On Antibiotics? Eating Less Sugar Might Shield Your Microbiome

FRIDAY, Oct. 2, 2026 (HealthDay News) — Many people understand that antibiotics' bacteria-killing properties can disrupt the gut's healthy microbiome, even as the drugs treat illness.  

Now, research in cancer patients suggests that cutting back on sugar while you're taking an antibiotic might help preserve microbiome health.

“No matter how we analyzed the data, the same strong signal kept appearing,” said study co-senior author Dr. Marcel van den Brink. “The patients who consumed more sweets while taking antibiotics were more likely to experience a loss of microbiome diversity as aggressive microbes crowded out other strains of bacteria.”

Van den Brink is chief physician executive at City of Hope hospital in Los Angeles. His team published its findings Sept. 30 in Nature.

“Antibiotics disrupt the diversity of our gut bacteria, or microbiomes, which play an essential role in supporting our immune systems and overall health,” van den Brink explained in a hospital news release. 

The new research was targeted at cancer patients, he said, because "preserving microbiome diversity has previously been linked with improved clinical outcomes for patients with cancer."

The study involved 173 patients hospitalized with blood cancers. Most underwent chemotherapy, often alongside radiation therapy, to dampen their immune systems ahead of stem cell transplant.  

All patients also got at least one antibiotic (often a very powerful variety) to help protect them against infections. 

Over five years, the City of Hope team used stool samples from 158 of the patients to profile the diversity of bacteria in their respective gut microbiomes.

They also enlisted the hospital's kitchens to get a detailed analysis of 40,000 food items in the more than 9,400 meals eaten by the patients as they underwent treatment.

“Unlike past nutrition studies that rely on patients’ memories of what they ate or other imprecise surveys, we had meticulous dietary records collected in real time,” van den Brink noted. “This enabled us to look for patterns that otherwise might be impossible to detect.”

One clear pattern emerged: Patients who cut back on sweets tended to have more diverse, better-preserved gut microbiomes despite antibiotic therapy, the team found. 

Folks with sweet tooths fared worse, especially when it came to the growing dominance in the microbiome of the Enterococcus bacteria. 

For every 100-gram increase in daily sugar (equivalent in sugar content to a large milkshake), microbiome diversity fell by 24% in patients taking antibiotics, the team found.

These changes could greatly impact patients' health. Researchers said if germs like Enterococcus are allowed a foothold in the gut, risks rise for bloodstream infections, graft-versus-host disease and other life-threatening complications for cancer patients.

Much of this had been mirrored in studies the City of Hope team had conducted in mice. 

Mice put on both antibiotics and a high-sucrose diet showed rapid growth of Enterococcus in their guts, the researchers noted. However, mice who got the high-sugar diet without getting an antibiotic maintained a healthy microbiome. 

“The fact that animal studies support associations we found in patient data provides a compelling rationale for future clinical trials to evaluate whether reducing sugar intake during antibiotic use would protect the microbiome,” van den Brink said.

He thinks the findings could affect hospital menus going forward.

“Many of the foods and drinks we encourage hospitalized patients to consume, like nutritional shakes, smoothies and sports drinks, are high in sugar content,” van den Brink noted. “That’s something we’ll need to evaluate when prescribing antibiotics.”

Should people without cancer cut back on sugar if they're on an antibiotic?

“It’s premature to recommend everyone taking antibiotics avoid sugar," van den Brink said. "On the other hand, we don’t have strong evidence that probiotics help preserve or restore microbiome diversity and many people choose to take them anyway."

His take-home message: "There’s no harm in limiting sugar in your diet, and it’s an easy thing to try."

More information

Find out more about the microbiome at the U.S. National Human Genome Research Institute.

SOURCE: City of Hope, news release, Sept. 30, 2026