New analysis suggests ‘diet soda paradox’ – less sugar, more weight


Diet soft drinks are certainly as popular as the sugar-laden versions nowadays, but are the people who are drinking them getting any thinner? It may be exactly the opposite.

Statistics from the San Antonio Heart Study, a quarter-century-long community-based epidemiologic study conducted at The University of Texas Health Science Center at San Antonio, paradoxically suggest that the more diet sodas a person drinks, the greater the chance that he or she will become overweight or obese. Extra weight is a strong risk factor for the development of type 2 diabetes.

“On average, for each diet soft drink our participants drank per day, they were 65 percent more likely to become overweight during the next seven to eight years, and 41 percent more likely to become obese,” said Sharon Fowler, M.P.H., faculty associate in the division of clinical epidemiology in the Health Science Center’s department of medicine. She presented the finding June 12 in San Diego at the American Diabetes Association’s 65th Annual Scientific Sessions.

Can diet soft drinks increase your risk of becoming overweight?

Fowler’s colleague, Ken Williams, M.S., assistant professor of clinical epidemiology, analyzed questionnaire and health outcome data from participants in the San Antonio Heart Study, which began in 1979, to obtain the results being reported this week. Michael P. Stern, M.D., professor and chief of clinical epidemiology at the Health Science Center, is the principal investigator of the study and a co-author, along with Kelly Hunt, Ph.D, Helen Hazuda, Ph.D., and Roy Resendez, M.A.

Three years ago, Fowler and Williams began analyzing data from a group of 1,177 participants who were either normal-weight or overweight (but not obese) at baseline. Upon entering the study, these individuals were asked how many cans or bottles of soft drinks they usually drank per week, and whether they usually chose sugar-free soft drinks, regular soft drinks, or about half and half of each. The results of early analyses of these data were surprising.

“I was expecting to see that drinking regular soft drinks increased the risk of becoming overweight or obese – and that did seem to be the case early on,” Fowler said. “But it didn’t matter whether people were drinking diet or regular soft drinks: drinking sodas of any kind seemed to increase the risk of weight gain. In fact, drinking diet soft drinks seemed to be much more closely related to the incidence of becoming overweight or obese. That didn’t make sense to me at the time, and I set the results aside.”

But the unexpected finding was intriguing, and the investigators revisited their analyses this year. This time, Williams detected a “dose-response” effect: those who drank the most diet soft drinks had the highest incidence of weight gain. This was a particularly interesting finding. After adjusting for age, sex and ethnicity, Williams found that regular soft drinks were no longer significantly linked to the incidence of becoming overweight or obese, but diet soft drinks were.

Fowler’s job is to try to interpret these results.

“It may be that normal-weight people in our study whose weight had been increasing had switched to diet soft drinks in an attempt to stop their weight gain,” she said. “That’s a very real possibility. Another is that drinking soft drinks, either regular or diet, is part of a lifelong ‘Obey your thirst’ nutritional pattern that sets a person up for weight gain later in life. Whatever the case, our results definitely raise more questions than they answer.”

But Fowler pointed out that whenever someone is drinking a diet soda, he or she is drinking it to the exclusion of healthier alternatives such as milk, water, or juice. “Can you think of one good thing that comes from a diet soft drink can for your body? You’re giving yourself the taste of nourishment without any at all, so it may be that you then seek it from other foods, such as high-calorie desserts,” she said. “Even though you fool your tongue, you don’t fool your brain. It is not satisfied. I’ve seen people plunk down a doughnut and a diet soda on a convenience store counter. What our analyses indicate for sure is that drinking diet soft drinks will not protect a person from the health effects of the rest of his or her lifestyle.”

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