The Truth About Sugar-Free and Diet Drinks

Start with what is not controversial. A regular can of cola carries around 35 grams of sugar and roughly 140 calories. The diet version has near zero of both. If you drink one or two sugary drinks a day, swapping to the sugar-free versions removes a large, easy chunk of calories and free sugars without changing anything else about your diet.

It matters for your teeth too. Sugar feeds the bacteria that cause decay, and sipping sugary drinks across a day bathes your teeth in it repeatedly. Sweeteners do not feed those bacteria the same way, so the switch is a clear dental win. For someone trying to cut sugar without feeling deprived, a diet drink can be the difference between sticking with a change and giving up on it. This is the strongest case for them, and it is a good one.

In 2023 two arms of the World Health Organization reviewed aspartame, the sweetener in many diet drinks. The cancer-research agency classified it as possibly carcinogenic to humans, a category that signals limited and uncertain evidence rather than proof of harm. The same category includes things like aloe vera extract and pickled vegetables. It describes how confident scientists are that something could pose a hazard, not how dangerous it is at the doses people actually consume.

The other body, which assesses real-world risk, looked at the same evidence and kept the acceptable daily intake unchanged at 40 milligrams per kilogram of body weight. To put that in human terms: a 70 kg adult would need to drink somewhere between 9 and 14 cans of diet drink every day, with no other source of aspartame, to exceed that limit. Almost nobody comes close. The headline said cancer, the actual finding said the evidence is thin and the safe limit stands.

This is where the picture gets less flattering. In 2023 the World Health Organization issued guidance advising people not to use non-sugar sweeteners as a way to control body weight. Reviewing the long-term evidence, it concluded that sweeteners do not reliably help people lose fat over time, and it flagged possible associations with a higher risk of type 2 diabetes and heart disease in the longer term.

Read that carefully, because it is easy to misread. Those associations come mostly from observational studies, which struggle with a basic problem: people already worried about their weight or health are more likely to choose diet drinks in the first place. That makes it very hard to tell whether the drinks contribute to the risk or simply mark out people who were already at higher risk, a trap called reverse causation. The safe reading is not that diet drinks cause disease. It is that they are not a weight-loss tool, and swapping sugar for sweetener while everything else stays the same rarely moves the scale on its own.

A popular claim is that sweeteners keep your sweet tooth alive, or even trick your body into storing fat by promising sugar that never arrives. The evidence for this is much weaker than the confidence with which it gets repeated. Controlled studies have not shown that diet drinks reliably drive people to eat more sugar overall, and the metabolic trickery theory has not held up well in humans.

That said, there is a fair behavioural point buried in the myth. Drinking sweet things all day, sugary or not, keeps you accustomed to a high level of sweetness, which can make unsweetened water, tea, and coffee feel bland by comparison. If your long-term aim is to enjoy less sweetness across the board, leaning heavily on diet drinks can quietly work against that goal, not through any chemical effect but through simple habit.

Put it all together and diet drinks land in a sensible middle. They are clearly better than sugary drinks and clearly worse than water. Use them as a bridge, not a base. If you are cutting down from several sugary drinks a day, a diet drink is a smart, low-effort swap that saves real calories and protects your teeth while you build steadier habits.

• If you drink sugary drinks now, switching to diet versions is a genuine upgrade worth making today.
• If you already drink diet drinks, there is no need to panic about safety at normal amounts.
• If your goal is fewer cravings, treat diet drinks as a step toward water and unsweetened drinks, not the finish line.
• Do not count on them for weight loss. What you eat and drink around them decides that, not the swap itself.

The best drink for almost everyone remains water, with unsweetened tea and coffee close behind. Diet drinks are a reasonable, safe-in-normal-amounts tool for getting off sugar, and that is exactly how to use them: as help on the way somewhere, not as the place you want to end up.