Sugar-free does not mean effect-free: what was measured in 120 people

People choose the sugar-free label for a reason. Anyone watching their blood sugar, anyone who got a result they did not like, anyone who simply does not want to overdo it, reaches for the drink or the bar that says zero calories and considers the matter settled.

Zero calories, it turns out, does not mean zero effect. And that is not a headline from the internet but the finding of a trial on a hundred and twenty people.

Four sweeteners, two of which moved blood sugar

A study published in Cell in 2022 took a hundred and twenty healthy adults and divided them at random. For two weeks they took saccharin, sucralose, aspartame or stevia, while one group received only the carrier without any sweetener. What matters is that the doses were below the acceptable daily intake, in other words within what is considered safe.

All four sweeteners changed the make-up of the bacteria in the mouth and in the gut. But only two, saccharin and sucralose, also measurably worsened glucose tolerance, meaning blood sugar climbed higher after meals than it had before the trial. Aspartame and stevia showed no such effect.

The most interesting part is that the effect was not the same in everyone. It depended on which bacteria a person started with, so the same sweetener moved one person’s blood sugar and left another’s alone. To check that this was not chance, the researchers transplanted the bacteria of the strongest responders into mice. The mice responded the same way.

That is firmer than most of what gets written about food, because it is an experiment rather than an observation, and because the mechanism was shown twice, in people and then in animals.

A study that has been quoted backwards for years

Whenever sweet drinks come up, someone reaches for a 2014 paper in the American Journal of Public Health. It followed 5,309 American adults and measured the length of their telomeres, the protective ends of chromosomes that shorten as a cell ages.

The finding is real. Half a litre of sugary soda a day was associated with roughly 4.6 years of additional cellular ageing, which the authors themselves compared to the effect of smoking. The number is striking, which is why it travels.

But the same study found no association at all for diet sodas or for non-carbonated sweet drinks, and 100 per cent fruit juice was marginally associated with longer telomeres. Anyone citing that paper against fruit juice, and it happens regularly, is citing it against what it says.

Experiment or observation, that is the whole difference

The two studies do not carry the same weight, and that is the most useful thing to take from all of this. With the sweeteners, people were assigned at random, given a specific substance and measured before and after. That is an experiment, and from it you are allowed to talk about cause.

With the sodas, nobody was assigned to anything. The state of more than five thousand people was measured at a single moment and an association was found. People who drink half a litre of soda a day differ from everyone else in income, exercise and much else besides, and that can never be fully calculated away.

So when you meet a headline about food, one question is worth asking: did somebody try this, or did somebody merely notice it. Both kinds count, but they do not speak with the same authority, and newspapers print them in the same size type.

Artificial sweeteners, then, are not neutral, but they do not behave the same way in everyone. Sugary soda has the worst record. Fruit juice, for all the times it gets named, is not condemned in this story.

This is not medical advice. If you have reason to watch your blood sugar, the substitutes are a conversation for a doctor rather than an ingredient list.