Sweeteners and sugar alcohols

Healthy state

Sweeteners taste far sweeter than sugar and provide almost no calories. Sugar alcohols Sugar alcohols (polyols) Sweeteners such as sorbitol, xylitol or maltitol, which also occur in fruit. They are barely absorbed and can cause bloating and diarrhoea. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” are a second group: less sweet, also almost calorie-free, and only partly absorbed. The ingredients list gives the name, in the European Union and many other countries also an E number. What they do to the bacteria in the gut is a second, often overlooked question: not every substance is fully absorbed, and part of it reaches the large intestine and meets the microbiome Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” there.

Sucralose (E955) is made from sugar and is around 600 times sweeter. The body absorbs very little of it, and much of it leaves the gut unchanged. For the microbiome, sucralose is the best-studied sweetener: around 85 percent of it reach the large intestine. Reviews of randomised trials Randomised trial A study in which chance decides who gets the treatment and who does not, for comparison. This makes results particularly reliable. in humans find no clear average change in diversity or blood sugar; individual studies and many animal experiments do show shifted bacterial communities and a weakened gut lining. In a test tube, a sucralose-containing sweetener promoted biofilm Biofilm A community of microbes that sticks to a surface and embeds itself in a self-made protective coat, such as dental plaque. More in the chapter “Biofilms in the gut and “biofilm busters”” formation and the exchange of resistance genes in some gut bacteria. It is found in zero and light drinks, protein powders and bars, sugar-free chewing gum and table-top sweeteners.

Saccharin (E954) is the oldest sweetener and about 300 times sweeter than sugar. On the ingredients list it usually appears as the salt sodium saccharin. Some of it is absorbed and excreted through the kidneys, some reaches the large intestine. For the microbiome, saccharin carries the oldest finding: in a much-cited 2014 study it changed the gut flora in mice and humans and worsened blood sugar control, and the effect could be passed on to germ-free mice by stool transfer. How well this translates to everyday life is still disputed. It is found in sweetener tablets, light and zero lemonades, sugar-free sweets and, in some countries, in toothpaste.

Aspartame (E951) is around 200 times sweeter and is broken down into its building blocks already in the digestive tract; hardly any of it reaches the large intestine intact. Because so little of it arrives in the large intestine, it was long considered harmless to the gut flora; in the large Cell study of 2022 it still changed the microbiome, and solid microbiome studies in humans are lacking. It is found in light and zero lemonades, chewing gum, light yoghurt and table-top sweeteners. People with the metabolic disorder phenylketonuria have to avoid it, which is why there is a note on the packaging.

Steviol glycosides from the stevia plant (E960) are around 200 to 300 times sweeter. They pass through the small intestine largely unchanged, and only in the large intestine do bacteria split them into steviol. There they inhibit individual bacterial species and change the microbiome only cautiously in experiments. One finding concerns viruses: stevia and its main component rebaudioside A woke dormant bacteriophages Bacteriophages Viruses that only infect bacteria. In the gut there are probably about as many phages as bacteria, perhaps more, and they help determine which bacteria prevail. More in the chapter “Bacteriophages: the hunters in the gut” inside gut bacteria – viruses that attack bacteria – and thereby changed which microbes can multiply; rebaudioside A also bound directly to phage proteins and changed their infectivity. What this means for humans is open. They are found in table-top sweeteners, drinks, dairy products and sweets advertised with “stevia”; the dried leaves are used to sweeten tea.

Acesulfame K (E950) is named after the fact that it occurs as a potassium salt — the K stands for potassium. Most of it is absorbed and excreted unchanged through the kidneys; but part of it does reach the large intestine. There, bacteria can break it down: in a gut model with children’s microbiota this produced more butyrate Butyrate A short-chain fatty acid and the main energy source for the cells of the colon wall. It strengthens the barrier and dampens inflammation. More in the chapter “Butyrate and other short-chain fatty acids”, while breakdown products were linked to a more permeable gut lining. A small study in young adults found shifted bacterial proportions after diet soda with acesulfame K and sucralose. It is found in table-top sweeteners, light and zero lemonades, soft drinks and sugar-free chewing gum.

Cyclamate (E952) appears on the ingredients list usually as the salt sodium cyclamate. It is about 30 to 50 times sweeter than sugar, and so clearly weaker than the other sweeteners. Gut bacteria can form cyclohexylamine from cyclamate, and it is not permitted in every country. Studies on what cyclamate does to the microbiome or to bacteriophages are still largely lacking. It is found in light and zero lemonades and in table-top sweeteners.

Acesulfame K and cyclamate rarely appear alone on an ingredients list. Almost always they come as a mixture with aspartame or saccharin, because the combination tastes rounder and each single substance can then be dosed lower. The newer neotame (E961) fills the same role. What such mixtures do in the gut has hardly been studied; most studies examine single substances, while labels often list several together.

Sugar alcohols such as sorbitol (E420), mannitol (E421), isomalt (E953), maltitol (E965), lactitol (E966), xylitol (E967) and erythritol (E968) are absorbed slowly and only in part. The rest draws water into the gut and is fermented by bacteria. Erythritol is the exception: most of it is absorbed and excreted unchanged, so it usually causes no symptoms. Sorbitol and maltitol are FODMAPs FODMAP Short for fermentable sugars and sugar alcohols that are poorly absorbed in the small intestine, such as fructans, lactose, fructose and sorbitol. More in the chapter “Doing low-FODMAP right”. In the large intestine they are food for the bacteria: erythritol scored well in a gut model and promoted butyrate production, while sorbitol promoted inflammatory signals in a mouse study. In practice the amount matters most – the more arrives undigested, the more gas and bloating. Sugar alcohols are found in sugar-free chewing gum, sweets and cough drops, protein bars, sugar-free chocolate, “keto” and “zero” products and some toothpastes.

Disrupted state: causes and effects

In a randomised trial with 120 people, all four sweeteners studied changed the gut microbiota within two weeks, differently from person to person. Saccharin and sucralose also made blood sugar rise slightly more after a sugar drink. Whether this harms health is open: the study looked at healthy adults over two weeks.

When the researchers transferred the participants' gut microbiota to germ-free mice, these mice also showed the altered blood sugar values. This suggests that the gut microbiota mediates the effect.

Sugar alcohols draw water into the gut and are fermented by bacteria. Larger amounts lead to bloating and diarrhoea. Xylitol causes considerably more symptoms than erythritol.

Sugar-free is not automatically gut-friendly, and “naturally sweetened” is no opposite of it: honey, agave syrup, coconut blossom sugar, maple syrup and fruit juice concentrate are, for the gut, essentially sugar made of glucose and fructose — with just as many calories as household sugar.

In its 2023 guideline, the World Health Organization advises against using sweeteners for weight control. The reason is analyses that found no long-term benefit and, in observational studies, a slightly higher risk of type 2 diabetes; the recommendation is graded conditional, so the evidence is limited. The guideline does not question the safety of the permitted maximum amounts.