Are artificial sweeteners and diet soda bad for you or do they cause weight gain?

The myth hides two questions, and they have different answers.

Do they make you fat? Here the intervention trials are fairly consistent. Rogers and colleagues pooled human and animal studies and found that low-energy sweeteners do not increase energy intake or body weight. In short-term trials they produced on average 94 kcal lower intake than sugar-sweetened products, and no measurable difference versus water. In longer trials, people who swapped sugar for sweeteners weighed on average 1.35 kg less. The idea that sweetness tricks the brain into eating more does not show up in the pooled trial data.

Are they harmful over time? Here the picture is more divided. Toews and colleagues' review in the BMJ found no clear health differences between people who used and did not use sweeteners, but the underlying studies were small, short and of low quality. In 2023 the WHO issued a conditional (weak) recommendation not to use sweeteners for weight control, because they showed no long-term reduction in body fat and because observational studies have linked them to higher risk of type 2 diabetes and cardiovascular disease. That link is hard to read: people already gaining weight or with rising blood sugar often switch to diet products, which makes the sweetener look like a cause rather than a consequence. In short: the claim that they make you fat does not hold, while their long-term health effects are not yet fully settled.

Two things drive the myth. One is that 'unnatural' feels dangerous: a lab-made compound with a long chemical name draws more suspicion than sugar, which we recognise, even though sugar is the known risk here. The other is headline logic. An observational study linking diet soda to diabetes makes a dramatic headline, while 'reverse causation makes the association hard to interpret' never spreads the same way. On top of that, the sugar industry benefits from doubt about its competitors, and some health influencers build reach by naming a hidden villain. A neutral 'they look safe but we aren't fully certain long term' sells neither supplements, nor fear, nor clicks.

If a diet soda helps you drink less sugar-sweetened beverage, that's a sensible trade, not an own goal. Sweeteners are a tool for cutting sugar intake, not a health supplement you need to seek out. If you already drink mostly water, there's no reason to start. If you treat diet products as a licence to eat freely otherwise, that's where the problem sits, not in the sweetness itself. To see how the calories you save land against your goal, calculate a deficit rather than guessing.