''Our study could not show cause and effect," says Esther Krug, MD, an endocrinologist at Sinai Hospital of Baltimore and assistant professor of medicine at Johns Hopkins University School of Medicine, Baltimore, who presented the findings at ENDO 2010, the annual meeting of The Endocrine Society, in San Diego.
But she did find that vitamin D deficiency was common in her study, with more than 91% of participants deficient. As the deficiency worsened, so did diabetes control. Only eight of the 124 participants took vitamin D supplements, she found.
About 18 million people in the U.S. have been diagnosed with diabetes, according to the American Diabetes Association, and about 6 million more are believed to have the condition but are undiagnosed.
Low Vitamin D, Poor Diabetes Control: The Study
So they evaluated the medical charts of 124 people with type 2 diabetes (in which the body doesn't make enough insulin or the cells ignore the insulin) seen at an outpatient clinic from 2003 to 2008. The charts contained information on the patients' age, race, vitamin D levels, calcium intake, family history of diabetes, and results of their hemoglobin A1c blood test. The A1c provides an average measurement of blood sugar control over about a 12-week span. (For people with diabetes, the goal is 7%; for people without, the normal range is 4%-6%.)
Krug's team divided the vitamin D levels they found into four groups: normal (defined in the study as above 32 nanograms per deciliter), mild deficiency, moderate deficiency, or severe.
In all, 113 of the 124 patients (91.1%) were vitamin D deficient -- 35.5% severely, 38.7% moderately, and 16.9% mildly.
The average A1c was higher in patients with severe vitamin D deficiency compared to those with normal levels of vitamin D. Those with severe deficiency had an average of 8.1%; those with normal vitamin D levels averaged 7.1%.
Krug found racial differences. ''In people of color, vitamin D levels were even lower than in Caucasians and they were associated with even poorer diabetes control," she tells WebMD.
Only 6.4% were on vitamin D supplementation. This was true, Krug says, even though they had medical coverage and saw their doctors. She suspects a lack of awareness on the part of the physicians partly explains the frequent deficiencies she found.
Aggressive screening of vitamin D levels is crucial for people with diabetes, Krug says. Once a supplement is recommended, she says, the blood levels should be rechecked to see if the supplement sufficiently increases vitamin D levels.
Vitamin D Facts
Vitamin D is crucial not only to maintain bone strength, but research also suggests it plays a role in immune system functioning, cancer prevention, and cardiovascular health. It is produced when ultraviolet rays from the sun strike the skin and is also found in fish, eggs, fortified milk, cod liver oil, and supplements.
Adequate intakes, set by the Institute of Medicine of The National Academies, are 200 international units (IU) a day for adults up to age 50, 400 IU for people aged 51-70, and 600 IU for people 71 and older. But some experts say much more is needed; the recommendations are under review, with an update expected in 2010.
The new study lends support to a growing body of scientific and clinical data linking vitamin D with insulin and glucose, says Ruchi Mathur, MD, an endocrinologist and assistant professor of medicine at Cedars-Sinai Medical Center, Los Angeles, who reviewed the study for WebMD.
Other research has shown that supplementing with vitamin D and calcium slows the progression to type 2 diabetes, Mathur says. Even so, she tells WebMD, ''At present, a direct link between vitamin D and type 2 diabetes is not conclusively established."
She has another caveat. ''One important point that is missing ... is the prevalence of vitamin D deficiency in the general population" compared to those in the study. As vitamin D deficiency is being noted with ''an alarming increase in frequency'' overall, she says, ''it may shed doubts on the authors' conclusions."
It's also possible, she says, that people with poor glycemic control have it because of a general unhealthy lifestyle, not just their low vitamin D status. They may engage in less outdoor exercise, for instance, or have unhealthy eating habits.
Because of the possible link, however, she agrees that screening for vitamin D deficiency in people with type 2 diabetes may be warranted.
This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.