Most people know that eating too much sugar isn't good for them. But a new review published in Gut Microbes by researchers at Albert Einstein College of Medicine and the Icahn School of Medicine at Mount Sinai adds important depth to that familiar warning, arguing that much of sugar's harm to the body may actually travel through the gut microbiome first.

Study Overview

Researchers conducted a comprehensive review of animal and human studies examining how added sugars, including glucose, fructose, and sucrose, affect the trillions of bacteria living in the gut, and how those microbial changes in turn influence long-term health. The review draws a clear distinction between naturally occurring sugars found in whole foods like fruit and dairy, where fiber, water, and other nutrients slow absorption, and added sugars found in processed foods and beverages, which are absorbed faster, produce larger blood sugar spikes, and deliver more undigested sugar to the lower intestine where most gut bacteria reside. That difference, the authors argue, makes added sugars a uniquely disruptive force for the microbial ecosystem.

Key Findings

• Added sugars consistently alter the composition of gut bacteria. High intake tends to enrich bacteria that thrive on simple sugars, including members of Enterobacteriaceae (such as E. coli and Klebsiella) and Helicobacter,  while depleting bacteria that produce short-chain fatty acids (SCFAs), a class of compounds critical to gut barrier health, immune regulation, and metabolic function. Depleted SCFA producers include Clostridium, Ruminococcus, Eubacterium, and Lachnospira.
• The form sugar takes matters. Liquid sugars (sodas, juices, sweetened beverages) appear more disruptive to the microbiome than the same sugars consumed in solid food. This likely reflects the faster absorption and greater colonic delivery of liquid sugars. Sugar dose also plays a role, with higher intakes producing more pronounced microbial shifts.
• Gut barrier integrity can be compromised. Animal studies found that added sugar reduced the expression of tight junction proteins that keep the gut lining sealed. When that lining becomes "leaky," bacterial endotoxins can pass into the bloodstream triggering systemic inflammation and contributing to insulin resistance, weight gain, and liver fat accumulation.
• Downstream metabolic effects are wide-ranging. Through changes in gut bacteria, added sugar intake has been linked to disruptions in bile acid metabolism (affecting fat and glucose regulation), reduced production of beneficial SCFA metabolites, altered amino acid metabolism tied to diabetes risk, and increased liver lipogenesis, particularly from fructose, which the review identifies as especially potent in driving fatty liver disease.
• Results across studies were not uniform. Several human studies found no significant change in gut microbial diversity with added sugar intake. Individual variation in genetics, background diet, and existing gut microbial ecology all appear to shape how any one person's microbiome responds - an important caveat throughout the review.

Why It Matters

Added sugars remain a major feature of the modern diet. In the US, the average adult still gets roughly 12–14% of daily calories from added sugars, above the 10% ceiling recommended by the WHO and US dietary guidelines, and adolescents consume even more. The review highlights that added sugar's harms likely don't stop at blood sugar and calorie counts. By reshaping the gut microbiome in ways that promote inflammation, weaken the gut barrier, and disrupt metabolic signaling, high sugar intake may contribute to irritable bowel syndrome, obesity, type 2 diabetes, fatty liver disease, and cardiovascular disease through a mechanism most people, and many clinicians, aren't yet thinking about.

The authors also note a meaningful demographic dimension: in the US, non-Hispanic Black adults and lower-income adults have higher added sugar intake than other groups, pointing to the role that food access and structural inequity play in diet-related health disparities.

Takeaways

Reducing added sugar is one of the most evidence-backed dietary changes available, and this review adds another reason to take it seriously. Practical steps include:

• Prioritize whole food sources of sweetness like fruit, dairy, and root vegetables over processed foods and beverages with added sugars. The fiber and nutrients in whole foods buffer sugar absorption and protect the microbiome in ways that isolated added sugars don't.
• Pay particular attention to sugary drinks. Liquid sugars appear more disruptive to gut bacteria than solid sources. Swapping sodas, sweetened coffees, juices, and sports drinks for water, unsweetened tea, or sparkling water is one of the highest-impact changes you can make.
• Read ingredient labels for hidden added sugars, which appear under names like high-fructose corn syrup, cane juice, dextrose, maltose, and agave nectar. The closer to the top of the ingredient list, the more a product contains.
• Support your SCFA-producing bacteria by eating plenty of fiber-rich foods (legumes, oats, vegetables, and whole grains)  which feed the exact microbes that added sugars tend to deplete.
• Recognize that individual responses vary. Your microbiome is shaped by your genetics, your history with antibiotics, your overall diet, and more. Broad dietary patterns over time matter more than any single food or meal.

Read the Research: Zhang Y, Walker RW, Kaplan RC, Qi Q. Added sugars, gut microbiota, and host health. Gut Microbes. 2025;17(1):2592431. https://doi.org/10.1080/19490976.2025.2592431