What are the effects of non-nutritive sweeteners on the human gut microbiome?

In a recent study published in Cell, researchers evaluated the short-term effects of four non-nutritive sweeteners (NNS) on the human microbiome, glucose tolerance, and additional health parameters in a randomized-controlled trial (RCT) encompassing 120 healthy adults.

Background

Dietary strategies for obese and hyperglycemia patients involve the replacement of dietary sugar, which increases weight, with NNS. These strategies presume NNS to be metabolically inert and incapable of eliciting a post-prandial glycemic response. Though popular, the efficacy of such dietary strategies remains unknown.

Moreover, there is a lack of studies demonstrating how metabolically inert substances can affect human metabolism; therefore, NNS use is still widely endorsed by dietitians for adults worldwide. However, a more cautious approach is needed, especially for children.

About the study

The present trial featured four arms, where the researchers gave two sachets of each NNS supplement – aspartame, saccharin, sucralose, and stevia – three times a day, with glucose as a bulking agent, corresponding to their acceptable daily intake (ADI) of 8%, 20%, 34%, and 75%, respectively. The participants of the fifth arm received five grams per day of glucose, and the sixth arm did not receive any supplement (the NSC group).

The study had three distinct periods, ranging from the first seven days of baseline measurements of metabolic, metabolomic, and microbial parameters to 14 days of exposure to four NNS, following which the researchers ceased the supplementation to follow up with the participants for additional seven days.

The researchers instructed all the study participants aged 18 to 70 to wear a continuous glucose monitor (CGM) throughout the 29 days trial using which they performed glucose tolerance tests (GTTs) on pre-determined study days. For instance, they self-performed anthropometrics and blood tests on study days zero, 14, and 28. All the participants performed nine GTTs at home; CGM recorded interstitial glucose every 15 minutes, data from which helped the researchers compute the incremental area under the glucose curve (iAUC).

Further, the team used two linear-mixed-effects models (LMMs) to assess the impact of NNS consumption over time for all GTTs. Likewise, the researchers collected microbiome samples from the oral cavity and stool at pre-determined time points. Additionally, the researchers asked all the participants to log their food intake and physical activity in real-time using a mobile application.

The study participants provided their NNS consumption frequency (never; daily; weekly, monthly) based on intake of NNS-containing products available in the Israeli market, e.g., carbonated diet drinks; sugar-free energy drinks; any product labeled as diet/low/sugar-free, and many more.

Study findings

The researchers screened 1,375 individuals between 2018 and 2020, but 20 participants in each group (a total of 120) completed the trial and provided enough glucose measurements for analysis. The median age of the study participants was ~30 years, and 65% were women.

Saccharin, sucralose, aspartame, and stevia significantly altered the human intestinal and oral microbiome. During the first week of exposure, the normalized glycemic response was significantly higher in the sucralose and saccharin groups than in the glucose vehicle and NSC groups, as assessed via two-way analysis of variance (ANOVA). This elevated glycemic response persisted during the second week of exposure but returned to baseline during further follow-up. The authors observed a considerable inter-individual heterogeneity ranging between 1,225 and 7,458 mg dL⁻¹ min⁻¹ in the GTT-iAUCs of each participant. However, the baseline GTTs of the same individual were similar and correlated with each other, with a Spearman coefficient (r) of 0.44.

Intriguingly, the authors also noted inter-subject variability in fecal excretion. Sucralose is poorly absorbed; thus, orally supplemented sucralose reaches the colon and is excreted unchanged in feces. Furthermore, while the sucralose exposure reduced the abundance of nucleotide biosynthesis genes, its supplementation increased the abundance of genes related to the tricarboxylic acid (TCA) cycle. Elevated plasma TCA metabolites during sucralose supplementation suggested microbiome contributions and impaired glycemic control.

Conclusions

The study revealed that commonly consumed NNS by humans might not be physiologically inert. In fact, it turns the human gut microbiome into a responsiveness hub by affecting its several distinct configurations. For instance, the study participants consuming saccharin or sucralose with glucose as a vehicle experienced a blunting of glucose-stimulated insulin secretion, leading to elevated glycemia.

Future studies should uncover molecular mechanisms and clinical consequences of NNS consumption on the human microbiome. Further, these studies should investigate the effects of the coupling of NNS with a carbohydrate moiety on insulin sensitivity compared with NNS alone. This data could greatly help optimize dietary recommendations for hyperglycemic and obese patients.

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.