Diet Soda Makes People Fat. Here’s Why.

Here’s an interesting bit of science. There are taste receptors that are sensitive to “sweet” substances in the intestine ( They are similar to the ones on your tongue. When the intestinal receptors interact with sugar in consumed foods, they trigger a release of insulin from the pancreas. Then as you start absorbing glucose from a sweet meal, there is insulin waiting to drive that glucose into fat and muscle cells, getting preemptive control of a potential spike in blood sugar. That’s good, but it turns out that just like sweet receptors on the tongue, the intestinal sweet receptors respond to non-caloric artificial sweeteners the same as they do to real sugar.

This insulin release in response to artificial sweeteners explains the odd fact, first reported at least ten years ago, that people who drink diet soda are fatter than people who get the same daily calories but not the diet soda.

Insulin has three effects that contribute to fat gain. 1) When insulin is circulating, fat and muscle tissues take up sugar from the blood. If glycogen stores are full or nearly full, most of the sugar taken up goes to make new fat. Even if muscle glycogen stores are low, much of the circulating blood sugar goes to make fat. So, effect 1 is that blood sugar is driven into fat when one drinks diet soda. 2) Reduced blood sugar registers as hunger, even if fat and muscle cells contain plenty of stored energy. So, effect number two is that drinking diet soda makes one hungry. If one has sweet snack to cure the hunger or washes down a healthy snack with diet soda, the cycle starts again. 3) Insulin suppresses fat metabolism. For the next three hours or so after an insulin release, less fat will be metabolized than if that insulin release had not happened. One study I looked at found that in a long exercise session after an insulin release, roughly 19 grams less fat are metabolized than if one exercises without that insulin release. 19 grams a day for a year makes about two pounds of fat. So, effect number three is that drinking diet soda not only makes one make more fat, it also prevents one from using fat that is already stored.

Of course, having actual sugar-sweetened soda or anything else that delivers a big dose of sweet-tasting experience has a similar effect. I’m not saying that diet soda is worse than regular, but that it is almost as bad. I’ll stick with sparkling water.