Diet (?!) Soda and Obesity

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Diet sodas, which have no or very few calories, should be a boon to the overweight in their attempts to lose excess pounds. What, then, can we make of a recent report about a study purportedly showing that people who drink diet sodas gain rather than lose weight? The answer might lie in the report, rather than in biology, and is a great example of the drawbacks of presenting preliminary reports (those which have not been peer-reviewed) to the public.

The study in question was presented at a scientific meeting of the American Diabetes Society by Sharon Fowler from the University of Texas Health Science Center. After following the body weights and dietary intake of over 1500 individuals for seven to eight years, the researchers looked at the relationship between the type of soda the participants said they drank and their body weight changes.

People who reported drinking half a can of regular soda per day were 26% more likely to become overweight or obese than those who drank none. The risk of gaining weight increased even more -- to 47% -- for those who drank over two cans per day.

Oddly, however, people who reported drinking diet soda had an even greater risk of weight gain. The risk was nearly 37% for half can per day consumers, and a whopping 57% for those who drank over two cans per day.

Could this really be happening? The truth may lie in what the report failed to describe -- that is, what changes the participants made in other aspects of their lifestyles over the course of the study. Although switching from a full-calorie soda to an equal amount of diet soda will lower calorie intake, such a change is only part of the story. At the same time, a person might be increasing consumption of other foods or drinks, thus negating any decrease in calories from sodas. Or people might decrease physical activity and thus the number of calories used per day.

As reported, the story does quote a nutritionist who points out the importance of looking at the whole diet rather than just one component of it. But without appropriate scientific support, it ends by speculating that sweet, noncaloric diet drinks might make people actually want to eat more. The report cites evidence that this can occur in baby rats, but it's a far cry from young, growing animals to adult humans -- human data would have made this suggestion more convincing.

It's quite likely that there was much more to this study than was reported in the online story -- such as data on overall food intake and activity levels -- that would help the public make sense of evidence that seems to contradict common sense. But if such data were not presented, that fact should also have been part of the report to let readers know that the whole story was not presented at the meeting.

Unfortunately, reports like this dwell on the more sensational, less likely aspects of research and don't give enough context (for more on this issue, see: http://www.acsh.org/news/newsID.1131/news_detail.asp). It certainly should have indicated that the presentation was preliminary in nature.

The most realistic assessment one can make of such a report is that it's interesting, although contradictory to well-established principles of weight gain and loss. But it fails to give the consumer an appropriate context in which to evaluate the study.

Ruth Kava, Ph.D., R.D., is Director of Nutrition at the American Council on Science and Health.