People with a low level of vitamin D in the blood have poorly functioning insulin-producing cells and show a poor response to insulin, even when their blood sugar levels are normal, according to California-based researchers.

“Low vitamin D is extremely common,” lead investigator Dr. Ken C. Chiu told Reuters Health. Moreover, “our observations indicate that low vitamin D has a small but significant impact” on blood glucose metabolism and diabetes, he said.

Chiu and colleagues at the University of California, Los Angeles came to this conclusion after a study involving 126 healthy, glucose-tolerant subjects.

As reported in the American Journal of Clinical Nutrition, the team found that the lower the vitamin D concentration, the longer it took for blood glucose levels to decline during oral glucose testing.

Overall, subjects with low levels of vitamin D were more likely than did those with higher concentrations to exhibit components of the so-called metabolic syndrome, such as obesity, high blood pressure and insulin resistance.

In particular, 14 of 49 participants classified as having vitamin D were deemed at risk of the syndrome versus 9 of 79 without such deficiency.

“Now, we have one more reason to keep up vitamin D,” Chiu added. However, he also emphasized that “correction of low vitamin D itself is not sufficient in the treatment of these conditions and too much vitamin D could also be harmful.”

SOURCE: American Journal of Clinical Nutrition, May 2004.