Vitamins and supplements are big business in the US Half of Americans routinely take them to the tune of about $23 billion dollars every year.

Yet the science demonstrating health benefits to support such widespread use is often contradictory or lacking. In the case of a large NIH-backed study published in the journal Diabetes Care, the science shows little benefit, at least in reducing an adult’s risk of developing type 2 diabetes.

An international group of researchers from the US and China were interested in exploring the potential benefits of regular vitamin and supplement use in reducing a person’s likelihood of developing diabetes. Existing research had suggested that some of the same biological mechanisms involved in developing both heart disease and diabetes might be offset by antioxidant vitamins and minerals.

To see whether vitamins could protect against type 2 diabetes, researchers from the National Institutes of Health, AARP, Harvard Medical School and the Chinese Academy of Medical Science analyzed health data from 232,007 participants in the NIH-AARP Diet and Health Study, gathered in 1995-1996 and followed up in 2000.

This large group of older Americans, ranging from 50 to 71 years old and all diabetes-free at the start of the study, answered questions about regular vitamin and supplement use, general health, weight, race, age, gender, education, marital status and lifestyle habits such as exercise, diet, and smoking.

More than half of the participants said they took multivitamins and/or supplements routinely and most of those routine users took them daily.

By the end of the study period in 2000, 14,130 cases of diabetes had been diagnosed among the participants.

After taking traditional diabetes risk factors into account, the lead researcher, Dr. Yiqing Song of Brigham & Women’s Hospital in Boston and his colleagues compared the vitamin users and non-users and concluded that taking multivitamins neither increased nor reduced “future diabetes risk.”

Of note, however, was the finding that users of either vitamin C or calcium had a lower risk of diabetes than non-users, they wrote.

“This result was surprising,” Dr. Song told Reuters Health.

“The evidence suggests a benefit but the evidence is marginal” and a more powerful clinical trial is needed to confirm what can only be a suggestion from this observational study, he said.

“There’s a possibility that (calcium or vitamin C) might protect, but we don’t know for sure, we need more data,” he said.

SOURCE: Diabetes Care, online October 26, 2010.