Are artificial sweeteners actually bad for you?

Short answer

No. Artificial sweeteners don't cause cancer in people, and in randomized trials that swap sugar-sweetened drinks for diet versions, people lose a couple of pounds rather than gain. The observational link between diet soda and obesity is largely reverse causality and confounding: stevia, the sweetener favored by college-educated consumers, shows no weight association at all. The one group worth watching is the sugar alcohols like erythritol, which you consume in gram quantities and which will reliably wreck your afternoon at high doses.

A listener wrote in to ask whether she should switch from Diet Coke to regular Coke, because she was more afraid of the artificial sweeteners than the sugar. That question is the whole problem in one sentence. Artificial sweeteners exist because sugar isn't great for you, and the fear around them has gotten strong enough that people are willing to go back to the sugar.

So let's take the fears in order.

The observational data says diet soda makes you fat. The observational data is wrong.

Start with the association, because it's real and it's big. Across large cohorts, millions of people, artificially sweetened beverage consumption is consistently linked to higher rates of obesity and type 2 diabetes. That is about as established as an association gets.

But before the epidemiology, do the plausibility check. I am a believer in the laws of thermodynamics. Weight change is fundamentally driven by calories in versus calories out. I know that's a bit pat, like saying you get rich by earning more than you spend. Technically true, doesn't capture everything. Still, sucralose and aspartame deliver zero calories. If they cause weight gain, they have to do it by bringing in calories some other way, because they aren't calories themselves.

The proposed mechanisms are thin. Maybe they alter the microbiome in a way that changes calorie absorption, which is a lot of hand-waving on top of a field we don't understand well. Maybe there's something to sweetness tolerance, which I do think is a real phenomenon. Emily has chastised me for how much Splenda goes into my coffee, and I wonder whether my kids would prefer fruit over Kit Kats if they'd had fewer Kit Kats. But none of that gets you to "a Coke and a Diet Coke contribute equally to your weight."

What does explain the association is a pair of problems that artificial sweeteners illustrate better than almost any other exposure in nutrition.

The first is reverse causality. People start drinking diet soda because they want to lose weight. So out in the world, people struggling with obesity are more likely to be drinking diet soda, and the weight is causing the soda, not the other way around. Physicians know this as confounding by indication, the same reason people who take baby aspirin have more heart attacks than people who don't. We tell them to take it after the heart attack. The economists' term is better, as usual.

The second is what we call residual confounding and economists call omitted variable bias. Artificial sweetener consumption is a lifestyle marker. It tracks with lower education and lower income, and with smoking and less physical activity, all of which affect health on their own.

Here's the detail that closes the case for me. Sweetener choice is socially stratified. Emily ran the data herself before we recorded: about 53% of stevia consumers have a college degree, versus about 30% of people using other sweeteners. Stevia is the bougie sweetener. And in the observational data, stevia is not associated with weight gain, while the others are. Same lack of calories, same sweet taste, different customers, different result.

Now do the randomized trials, which is how you actually settle this. When you replace sugar-sweetened beverages with artificially sweetened ones, people lose a little weight. A couple of pounds. The trials are short, and some of the adolescent studies are really substituting water rather than diet soda. But that's exactly what you'd predict from pulling 250 calories a day out of someone's diet.

So, to the listener: no, don't switch to regular Coke. If you're worried about your weight, don't drink your calories. Drinking calories is just too easy a way to take in too many of them. The exception is when calories are the point. People in the middle of an ultramarathon drink Coke, not Diet Coke, because they need the glucose to finish.

The cancer question, and the genuinely ugly history behind it

First, how sweetness works, because it explains why these molecules exist at all. Taste is receptors on your tongue talking to nerves talking to your brain. The sour receptor is essentially a hydrogen channel, so acid goes through and your brain calls it sour. The salt receptor does the same with sodium. Sweet is more elaborate: a two-protein dimer that, when something binds it, opens a channel and lets calcium in. That's the sweet signal. Sucrose does it. Fructose does it. And like any receptor, you can find other chemicals that bind it.

