The Diet Coke Weight-Gain Paradox

Artificial sweeteners probably don't cause weight gain, when used strategically.
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As pervasive as aspartame, sucralose, stevia, and other low-calorie sweeteners have become, understanding of their health effects is relatively basic. Not to be scary about it; most signs point to no serious concerns, and eating too much sugar does point to serious concerns, and life demands sweetness. But the scale on which low-calorie sweeteners been introduced to our diets in recent decades is massive, and they are evolutionarily novel, so it's good to keep an eye on the latest research on what these chemicals are doing to us, which is this. 

The September issue of The American Journal of Clinical Nutrition just published a meta-analysis of the existing research on artificial sweeteners and weight gain. The conclusion lands in support of artificial sweeteners in the right context, specifically when they are substituted for sugar. People tend to see "modest weight loss," suggesting that low-calorie sweeteners (LCSs) indeed "may be a useful dietary tool to improve compliance with weight-loss or weight-maintenance plans."

That might seem obvious, but several studies have suggested that eating/drinking these nutritive sweeteners actually leads to weight gain. That has to do with satiety signals, effects on insulin levels, changes in the body's fluid balances, and other not-immediately-apparent downstream factors.

In the 1970s, one study of 31,940 women found that saccharin users gained more weight than non-users. A 2008 study titled "Fueling the Obesity Epidemic? Artificially Sweetened Beverage Use and Long-Term Weight Gain" looked at 3,682 participants in the famous San Antonio Heart Study and found that people who used artificial sweeteners gained more weight than non-users over the course of the trial. And a widely cited prospective study of 78,694 middle-aged women in the 1980s found that those who used artificial sweeteners were more likely to gain weight.

Those are all just correlations, but consuming artificial sweeteners in isolation has also been shown to make people hungrier later on. Dr. Barry Popkin, a distinguished professor of global nutrition at the University of North Carolina, wrote in a recent literature review that since most artificial sweeteners aren't consumed in isolation, that's not really an issue. So the key distinction in studying and using these sweeteners is the idea of replacement as opposed to addition. 

In addition to the sweeteners I mentioned, the U.S. Food and Drug Administration also allows use of acesulfame potassium, luo han guo extract, neotame, saccharin, erythritol, hydrogenated starch hydrolysates, isomalt, lactitol, maltitol, mannitol, sorbitol, and xylitol; other countries also use cyclamate, thaumatin, neohesperidin dihydrochalcone, alitame, and polyglycitol syrup. More are on the way. Still the largest randomized-control trial to look at weight gain as a function of replacement of sugar with low-calorie sweeteners involved 641 people, which is not a ton of people, and the study only lasted 18 months, which is not a ton of months. It did find that replacing sugar-sweetened beverages with low-calorie sweetened beverages reduced weight gain and fat accumulation in normal-weight children. 

"It would not be expected for a single dietary change, i.e., replacement of sugar with low-calorie sweeteners, to cause clinically meaningful weight loss," the current study reads. Weight management is really about overall dietary and lifestyle patterns. But it's worth considering if you think of an afternoon Diet Coke as a bonus, as opposed to replacing a regularly scheduled Coke heavy.

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James Hamblin, MD, is a senior editor at The Atlantic. He writes the health column for the monthly magazine and hosts the video series If Our Bodies Could Talk.

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