Pantuso, a naturopathic medicine student who worked on the study through a T32 training grant from the National Institutes of Health. Under Dr. Bradley's guidance, she examined vitamin D's role in activating white blood cells and aiding the immune system. Pantuso hopes to publish her results after she graduates this summer.
John Finnell, ND, a T32 postdoctoral research fellow, played a major role in the three-arm, multisite trial with Dr. Bradley. He will analyze more data on preventive properties and chronic diseases, including a novel biological indicator never measured in humans before and after vitamin D replacement.
In a separate project, nutrition undergraduate student Rebecca Oshiro will look for a relationship between vitamin D and muscle strength. She plans to provide supplements to subjects who are vitamin D deficient but otherwise healthy, then test their grip strength and vertical jumping ability to see if they improve. Her project is funded by Bastyr's Center for Student Research, which encourages students to learn research skills while in school.
A major report last year from the Institute of Medicine recommended lower vitamin D levels than many doctors supported, drawing media coverage and ensuring that the issue will remain contentious for the near future. As Bastyr researchers contribute to the body of evidence with each published study, the big picture grows clearer, bit by bit.