October 8, 2026, 3:42 pm | Read time: 5 minutes
Antibiotics work against harmful bacteria but can also alter beneficial bacteria in the gut. The extent of this change may depend on what patients eat during treatment. Scientists have therefore examined the role of diet, particularly sugar, in this process.
What Sugar Does to Gut Flora
The study suggests that high sugar intake could amplify the effects of broad-spectrum antibiotics on gut bacteria. Broad-spectrum antibiotics are medications that target many different types of bacteria simultaneously. Patients who consumed more sugar, sweets, and sugary drinks (including medical nutritional drinks) during treatment had a significantly lower diversity of bacterial species in their gut. At the same time, certain bacteria from the Enterococcus group–specifically the potential pathogen Enterococcus faecium–multiplied more.
How the Study Was Conducted
Researchers at the Memorial Sloan Kettering Cancer Center in New York, along with colleagues from other U.S. research institutions, studied blood cancer patients who received stem cells from a donor. In such treatments, patients often receive strong broad-spectrum antibiotics that target many different bacteria to treat or prevent infections. These can significantly alter the composition of gut bacteria.1
The scientists wanted to know if diet plays a role in this. They documented a total of 9,419 meals from 173 patients. The patients reported how much they actually ate.
From 158 patients, the researchers also examined a total of 1,009 stool samples. This allowed them to determine which bacteria were present in the gut and how their composition changed during treatment. They then compared these results with diet and antibiotic intake.
The researchers also investigated the relationship between sugar and antibiotics in experiments with mice.
Sugar Amplified Changes Under Antibiotics
What was particularly interesting was the comparison with times when patients were not receiving broad-spectrum antibiotics. Then, the researchers found no link between higher sugar consumption and the number of different bacterial species in the gut. The observed connection only appeared during antibiotic treatment.
The scientists’ calculations also showed how significant the difference could be: During treatment with broad-spectrum antibiotics, an additional 100 grams of sugar (based on the adjusted dry weight of the food) was associated with about 21 percent less diversity among gut bacteria.
The experiments with mice supported these observations. When the animals received both an antibiotic and sugar (sucrose), Enterococcus bacteria multiplied significantly more and persisted longer than with antibiotic treatment alone. Sugar without antibiotics did not have this effect, as it only serves as a nutrient for Enterococcus when healthy competitors are eliminated by the antibiotic.
The researchers also considered patient mortality. Among patients who consumed relatively high amounts of sugar, each additional day of treatment with broad-spectrum antibiotics was associated with about a 12 percent higher risk of death. This correlation was not statistically significant for patients with lower sugar consumption.
However, this does not mean that sugar was responsible for the higher risk of death. The study can only show correlations, not prove causation. Other factors, such as the severity of the underlying disease, could have influenced the results.
The study does not suggest that patients should immediately stop consuming sugar during antibiotic treatment on their own. Whether a low-sugar diet actually offers clinical benefits needs to be clarified by further studies.
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Strengths and Weaknesses of the Study
A strength of the study is that the researchers precisely recorded what patients actually ate in the hospital. They also examined stool samples multiple times during treatment. Additional calculations yielded similar results. Experiments with mice supported the observed link between sugar and antibiotics.
However, the study cannot prove that sugar combined with antibiotics caused the changes in gut bacteria. Patients received broad-spectrum antibiotics for medical reasons, such as fever or suspected infection. Therefore, their health status could have influenced the results. Additionally, almost all patients were already receiving preventive antibiotics.
Diet could not be fully captured either. Foods consumed outside the hospital were partially missing. Complaints like nausea or loss of appetite could also have influenced what and how much patients ate.
Since only patients with a stem cell transplant at a single hospital were studied, it is unclear whether the results can be applied to other patients. The study was funded by U.S. research institutions, foundations, and cancer research programs. Several authors also reported financial ties to companies in the microbiome and biotechnology sectors.
Also interesting: Where sugar hides in products–and when it becomes dangerous
What Similar Studies Show on the Topic
The importance of gut bacteria diversity in stem cell transplants was already demonstrated by a large study in the journal “New England Journal of Medicine.”2 Researchers analyzed nearly 8,800 stool samples from around 1,360 patients in clinics in the U.S., Germany, and Japan. Patients with more diverse gut flora had a lower risk of death. However, this study could only show correlations.
An earlier study by the same research group, which included the University Hospitals of Regensburg and Frankfurt, also focused on a sugar: lactose.3 Enterococcus bacteria use it as food to multiply. In mice, a lactose-free diet slowed the growth of the bacteria. Patients who genetically poorly digest lactose struggled to eliminate Enterococcus after antibiotic treatment.