Middle-aged and older adults who drink diet soda everyday may have a heightened risk of developing type 2 diabetes, a new study suggests.

The study, of more than 5,000 U.S. adults, found that those who drank diet soda daily were 67 percent more likely than those who did not to develop type 2 diabetes over the next several years.

They also had higher odds of blood sugar elevations and weight gain around the middle, according to the researchers, led by Dr. Jennifer A. Nettleton of the University of Texas Health Sciences Center in Houston.

The findings, published in the journal Diabetes Care, add to those from previous studies linking diet soda consumption to metabolic syndrome - a cluster of risk factors for diabetes and heart disease that includes abdominal obesity, high blood sugar and unhealthy cholesterol levels.

But it remains unclear whether diet soda, per se, is to blame.

One problem is that many people who choose diet drinks may already be overweight and have a less-than-healthy lifestyle.

In their study, Nettleton and her colleagues accounted for participants’ self-reported diet habits, initial body weight, education and other health factors. And they found that the link between diet soda and diabetes risk remained.

In fact, diet-soda drinkers generally ate more whole grains, fruit and low-fat dairy, and less high-fat dairy, processed meat and refined grains than other study participants.

One theory is that artificially sweetened drinks and food may whet people’s appetites for sweets, causing them to overindulge in high-sugar fare when given the chance. Whether that is the case, however, is unknown.

The current findings, note Nettleton and her colleagues, do not prove cause-and-effect, and further research is needed to understand why diet soda and diabetes are connected.

SOURCE: Diabetes Care, April 2009.