Almost none of the artificial sweeteners were designed to do that. The history of artificial sweeteners is the history of bad lab hygiene. Saccharin, 1879: Constantin Fahlberg was working on coal tar derivatives, went to dinner, and noticed his hands tasted sweet. Aspartame, 1965: James Schlatter licked his finger to pick up a piece of paper while trying to synthesize an anti-ulcer drug. Sucralose, 1976: Shashikant Phadnis was working on a chlorinated sugar when a colleague asked him to test it, he heard "taste it," and he did. Wash your hands, everyone.

My favorite receptor story isn't a sweetener at all. Miraculin, from miracle fruit, binds the sweet receptor without activating it, and binds tightly, which is why the effect lasts about an hour. Drop the pH in your mouth and the miraculin changes shape and opens the channel. Put acid in your mouth and your sweet receptors fire. The sour receptors still work normally, so lemon juice tastes like lemonade rather than pure sugar. People throw miraculin parties with pickle juice and lemons, and a Guinness tastes like a milkshake. People have also given themselves mouth ulcers drinking vinegar because it tasted fine. Don't go crazy.

Now aspartame, because the regulatory process was bad. Searle discovered it in 1974 and the FDA approved it fast. Then the approval was frozen after consumer attorneys raised concerns about related compounds and brain tumors. The FDA investigated, found the animal testing was poor, and requested a criminal investigation of the company over possibly falsified test results. Criminal investigations of pharmaceutical companies are very unusual in this country.

Then the US attorney investigating the case left government and took a job with Searle. The investigation stalled and ran into the statute of limitations. The FDA, facing predictable public anger, convened an independent panel to review the aspartame data, and that panel recommended against approval pending more brain tumor data. A year later, Ronald Reagan appointed Arthur Hayes as FDA commissioner, and Hayes overruled the panel. Aspartame was approved. Hayes later left the FDA for a public relations firm whose client list included Searle.

That is classic revolving-door corruption. It is not, however, the molecule's fault. The brain tumor data eventually arrived, replicated many times over, and aspartame does not cause brain tumors. There is arguably nothing in the nutrition literature more thoroughly studied than aspartame toxicology, and it does not appear to cause cancer. But the origin story stuck, the way the early mistreatment of Lyme disease patients still fuels Lyme conspiracy theories decades later.

The narrative caught fire in 1998, well after the follow-up studies came back clean, via a chain email attributed to a talk by "Nancy Markle" at the World Environmental Conference. The email claimed aspartame causes multiple sclerosis, lupus, chronic fatigue syndrome, seizures, brain tumors, Alzheimer's disease, blindness, birth defects, and death. Some versions added Gulf War syndrome. It was heavy on scientific jargon. It described audience members standing up mid-talk to say they'd been harmed, which is the 1998 equivalent of "and then everybody clapped." Nancy Markle does not exist. Neither does the conference. Reporters have traced the email to an activist named Betty Martini, who styles herself Dr. Betty Martini on the strength of an honorary humanities doctorate from an unaccredited religious institution. Read the claim list again and tell me it doesn't have the exact cadence of an Instagram wellness reel.

The other recurring move is chemical scariness. Aspartame is a dipeptide held together by a methyl ester. Break it down and you get aspartate, phenylalanine, and methanol, and methanol converts to formaldehyde. All true. Then put numbers on it. Break down all the aspartame in a 12-ounce diet soda and you get about 20 mg of methanol. A typical alcoholic drink has about 35 mg. Fresh-squeezed fruit juice has about 15 mg, and after a few hours sitting around, about 30 mg. Yeast make methanol alongside ethanol, which is part of why distilleries work so hard to separate the two. We evolved eating fruit that ferments. Your body processes small amounts of methanol routinely. The dose makes the poison.

