A new study has found that consuming sugary foods during antibiotic treatment may increase patients’ vulnerability to dangerous infections by reducing beneficial gut bacteria and allowing potentially harmful organisms to multiply.

Published in Nature, the study found that cancer patients who consumed more sweets and sweetened foods and beverages while receiving antibiotics experienced greater losses in gut bacterial diversity, alongside an increase in potentially harmful bacteria, including Enterococcus faecium.

The findings highlight the potential influence of diet on antibiotic-related disruption of the gut microbiome, particularly among vulnerable patients undergoing stem cell transplants, who face increased risks of serious infections and other complications.

However, the researchers cautioned that the findings did not establish a cause-and-effect relationship, calling for further studies to determine whether the results also apply to people taking antibiotics outside hospital settings.

The researchers examined how diet influenced gut bacteria in cancer patients receiving prolonged antibiotic treatment before stem cell transplants. They analysed more than 1,000 stool samples from 173 patients who consumed over 9,400 hospital-prepared meals, allowing them to assess food intake more accurately.

Lead author, Dr Jonathan Peled, said the research provided an opportunity to investigate dietary effects on patients whose gut microbiomes were already being disrupted by antibiotics and cancer treatment.

The team assessed the relationship between nutrients, including fats, carbohydrates and fibre, and different gut bacteria. They also examined individual foods such as chicken, apples and chocolate cake.

Higher consumption of sugary foods repeatedly emerged as a factor associated with greater microbiome disruption, with particularly harmful effects linked to added sugars.

According to the study, the intensity of antibiotic treatment was associated with greater losses in bacterial diversity. While most food categories showed no significant association with changes in diversity, sweets, defined as sugars and sweetened foods and beverages, stood out.

For every additional 3.5 ounces of sweets consumed during antibiotic treatment, bacterial diversity in patients’ stool samples decreased by approximately 24 per cent.

This decline was associated with an increase in potentially harmful organisms, including Enterococcus faecium, which can live harmlessly in the gut when other microorganisms keep its growth under control.

When antibiotics eliminate competing bacteria, however, Enterococcus can multiply excessively and, in some cases, cross the intestinal barrier into the bloodstream, potentially causing sepsis, a life-threatening response to infection.

Peled said the findings were particularly significant for bone marrow transplant recipients because Enterococcus could worsen complications such as graft-versus-host disease, in which donated stem cells attack the recipient’s healthy tissues.

The study also found that patients who consumed more sugar than average had approximately 12 per cent higher risk of dying after stem cell transplants than those who consumed less.

Peled described the difference as a modest change in patients’ risk profiles, adding that the expansion of Enterococcus bacteria persisted for about eight days after antibiotic treatment.

He cautioned that the research was not a randomised controlled clinical trial in which different diets were directly compared to determine which best protected patients receiving antibiotics.

Nevertheless, he said limiting sugar intake during antibiotic treatment could be reasonable, given the general benefits of avoiding excessive sugar consumption. The findings, he added, did not necessarily call for eliminating sugary foods from the diet entirely but suggested that reducing their consumption while taking antibiotics might be beneficial.