A new open-label, randomized, controlled study published in Cell showed that zero-calorie sweeteners are metabolically active and some might impair glycemic response in healthy adults .
What does “sugar-free” mean?
The human brain is the main consumer of glucose in the body, and glucose availability is paramount for neurons to generate action potentials and release neurotransmitters. This is why humans love sweets and develop sugar cravings: for the brain, sugar sends a strong message of more energy coming in. However, excessive dietary sugar consumption is associated with obesity and type 2 diabetes, which lead to accelerated aging.
One strategy to reduce sugar consumption entails substituting it with supposedly metabolically inert zero-calorie, or non-nutritive, sweeteners. These substances are not broken down to produce energy, but their sweet taste is used to flavor food and satisfy sugar cravings. Products with non-nutritive sweeteners can be recognized by their “diet” or “sugar-free” labels.
There is a wide range of artificial (chemically synthesized) and natural non-nutritive sweeteners, but it is still unclear if they actually promote a lower consumption of sugar. It is also largely unknown if these substances interfere with metabolism or have harmful or beneficial effects.
In this study, the researchers explored the effects on human microbiome and metabolic health of three artificial sweeteners, saccharin, sucralose, and aspartame, alongside a natural sweetener, stevia, which is derived from the Stevia rebaudiana plant.
Zero calorie sweeteners are not the same
The participants were healthy Israeli males and females of 18-70 years of age who did not consume any foods containing zero-calorie sweeteners in the six months prior to the trial. The trial participants were divided into six groups of 20 people each, which were given commercially available sachets with the following supplements for two weeks: saccharin, sucralose, aspartame, stevia, glucose, and no supplement (control). All the non-nutritive sweetener sachets contained an equivalent amount of glucose, which served as a formulation filler agent, as the glucose group (~5 g/day).
Throughout the study, the participants were logging their food consumption and physical activity using a smartphone application. Conveniently, glucose tolerance tests were conducted at home using a continuous glucose monitor. The oral glucose tolerance tests showed that both saccharin and sucralose impair glycemic response: these sweeteners lead to a more rapid rise of blood glucose following caloric intake. This effect was reversed once the participants discontinued the supplementation.
Next, the researchers checked if the zero-calorie sweeteners affected the production of insulin and glucagon-like peptide-1 (GLP-1). These two important hormones are secreted after a meal (or oral glucose test): insulin ensures glucose uptake by the cells, while GLP-1 enhances insulin secretion.
Following the test, it was shown that the plasma insulin levels increased only in the stevia and glucose groups but not in the saccharin and sucralose groups, which probably explains the elevated glycemic response in the latter two groups. No GLP-1 changes were observed in any of the groups.
The researchers then collected stool and oral samples from the participants to assess if zero-calorie sweeteners affect metabolism by altering the microbiome. They sequenced the bacteria present in the feces and oral cavities of the participants and analyzed the changes in their composition and function over the course of the trial. Sucralose and saccharin significantly changed the microbiome’s composition, while all the sweeteners changed the microbiome’s function.
Interestingly, the microbiome changes observed were sweetener-specific. For example, sucralose altered purine metabolism, which was also demonstrated by the blood tests, while stevia-induced changes were related to fatty acid biosynthesis. Bacterial composition changes were also observed in the supplementation groups based on the oral microbiome analysis. Overall, of all the sweeteners, sucralose supplementation led to the most prominent fecal microbiome changes.
As expected with dietary interventions, there were person-to-person response variations within groups. Hence, some participants in each group had a reduced glucose tolerance and some did not. The researchers leveraged these results to identify the metabolites that would explain this heterogeneity.
They identified the abundances of different bacterial species and metabolites that correlated with the glycemic response to different sweeteners. This means that the way the body of a specific person would respond to a specific sweetener depends on the initial microbiome composition and metabolomic features of that person.
Next, the researchers transplanted human fecal microbiomes into germ-free mice lacking microbiomes of their own to explore if the glycemic response changes observed in some supplementation groups were indeed caused by the microbiome composition. The mice were transplanted with microbiomes taken from 42 trial participants, either at the beginning of the trial or after the supplementation. This research was conducted using only the microbiomes from the participants showing either the strongest or the weakest response to the supplementation within their respective groups.
These experiments mostly replicated the results in humans: mice that received samples from participants who responded strongly to the sweeteners demonstrated a higher glycemic response. In addition, the transplantation experiments showed that sucralose altered the microbiomes of the participants with the strongest response more than those with the weakest response.
Non-nutritive sweeteners (NNS) are commonly integrated into human diet and presumed to be inert; however, animal studies suggest that they may impact the microbiome and downstream glycemic responses. We causally assessed NNS impacts in humans and their microbiomes in a randomized-controlled trial encompassing 120 healthy adults, administered saccharin, sucralose, aspartame, and stevia sachets for 2 weeks in doses lower than the acceptable daily intake, compared with controls receiving sachet-contained vehicle glucose or no supplement. As groups, each administered NNS distinctly altered stool and oral microbiome and plasma metabolome, whereas saccharin and sucralose significantly impaired glycemic responses. Importantly, gnotobiotic mice conventionalized with microbiomes from multiple top and bottom responders of each of the four NNS-supplemented groups featured glycemic responses largely reflecting those noted in respective human donors, which were preempted by distinct microbial signals, as exemplified by sucralose. Collectively, human NNS consumption may induce person-specific, microbiome-dependent glycemic alterations, necessitating future assessment of clinical implications.
This study demonstrates that contrary to what is generally accepted, non-nutritive sweeteners are metabolically active and capable of changing the human gut microbiome composition even at dosages below the acceptable daily intake. Saccharin and sucralose seem to impair glycemic response in healthy individuals, but the response is person-specific. Unfortunately, all the tested sweeteners in this study were coupled with glucose, and, although compared to the glucose group, it is impossible to assess the effect of the sweeteners on their own without additional studies. The authors of the study stress that their results should not be interpreted as a call to consume more sugar. Perhaps the safest option would be to consume fewer sweets altogether, whether “sugar-free” or not.
 Suez, J., Cohen, Y., Valdés-Mas, R., Mor, U., Dori-Bachash, M., Federici, S., … & Elinav, E. (2022). Personalized microbiome-driven effects of non-nutritive sweeteners on human glucose tolerance. Cell.