A strawberry in a tall glass of sparkling water, bubbles clinging to the fruit

Do Diet Drinks Help You Lose Weight? What Evidence Says

8 min read

Switching from regular cola to a diet version feels like a clean win. The sugar goes. The calories go. The taste stays. For anyone in London trying to lose fat, that swap looks like the easiest change of the week.

The question is whether the switch does what people hope. Removing sugar is real progress. Yet a diet drink is not the same as water. The World Health Organization reviewed the evidence in 2023 and reached a careful answer. The short version: sweeteners help you cut sugar, but they are a step on the path, not the finish line.

~40%
of adults worldwide live with overweight or obesity (WHO, 2023)
30g
daily free-sugar limit for adults, about 7 sugar cubes (NHS, 2023)
9
sugar cubes at most in a can of cola, more than the whole daily limit (NHS, 2023)
9%
higher cardiovascular risk in higher sweetener consumers (NutriNet-Sante cohort, 103,388 adults, BMJ 2022)

Why people reach for diet drinks

Sugary drinks are the clearest source of free sugars in most UK diets. The NHS advises adults to have no more than 30g of free sugars a day, roughly 7 sugar cubes. A can of cola can hold as many as 9 cubes. One of those pushes an adult past the whole daily limit before food even enters the picture.

Free sugars add calories with no fibre and little fullness. Remove them and you remove an easy source of surplus energy. A diet drink removes that sugar load while keeping the sweet taste, so the appeal is obvious. As a way to reduce sugar, sweeteners work. The harder question is what happens to body weight and health over months and years.

What the short-term trials show

The WHO 2023 review pooled randomised controlled trials, the strongest design for cause and effect. In short trials, most lasting three months or less, using non-sugar sweeteners led to lower sugar intake, lower energy intake, and small reductions in body weight and BMI. The WHO rated that evidence as low certainty. A 2022 meta-analysis in JAMA Network Open by McGlynn and colleagues pooled 17 trials in 1,733 adults with overweight or obesity. Swapping sugary drinks for low or no-calorie versions gave an average of 1.06kg lower body weight, with small reductions in BMI and body fat. The authors described the effects as small.

That result makes sense. If you replace a sugary drink with a near-zero one, you remove calories. Over a few weeks the scale moves a little. The WHO review added an important qualifier. The benefit showed up mainly when sweeteners stood in for sugar. When sweeteners were compared with water or a placebo, the review found no effect on body weight. So the advantage comes from displacing sugar, not from any special power in the sweetener itself.

Sugar in sugar cubes: cola against the daily limit and diet cola
9 7 0 Regular cola NHS daily limit Diet cola
Source: NHS, Sugar: the facts, 2023. One can of cola can hold up to 9 sugar cubes against a 7-cube daily limit.

What the long-term evidence adds

Short trials tell only part of the story. Weight loss counts when it lasts. Here the picture shifts. The WHO 2023 review found no evidence of a long-term benefit on body fatness from sweetener use in adults or children. On that basis the WHO suggested that non-sugar sweeteners not be used as a way to control weight or to reduce the risk of noncommunicable diseases.

The observational data raised further caution. In large cohort studies with an average follow-up near 13 years, higher sweetener intake was linked to a higher risk of type 2 diabetes, cardiovascular disease, and death from any cause. The certainty of this evidence was very low to low, and cohort studies cannot prove cause. Reverse causation is one plausible explanation, since people already at higher risk often switch to diet products. Still, the pattern was consistent enough for the WHO to weigh it.

One of the largest single studies came from the NutriNet-Sante cohort in France, reported in the BMJ in 2022. Across 103,388 adults, higher intake of artificial sweeteners was associated with a 9% higher risk of cardiovascular disease, and an 18% higher risk of cerebrovascular events such as stroke. These are associations, not proof, and the absolute differences were small. The cardiovascular event rate was 346 per 100,000 person-years in higher consumers against 314 per 100,000 in non-consumers. The associations are still why health bodies no longer treat sweeteners as a free pass.

The WHO drew a wider point from all of this. Non-sugar sweeteners are not essential in the diet and carry no nutritional value of their own. Swapping sugar for a sweetener leaves the overall quality of the diet largely unchanged, since the sugary product is often processed to begin with. The review named a better route: replace sugary items with naturally sweet foods such as fruit, along with unsweetened foods and drinks. That path improves the diet rather than leaving it in place. The recommendation itself is conditional, which in WHO terms reflects the low certainty of the evidence behind it rather than a firm verdict against sweeteners.

Are sweeteners safe to drink

Safety and weight benefit are two separate questions. On safety, the sweeteners on UK shelves have passed toxicology review, and typical intakes sit well within the acceptable daily intakes set by the Joint FAO and WHO Expert Committee on Food Additives.

