Does Diet Soda Make You Fat?

Don't take the studies linking it to weight gain too seriously. The Daily Beast's Susan B. Roberts on the war over diet drinks—and how to make them taste better.

08.31.10 11:33 PM ET

For a dieter, the ultimate terror is hearing that the supposedly innocuous food or drink you’ve been consuming daily for God knows how long is actually a fat-filled Trojan horse. Which might be why the scare over diet soda—triggered every few years by some study that suggests that Diet Pepsi, Coke Zero, and their kin might actually make you gain weight—understandably creates some hysteria.

Controversy over whether the “high-intensity sweeteners” found in diet soda and low-calorie goods—like aspartame, sucralose, acesulfame potassium, saccharin, and Stevia—are helpful for weight control, or even safe, warrants an unbiased look at the research.

First, the good news: Reviews of safety continue to indicate that these products are safe (with one possible exception that I’ll get to shortly). While various older studies reported increased cancer risks and other significant health concerns, when biased study designs are excluded the evidence for detectable harm gets smaller and smaller. That doesn’t mean you have to use artificial sweeteners, of course—just that from the perspective of safety and health, there’s no need to feel guilty about a trip to the vending machine.

The bad news is that research on the benefits of high-intensity sweeteners for weight control is extremely conflicting. Here are two examples of studies that represent about the best out there. In one five-week study conducted in 2008, 27 rats that were provided with a saccharin-sweetened yogurt ate more total food and gained more weight than rats fed sugar-sweetened yogurt. And it wasn’t because they loved the saccharin-sweetened yogurt—in fact, they tended to eat less of it, but went back to their regular chow for second helpings more often that the rats given sugar. On the other hand, a 2004 study of 24 French men and women, also for about five weeks, found that the individuals randomly assigned to include high-calorie, sugar-sweetened beverages in their diet did the most overeating—and had no better hunger control compared to a similar group allocated to consume artificially sweetened beverages.

What to make of this? What we would all like, of course, is more research so that firm conclusions can be reached without the need to triage an inadequate number of underpowered low-budget studies. Unfortunately, it’s unlikely that a flood of good research is going to appear any time soon, because funding institutes like the NIH tend to neglect these topics, and because manufacturers of high-intensity sweeteners don’t like to risk money on independent research that might show their product in a bad light. And while we wait hopefully for more studies, based on my own experience doing comparative research on rats and baboons in nutrition graduate school, I’ll take the results from a careful study of 24 humans over a few cages of rats (however carefully studied) any day.

Using artificial sweeteners may actually make us like the real thing less over time.

I’m not taking the large “observational” studies linking diet-soda consumption to weight gain too seriously—the studies that are often behind the alarming reports about diet soda making you fat. We’ve seen too many instances where observational studies confuse cause and effect to such an extent that they appear to prove the opposite of what really happens. In the realm of diet beverages, is it any surprise, really, that people struggling with their weight are drinking more Diet Coke? Of course not. One thing these observational studies can never show: maybe those diet-soda drinkers would have gained more weight if they weren’t drinking Diet Coke.

Which brings us to the question of what exactly are high-intensity sweeteners good for, based on the best research so far? Extrapolating from the French study, it seems reasonable to conclude that they may help prevent weight gain if used wisely—for example, if you use them to replace sugary sodas that contain more calories than you would otherwise drink.

In my I Diet weight-loss program, I also see that high-intensity sweeteners are of substantial help to some, if not all, people during active weight loss. When you start to lose weight but have a history of craving sweets, going cold turkey on sweet treats usually doesn’t work, because cravings and temptation get in the way. In this case, replacing your favorite sugar fix with something sweetened with a high-intensity sweetener can stave off those cravings just well enough.

Although there are insufficient research studies available to say with certainty that high-intensity sweeteners help with weight loss as much as they appear to prevent weight gain, there is indirect evidence suggesting this may be the case. For example, magnetic resonance imaging studies tracking the brain’s responses to sugar and intense sweeteners show that in our unconscious brain we know they are different—even while we perceive both of them as “sweet-tasting” in our conscious brain. While this might seem like bad news, I view this as positive because it means we can still enjoy sweet taste without getting the neurological high that accompanies a rush of sugar calories.

In fact, over time, what seems plausible is that our brains can be retrained to learn that sweet taste doesn’t come with calories, and we can “decondition” the synthesis of addictive chemicals like dopamine that keeps us coming back for more. In other words, using artificial sweeteners may actually make us like the real thing less over time, and provide a bridge to a healthier low-calorie, low-sugar diet that still tastes sweet due to our increased sensitivity for the sugars in natural foods.

Of course, for many people, high-intensity sweeteners don’t taste the same as sugar even though they’re comparably sweet. Some people notice a bitter or cold taste in diet products that contain a single sweetener (aspartame is the one commonly used alone). My suggested fix: look for products that mix them up already (for example, Coke Zero contains two sweeteners, compared to one in Diet Coke). And when sweetening something like iced tea, make your own sweetener mix by combining two or more of the common types, such as Splenda (sucralose), Equal (aspartame and acesulfame potassium), and Sweet’N Low (saccharine) or Stevia in equivalent amounts.

If that doesn’t work, for drinks and other products you are sweetening yourself, try mixing artificial sweeteners with 5 or 10 percent sugar. The name of the game is fooling your brain into thinking that sweet taste is close to the real thing even when it’s not.

One significant qualification to my generally positive feelings about high-intensity sweeteners: In one admittedly older study of an aspartame-based product given to people suffering from depression, worsening mood was enough of a concern that the institutional review board overseeing the study stopped patient entry after only 13 subjects. That’s a big thing to happen in a research study—I have never heard of such a thing happening to anyone I know, and my institute is famous for its stringent overview process. To be sure, it’s only one study, and I would love to see it repeated, but in the meantime I’m mentioning the results to my dieters so that they can make an informed choice. Fortunately, different high-intensity sweeteners have different chemical structures and metabolic effects, so even if you are concerned that one of them affects your mental state, there is no reason to assume the same will be true of another.

I wholeheartedly endorse a natural low-sugar diet without high-intensity sweeteners. But as I tell my dieters, if the choice is between weight loss with sweeteners versus no weight loss without them, I’ll encourage the sweeteners any day.

Dr. Susan Roberts is a professor of Nutrition and professor of Psychiatry at Tufts University, and author of The “I” Diet ( which in dieters following the program in Boston is causing average sustainable weight loss of 30 pounds.