Predicted Function, Not Proven Benefit

What the biggest long-term sweetener study actually showed — and what it never measured.

By Dr. Eugene Capitano, DC, MSc 

A one-year study says sweeteners are fine. It is the strongest evidence in their favour, and I am not going to wave it away. But what it measured, and what it did not, changes the story.

Everyone in the SWEET study had just finished a diet. They were then split into two groups. One was allowed sweeteners and sweetness enhancers such as aspartame, stevia and others, and one cut them out. A year later, the group that kept using them had kept off about 1.6 kg more weight than the group that quit. Their blood-sugar and heart-health markers stayed normal. And their gut bacteria shifted in what looked like a good direction.

That is a real result, from a randomised trial, over a full year. It is the longest human data we have on this question, and it deserves to be stated plainly before anything else is said about it.

The sweetener arm also permitted sugar alcohols such as erythritol and xylitol, and sweet fibres including inulin-type oligosaccharides. Those are fermentable substrates, which they feed beneficial gut bacteria on their own, with or without a sweetener present.

So the favourable gut result cannot be credited to the sweeteners specifically. The study was not designed to separate the two, and it cannot tell us which ingredient moved the microbiome.

This is the part that matters most, and the part that tends to get lost. The study measured which bacteria were present, and that part is real, and it was done properly. What it did not do was measure what those bacteria actually produced. The beneficial compounds they are known for, the short-chain fatty acids, were never quantified. Their production was predicted from the bacterial line-up, not observed. The authors say as much themselves.

And even if those compounds had been measured, more is not automatically better. Butyrate, the best-studied of them, is meant to be burned by the cells lining your colon, it supplies most of their energy. If those cells are using it well, less of it is left over. If they are not using it, it accumulates. So a higher reading can mean the system is working, or that it is not. Without a measure of use, you cannot tell which.

The study measured neither. That does not mean anything went wrong in these participants — nothing in the trial suggests it did. It means the question was never asked.

Reassuring. For weight management over one year, within a structured healthy diet, sweeteners did no harm to what was measured, and modestly helped people keep weight off.

Unanswered. Gut barrier integrity, inflammation, whether these compounds are actually used by the body, and anything past twelve months.

That is why we take the cautious route. If a sweetener is not necessary, we leave it out, not because it has been shown to cause harm, because it has not, but because the questions we care about most have not yet been asked.

Related reading: The Sweetener Problem  ·  Why We Use Coconut Sugar

Dr. Eugene Capitano, DC, MSc, is a chiropractor, rehabilitation clinician, and researcher with more than 25 years of clinical experience. He earned an MSc in Psychology & Neuroscience of Mental Health from King’s College London and holds the ACSM Exercise is Medicine® (EIM) Credential and the ACSM Certified Personal Trainer® (ACSM-CPT®) certification. His research interests include the gut–brain–muscle axis, microbiome-targeted nutrition, resistance training, mitochondrial function, metabolic health, and healthy aging.

Affiliation:TLC NeuroMicrobiome Labs Inc., Winnipeg, Manitoba, Canada

Disclaimer. This article is scientific commentary intended for educational purposes. It is not medical advice and is not intended to diagnose, treat, cure or prevent any disease. 

Disclosure. TLC NeuroMicrobiome Labs Inc. formulates protein products that do not contain non-nutritive sweeteners. This commercial interest is disclosed so that readers may weigh it. The trial's favourable findings are reported here as the authors stated them.

Source. Pang, M. D., … Raben, A. (2025). Effect of sweeteners and sweetness enhancers on weight management and gut microbiota composition in individuals with overweight or obesity: the SWEET study. Nature Metabolism, 7. doi:10.1038/s42255-025-01381-z