Saccharin's bladder cancer warning label, added in 1977 and removed in 2000, has a similar shape. Rodents got Herculean doses, and male rats formed saccharin crystals in the bladder that irritated and damaged the epithelium, which raised cancer risk. Not in the females. Not in other species. It was a quirk of that animal's urinary tract. It doesn't happen in people, which is why the label came off. The rodent bioassays that generated the later carcinogenicity signal have been widely criticized for design and interpretation problems. And the doses involved are vastly higher than any human, even a devoted one, could consume.

Sugar alcohols are the different case

The high-potency sweeteners are hundreds of times sweeter than sugar, so you use a pinch. A diet soda has about 200 mg of aspartame. To reach 30 grams you'd need something like a hundred cans.

Sugar alcohols are not like that. Erythritol, xylitol, maltitol, lactitol, all the -itols, are actually about 70% as sweet as sugar. You have to load them in. A single serving of keto ice cream can get you 30 grams of erythritol without trying.

That difference in dose is why I'm slightly more cautious here as a matter of principle. The high-potency molecules are engineered to hit one receptor extremely well, and at a 200 mg dose the odds of them binding something else and producing off-target effects are lower. Not zero. Lower. When you need a hundred times the dose to hit the same receptor, there's a lot more molecule floating around to do other things.

The unambiguous effect is gastrointestinal. Sugar alcohols are largely not absorbed, and because you're taking in grams of the stuff, there's enough sitting in the intestinal lumen to pull water in behind it. They're osmotically active, like those desiccant packets in pill bottles. Which you also should not eat. This is why sugar-free gummies have a reputation, and the Amazon reviews for sugar-free Haribo gummy bears are a genuine literary genre. "These should come with a hazmat suit and a formal apology to your plumber." "I am a different person than I was 20 minutes ago." "A gummy-bear-shaped vessel for pure, unadulterated intestinal warfare." If you're constipated, a bag of sugar-free peach gummies works. It's like fiber, but very fast.

On erythritol specifically, there's an early signal worth saying out loud. A 2023 Nature Medicine study randomized 20 healthy volunteers in a crossover design to glucose or erythritol and measured thrombosis potential and platelet reactivity. Erythritol increased both. Twenty people, and both endpoints are surrogates. There are many steps between platelet reactivity and an actual clinical event. Later work has suggested similar effects for xylitol. I'm not telling you to panic. I'm telling you this is the one corner of the field where I'd want more data.

One practical warning. I went through my pantry and found monk fruit sweetener. Monk fruit's active sweetener, mogroside, is a high-potency compound, roughly 150 to 250 times sweeter than sugar. The first ingredient on my monk fruit sweetener was erythritol. Monk fruit extract was second. So I was buying a sugar alcohol with a bougie label on it. Worth reading the ingredient list.

Which brings up the last thing about the fancy sweeteners. Stevia and monk fruit read as natural, and it's true you can chew a stevia leaf and taste something faintly sweet. Getting from the leaf to the white powder in your coffee is an extensive industrial process. These are highly processed products with better marketing. You're paying for the packaging.

Bottom line

Artificial sweeteners aren't health food, but they are not something to be afraid of. The cancer worry doesn't hold up, and the observational link to obesity and diabetes is largely explained by who drinks diet soda and why they started. Randomized trials point the other direction: swap sugar-sweetened beverages for artificially sweetened ones and people lose a couple of pounds. If weight is your concern, don't drink your calories, and don't trade your Diet Coke for a regular one on the theory that sugar is the safer option.

Two real caveats. Sugar alcohols are a different animal, consumed in gram doses, with guaranteed GI effects and an early, unsettled signal on clotting for erythritol. And sweetness overload is probably worth thinking about. It might be useful to retrain your palate to enjoy things that aren't quite so sweet, which is advice I am ignoring in my own coffee as I type this.

Here's our discussion from the episode:

I covered this in depth on Wellness, Actually. Listen below.

Wellness, Actually Podcast

"What's the deal with artificial sweeteners?" — Listen to the full episode, including the week's health news and listener Q&A.

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