Eating Sugar While Taking Antibiotics May Disrupt Gut Bacteria, New Study Finds

Antibiotics are built to kill bacteria, but they rarely stop at the bad ones. Helpful gut microbes can get swept away too, leaving open space for other bacteria to move in.

A new study in Nature suggests that what patients eat while taking antibiotics may influence what happens next, and sugar kept surfacing in the results. Researchers from NYU Langone Health, City of Hope and Memorial Sloan Kettering studied hospitalized patients, where meals could be tracked in unusual detail instead of reconstructed from memory.

That makes the findings unusually specific, but not universal. The people studied were seriously ill, their microbiomes were under heavy pressure, and the researchers are careful not to extend the results to everyone who gets an antibiotic prescription.

Still, the study raises a practical question: some sugary foods and drinks are products hospitals routinely encourage patients to consume, but the findings do not show whether changing that would help. Before blaming dessert, it helps to see what the researchers actually found.

Why Hospital Patients Made a Revealing Test Group

Diet studies usually depend on what people remember eating. This one had far better records.

Published September 30 in the journal Nature, the study followed 173 patients with blood cancers and logged 9,419 meals, down to each food item.

Patients also completed questionnaires about how much they ate and drank, while kitchen staff helped establish the nutritional details.

Researchers collected microbiome samples daily, beginning 12 days before transplantation and continuing through 49 days afterward. Every patient received at least one antibiotic, and 80% received one or more broad-spectrum drugs.

That overlap mattered. Study author Jonathan Peled said these patients offered a rare chance to watch diet and antibiotic exposure together while cancer treatment was also disrupting nutrition. Their microbiomes, in other words, were already taking a beating.

Sweets Kept Showing Up in the Results

The researchers sliced the data several ways, and one theme refused to go away.

Higher sugar or sweets intake was linked to lower bacterial diversity and more Enterococcus faecium, a bacterium associated with hard-to-treat infections. Peled said to CNN that the pattern appeared regardless of the type of sugar patients consumed, although added sugars looked especially harmful.

The team also examined fat, carbohydrates, fiber, and individual foods ranging from chicken and apples to chocolate cake. Most other foods showed no relationship with diversity, while more intensive antibiotic regimens were tied to larger losses.

The effect size depends on the analysis. NYU estimated an additional 21% drop in diversity for every 100 grams of sugar consumed in the 48 hours before treatment.

A separate estimate based on sugars, sweets, and sweetened drinks after antibiotic exposure came to about 24% per 100 grams, or 3.5 ounces. The two figures measure different things and are not interchangeable.

Related: 8 Gut Health Lies You Need to Stop Believing Before It’s Too Late

Mice Showed How the Pieces May Fit Together

Enterococcus is harmless until its competition disappears, and the animal experiments show how quickly that can change.

Enterococcus can live quietly in the gut while other bacteria keep it in check. Once antibiotics thin out those competitors, it can expand and, in some cases, cross the gut wall into the bloodstream and cause sepsis. Peled also told CNN that it can worsen graft-versus-host disease, in which donated stem cells attack healthy tissue.

In mice, a high-sugar diet alone left the microbiome healthy and diverse. Add an antibiotic and Enterococcus surged. In NYU’s experiment, biapenem and dietary sucrose expanded it 16.3-fold by day three and 33.4-fold by day six.

Those numbers are from mice, not people. William Jogia, the paper’s second author, called sugar an amplifier of antibiotic damage, possibly because surviving harmful bacteria can use it to grow.

Whether sugar feeds Enterococcus directly or weakens its rivals is still unknown in patients.

The Hospital Drink Dilemma

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Some of the sugar in question may arrive in products hospitals encourage patients to consume.

Marcel van den Brink, a co-senior author and chief physician executive at City of Hope, pointed to an uncomfortable possibility. “Many of the foods and drinks we encourage hospitalized patients to consume, like nutritional shakes, smoothies and sports drinks, are high in sugar content,” he also said to CNN.

He added that it will need evaluating when antibiotics are prescribed, though the study does not show these products caused the microbiome changes.

Antibiotic choice matters too, as researchers noted that some drugs disrupt beneficial gut bacteria more than others.

The Catch Is Who Was Studied

The strongest evidence comes from a group that looks little like most antibiotic users.

These were hospitalized people with blood cancers receiving intensive treatment, not otherwise healthy people taking a routine outpatient course.

The human portion also was not a randomized diet trial, so it can show associations but cannot prove sugar caused the damage. The mortality finding needs the same care: patients who ate more sugar than average had about a 12% higher risk of dying after transplantation than those who ate less.

That is a relative increase, not a 12-point jump in deaths.

So Should You Skip Dessert?

Even the authors urge restraint on what this means for the rest of us.

Researchers said that limiting sugar may work best during antibiotic treatment.

A 2018 Nature Microbiology study offers useful context. Twelve healthy men took a four-day course of three last-resort antibiotics, and their microbiomes returned to near baseline within about 1.5 months. Yet nine common species present in every participant beforehand were undetectable in most of them 180 days later.

That study did not test sugar, and it was small and male-only. But it shows a microbiome can look recovered while species remain missing.

If you’ve ever been prescribed antibiotics, did anyone mention what to eat or drink while taking them?