Do unhealthy people drink diet soda? Does diet soda make people unhealthy?

Those are some of the questions raised by a surprising new study that links consumption of soft drinks -- both the sugared and diet variety -- with a higher risk for a range of obesity-related health problems. High consumption of regular soda, which contains about 150 calories a can, has previously been linked with obesity and diabetes in kids and teens, as well as high blood pressure in adults. But the finding that diet-soda drinkers faced similar health risks is unexpected, because the zero-calorie drinks are often touted as a way to help people prevent weight gain and related health problems.

The results may simply signal that the diet-soda drinkers in the study were less healthy to start with, and they had turned to sugar-free beverages to help with weight loss or because they had diabetes. The study investigators, who oversee the respected Framingham Heart Study in Massachusetts, noted also that the type of person who drinks diet soda may be more likely to eat less-healthful foods. But they also cited research suggesting that artificially sweetened beverages may affect a person's satiety or cravings for sweets.

"There have been suggestions in the literature that diet soda may not be innocuous," says Vasan S. Ramachandran, associate professor at the Boston University School of Medicine and senior investigator on the study. "We have explanations that we offer as theories, but we need additional research."

The research was immediately criticized by the soft-drink industry and some nutrition researchers. "There is no plausible physiological mechanism to explain this and causes me to question the accuracy of the methodologies used in this study," says Dean Ornish, clinical professor of medicine at the University of California, San Francisco, and chairman of the PepsiCo Health & Wellness Advisory Board.