Diet soda and weight gain: The connection may be, in a way, what you think


Diet soda may indeed be associated with weight gain, as a new study suggests, but the fault may lie in your head, not necessarily your metabolism.

In a study that has sparked headlines along the lines of “Diet soda makes you fat,” researchers found that people who drank diet soda for nearly a decade gained more stomach pudge than diet-drink abstainers.

The study wasn’t huge or broad, assessing only 474 elderly participants from the San Antonio Longitudinal Study of Aging. But it concluded that those who drank two or more diet soft drinks a day had the largest waistline increases—about five times more than that of non-drinkers. The results, which haven’t been published or peer-reviewed, were announced this week at a meeting of the American Diabetes Assn. in San Diego.


The researchers, from the University of Texas Health Science Center at San Antonio, were fairly measured, writing in their abstract:

“Data from this and other prospective studies suggest that the promotion of diet sodas as healthy alternatives may be ill-advised: they may be free of calories, but not of consequences.”

Note that this doesn’t mean artificially sweetened sodas make you gain weight. (See “For the record” below.)* As Richard Mattes, a nutrition scientist at Purdue University, points out: Heavy people simply might choose to consume diet drinks more.

Mattes has studied how artificial sweeteners affect appetite and food intake. He believes that many studies reporting a link between diet soda and weight gain are actually hitting on a behavioral phenomenon—people think they can eat more calories because they’ve swapped their regular soda for a Coke Zero.

“That’s not a fault of the product itself, but it’s how people chose to use it,” he says. “Simply adding them to the diet does not promote weight gain or weight loss.”

The recent study didn’t track how many calories people consumed, though it did consider age, sex, education, neighborhood, diabetes status, leisure activity level and smoking status.


Study co-author Sharon Fowler, an epidemiologist, agrees – to a point. Yes, the diet drink association is partly psychological, she says, but she also believes there could be physiological explanations for why chemicals in diet sodas could lead to weight gain.

In rodent studies, researchers have observed artificial sweeteners stimulating cells in the pancreas to change insulin secretion, altering the pH in the gut, and affecting hunger-regulating cells in the brain. Fowler and colleagues also reported this week that diabetes-prone mice exposed to aspartame for three months had higher blood-sugar levels than those not exposed at all.

Previous studies have found links between obesity and diet drink consumption.

“My feeling is there is something going on here,” says Fowler.

But rodent studies and associations are a far cry from causation.

Dariush Mozaffarian, a Harvard epidemiologist who recently studied how a long list of food and drinks contribute differently to weight gain, says zero-calorie soda could be a good choice for people trying to lose weight, calling it an “intermediate step” to going “cold turkey” with sweets and soda.

“Artificial sweeteners may be a good short-term option to bridge people away from refined sugars, but consuming moderate or high amounts long-term should be avoided as possible,” he wrote in an email.

The main issue, he said, is again, less about biological reactions than about our perception of sweetness: “We don’t know how consuming artificially sweetened drinks and foods alter tastes and preferences for other healthy foods, especially in children. For example, do apples or carrots taste just as sweet to someone who consumes high amounts of artificial sweeteners?”

Obviously, the mind can play tricks on the tongue.

Mattes said when people are given either regular or diet products, but aren’t told which they received, they generally reduce their total number of calories.


This suggests one strategy for those who don’t want to switch to water or unsweetened coffee or tea: When you drink diet soda, try to convince yourself it’s regular.

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For the record: An earlier version of this story said: “Note that this doesn’t mean sweetened sodas make you gain weight.” Obviously, we meant “artificially sweetened sodas.”