Two weeks ago, researchers from the University of Texas claimed that diet soda may promote weight gain, and research released Tuesday suggested that there may be a link between diet soda consumption and increased vascular events (heart attacks and strokes). Such claims about the sugar-free beverages have the potential change the way Americans look at dieting and weight loss.
There's one problem, however: they're not true. There is very little evidence to support the assertion that diet soda is responsible for weight gain or poor cardiovascular health.
Whenever scientists postulate that a certain product or behavior can harm your health, remember that they can't demonstrate it by conducting observational studies, which prove very little about the subjects they investigate. In this case, both studies simply found that people who consume diet soda may also gain weight or experience heart attacks or strokes, not that drinking diet soda causes the latter two. At first glance the association does seem to suggest a causal link, but there are any number of variables that can explain observations of this nature.
Consider a hypothetical study in which scientists discover that girls who quit high school are more likely to become pregnant than girls who graduate. One conclusion the researchers could draw is that dropping out of school causes pregnancy. Most observers wouldn't accept that conclusion because they recognize that dropping out doesn't literally lead to pregnancy; there's some other factor that better explains the situation, such as basic biology.
The same is true of the two real studies cited above. Diet soda doesn't cause weight gain or vascular events; people who consume it tend to suffer from these health problems in spite of the fact that they're forgoing sugary beverages. This is a better explanation of the data for two reasons. First, neither study found a mechanism by which the artificial sweeteners in diet soda can cause weight gain or vascular events. Several theories have been proposed over the years, but none have held up.
A recent review of the primary sweetener used in diet soda, Aspartame, found that "The weight of existing evidence is that aspartame is safe at current levels of consumption as a nonnutritive sweetener." The federal Food and Drug Administration (FDA), certainly no friend to the beverage industry, agrees. "Aspartame was first approved in the United States in 1981 and is one of the most widely used artificial sweeteners. When metabolized by the body, aspartame is broken down into two common amino acids, aspartic acid and phenylalanine, and a third substance, methanol. These three substances are available in similar or greater amounts from eating common foods."
More importantly, however, sugar unquestionably causes weight gain and damages the cardiovascular system, and the average American's diet is loaded with it. Unlike the studies linking diet soda to negative health outcomes, numerous clinical trials, which are far more reliable than observational studies, have demonstrated that avoiding sugar is one of the best ways to prevent obesity and most of its related health effects. For those reasons, sugar is the much more likely culprit than the sweeteners in diet soda.
We should always be open to the possibility that the accepted wisdom about proper nutrition is flawed. In fact, it often is. But in the case of diet soda, we don't have good enough evidence to throw out the standard advice. And no amount of shrill news coverage or unsound science will serve as a proper replacement.