Are diet sodas and sweeteners bad for you, and if so, why?

Diet soda is marketed as a healthy alternative, but the research picture is nuanced. There are both possible benefits and genuine concerns, and for most risks conclusive causal evidence in humans is still lacking.

When it comes to weight and blood sugar, the picture does not look bad. Long-term use of sweeteners appears to be beneficial for body weight and blood sugar regulation in people with type 2 diabetes, and is better for your teeth than regular sugar¹. Some review studies also find no measurable negative effect on weight or blood sugar².

For cancer, the signals are more worrying. A large French study with nearly 103,000 participants linked above-average intake of artificial sweeteners to a 13% higher risk of cancer. Aspartame was associated with a 15% higher overall cancer risk and a 22% higher breast cancer risk; acesulfame-K with a 13% higher cancer risk³. The researchers themselves cannot fully rule out confounding factors, but the scale of the study makes the findings worth taking seriously.

One possible explanation runs through your gut bacteria. Research from the Weizmann Institute showed that saccharin, sucralose and aspartame can disrupt the composition of your gut bacteria and thereby cause blood sugar problems. That effect was transferable via a gut transplant to mice without their own gut bacteria, which makes a causal link plausible⁴. A smaller study in 17 healthy adults found no effect at realistic doses after 14 days⁵. One author of that study received a honorarium from PepsiCo, which limits its independence.

Large observational studies also link sweetener consumption to cardiovascular disease, higher mortality, depression and childhood obesity⁶,²,⁷. A prospective study over 8 years among more than 12,000 Brazilians linked high intake of sweeteners, including sugar alcohols such as erythritol and xylitol, to faster cognitive decline, particularly in people under 60 years of age⁸. Here too, these are associations and confounding factors have not been fully ruled out. The fundamental problem remains: people who drink a lot of diet products also live differently in other ways. Genuine causal evidence in humans is almost entirely absent.