Aspartame drew headlines in July 2023. The International Agency for Research on Cancer classified it as possibly carcinogenic to humans, its Group 2B category, based on limited evidence for liver cancer. In the same week the expert committee on food additives reviewed the risk and kept the acceptable daily intake at 40mg per kilogram of body weight. To reach that limit, a 70kg adult would need to drink between 9 and 14 cans of a diet soft drink every day, depending on the product, and from no other source. For typical use, regulators judged aspartame within safe limits. Group 2B signals a call for more research, not a proven everyday danger.

How to use diet drinks well

Read together, the evidence points to a practical middle path. Diet drinks beat sugary drinks for anyone cutting a heavy sugar habit. They are not something to build a diet around. Use them as a bridge, then keep moving toward drinks with no downside.

Step 1. Swap sugar first
If you drink sugary cola or energy drinks daily, move to the diet version. You remove a large sugar and calorie load in one change.
Step 2. Make water the default
Set water, sparkling water, or unsweetened tea and coffee as your main drinks. Keep the diet drink as an occasional swap, not the base of your day.
Step 3. Watch the food around it
A zero-calorie drink does nothing for a high-calorie meal. Fat loss still rests on total food, protein intake, and daily movement.
Step 4. Retrain the sweet habit
Dilute squash with sparkling water, then reduce the sweetness over time. The goal is fewer sweet drinks of any kind, not a like-for-like sugar substitute forever.

Common mistakes to avoid

The first mistake is treating a diet drink as a health food. It is a lower-sugar version of a treat, nothing more. The second mistake is drinking more of it because it feels free. Two or three litres of diet cola a day is still a habit worth trimming, and it crowds out water and other drinks that carry real benefit. The third mistake is ignoring the meal. People often reward a diet drink with extra food, which cancels the saved calories. Keep the drink change and the food change separate in your mind, and let each one earn its place.

Frequently asked questions

Do diet drinks break a fast?

A plain diet drink has close to zero calories and no sugar, so it adds almost no energy. If you fast for weight control, water remains the cleanest choice, with a diet drink as a backup.

Will switching to diet cola make me lose weight on its own?

In short trials, replacing sugary drinks with diet versions produced small weight loss (WHO, 2023). The WHO found no evidence of a long-term benefit on body fat, so the switch helps only as part of a wider change to food and activity.

Is aspartame dangerous?

In 2023 the cancer research agency rated aspartame as possibly carcinogenic on limited evidence, while the food additives committee kept the safe daily intake at 40mg per kilogram. A 70kg adult would need between 9 and 14 cans a day, depending on the product, to reach that limit.

Are diet drinks worse than regular cola?

For sugar and calories, diet versions are the better pick. Cohort studies link high sweetener intake to higher cardiovascular risk (BMJ, 2022), and the WHO rated that wider body of observational evidence as very low to low certainty. For most people, replacing sugary cola with a diet version is a sensible first step, with water as the destination.

What should I drink instead?

Water, sparkling water, and unsweetened tea or coffee cover most needs. Keep fruit juice and smoothies to 150ml a day, since they still count as free sugars under NHS guidance.

The bottom line

Diet drinks solve one problem well. They remove the sugar and calories of sugary drinks, and short trials show a small weight benefit when they replace sugar. They do not carry a proven long-term advantage, and cohort data hint at reasons to keep intake modest. Use them as a bridge away from sugary drinks, make water your default, and put your real effort into food, protein, and movement. That is where lasting results come from.

Want a plan built around your real habits
DT Fitness London builds nutrition and training around your week, not a template. Book a consultation at www.dtfitnesslondon.com and turn small swaps into lasting change.

Sources

  • World Health Organization. Use of non-sugar sweeteners: WHO guideline. Geneva, 2023. ncbi.nlm.nih.gov/books/NBK592245
  • Debras C, et al. Artificial sweeteners and risk of cardiovascular diseases: results from the prospective NutriNet-Sante cohort. BMJ 2022, volume 378, e071204. pubmed.ncbi.nlm.nih.gov/36638072
  • World Health Organization. Aspartame hazard and risk assessment results released, July 2023 (IARC Group 2B, JECFA acceptable daily intake 40mg/kg). who.int, aspartame assessment
  • McGlynn ND, et al. Low- and no-calorie sweetened beverages as a replacement for sugar-sweetened beverages and body weight: systematic review and meta-analysis. JAMA Network Open 2022, volume 5, issue 3, e222092. jamanetwork.com, article 2790045
  • NHS. Sugar: the facts. Reviewed 2023. nhs.uk, sugar the facts
Back to blog