What you eat when taking antibiotics could influence how much those drugs disturb the community of bacteria living in your gut.

The finding comes from an unusual, highly detailed look at the relationship between diet and the microbiome in people undergoing treatment for blood cancers.

The study followed 173 adult cancer patients hospitalized for allogeneic hematopoietic cell transplantation, a procedure in which a patient receives blood-forming stem cells from a donor after intensive treatment that suppresses the immune system. These patients routinely receive antibiotics to prevent or treat infections, making them particularly vulnerable to antibiotic-related disruptions of the gut microbiome.

Eating sweets and other sugary foods were associated with greater loss of bacterial diversity in the gut during exposure to broad-spectrum antibiotics.

The researchers, led by scientists at Memorial Sloan Kettering Cancer Center, NYU Langone Health and City of Hope, analyzed over 9,400 meals eaten by the patients and more than 1,000 stool samples.

The patients were hospitalized for weeks, allowing researchers to collect dietary and microbiome information repeatedly, rather than asking participants to remember what they had eaten.

That distinction is notable. Each time a patient ordered a hospital meal, researchers recorded what was consumed and in what amount. The team tracked between eight and 120 meals per patient, from 12 days before to 49 days after transplantation. They then compared dietary information with changes in the bacterial communities found in stool samples.

The analysis showed that sweets and other sugar-rich foods were associated with greater loss of gut bacterial diversity during exposure to broad-spectrum antibiotics. For every additional 100 grams of sweets consumed after broad-spectrum antibiotic exposure, the researchers estimated a nearly 25 percent decrease in the variety of bacteria in the gut.

They also found an increase in enterococcus, a group of bacteria that can become harmful when the normal balance of the microbe is disrupted. Enterococcus Faecium, in particular, is associated with difficult-to-treat (DTR) bloodstream infections and other complications in patients undergoing stem cell transplantation.

These findings in people were supported by experiments in mice. Animals given an antibiotic and supplemental sucrose developed a much larger and longer-lasting expansion of enterococcus bacteria than mice receiving the antibiotic without an extra sugar. Sucrose alone, without antibiotics, did not produce the same effect.

The results suggest a relatively simple way of reducing some antibiotic-related microbiome damage. “Our study, which used more than 9,000 recorded meals, suggests that modifying diet during and after antibiotics, specifically by reducing sweets intake, could help with that,” Jonas Schluter, assistant professor in the Department of Microbiology at NYU Grossman School of Medicine and a co-senior author of the study, said in a press release.

Still, the researchers caution against assuming that the finding applies equally to everyone who takes antibiotics. The participants were seriously ill adults undergoing stem cell transplantation, and their immune systems and microbiomes had been profoundly affected by cancer treatment. The paper also notes that the human study was observational and that factors such as the severity of illness and the reasons patients received broad-spectrum antibiotics could influence the results. That distinction matters for people taking a routine antibiotic for an infection.

The results suggest a relatively simple way of reducing some antibiotic-related microbiome damage. When an antibiotic is necessary, what you eat could help determine how much collateral damage it causes to the microbiome.

Also, the study does not establish that eating sugar while taking a short course of antibiotics will cause clinically important microbiome damage in otherwise healthy adults. The researchers say clinical trials are needed to determine whether deliberately reducing simple sugars during antibiotic treatment actually protects the microbiome or improves health outcomes. For now, the study adds another piece to the growing evidence that diet can influence how the gut responds to medical treatment.

For patients facing intensive cancer treatment, the question may be important because preserving microbial diversity in the gut has been linked to better outcomes and fewer serious complications. And while it may be too soon to prescribe a low-sugar diet alongside antibiotics, the researchers offer a question worth discussing with the doctor: When an antibiotic is necessary, could what you eat help determine how much collateral damage it causes to the microbiome?

The study is published in Nature.