Vitamins and supplements are big business, 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 online in Diabetes Care, the science shows little benefit, at least in reducing an adult's risk of type 2 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.
Type 2 diabetes
To see whether vitamins could protect against type 2 diabetes, researchers analysed 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 cohort 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.
Diabetes risk factors
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 said.
"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.
(Reuters Health, Rachael Myers Lowe, November 2010)