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Daily doses of certain probiotics appeared to keep upper respiratory tract symptoms somewhat at bay, especially in patients older than 45 and with a body mass index (BMI) in the obesity range, secondary results of a small trial indicated.

Overall reports of such symptoms -- including cough, sore throat, headache, muscle ache, and wheeze -- were 27% lower among 110 people taking a probiotic mix called Lab4P than in 110 assigned to placebo in the randomized study, said Benjamin Mullish, MD, of Imperial College London, during a press briefing held in advance of the annual meeting.

The findings were also in Gut Microbes.

The reduction was more pronounced in subgroups defined by age and BMI. In participants age 45 and older, Mullish and colleagues found an incidence rate ratio for respiratory symptoms of 0.60 (P<0.0001) for probiotics versus placebo, as opposed to 0.90 (not significant) in younger people.

And those with BMI values of 30-34.9, indicating obesity, showed an incidence rate ratio of 0.57 (P<0.0001) with probiotics versus placebo, compared with 0.78 (P=0.006) for those with BMI values of 25-29.9, considered overweight.

Kaplan-Meier curves showing time to first respiratory tract symptom indicated advantages for the probiotic group developing early in the 6-month study. In the 45-and-older group, nearly 80% of those taking probiotics remained symptom-free at day 50, whereas half of the placebo participants had shown symptoms at that point. A similar though less dramatic difference was seen among obese participants. Probiotics provided no benefit for this metric in overweight participants or in those younger than 45.

Mullish said previous studies had suggested that probiotics could protect against upper respiratory tract symptoms, but had not examined the effect in older or overweight people specifically.

Those earlier studies had established that there is a "gut-lung axis" connecting the intestinal microbiome to respiratory function, with which the current research firmly aligns.

The so-called randomized 220 patients to daily placebo or the Lab4P probiotic mix (including two Lactobacillus and two Bifidobacterium species), chosen on the basis of earlier findings that these were particularly capable of affecting overall metabolism and also had a good safety record. Primary outcomes were body weight and lipid parameters (results of which were ); however, patients also completed daily diaries tracking the five types of respiratory symptoms, analyzed for the current analysis.

Although these results document an apparent transaction whereby modifications to the intestinal microbiome affect respiratory health, Mullish emphasized in a press release that it's a two-way street: "It's not just the gut sending out signals that affect how the lungs work. It works in both directions."

Notably, PROMAGEN did not examine immune parameters nor identify pathogens, allergens, or other proximate causes for the respiratory symptoms.

Nevertheless, the researchers did suggest in their Gut Microbes paper that probiotics should be tested specifically for preventing viral upper respiratory tract infections "and possibly also COVID-19."

"We feel that a compelling case exists for further randomized studies to prospectively explore the potential impact of probiotics on prevention of respiratory infection in particular for those at higher risk, including obese and older people," the team wrote.

During the DDW press briefing, Mullish noted that the case for probiotics as a COVID-19 preventive or treatment is built largely on "strands of circumstantial evidence."

Preclinical studies have pointed to "perturbation of the gut microbiome" as an influence on immune response, he said. But he also pointed to a small study from Italy in which COVID patients received probiotics, and none progressed to needing respiratory support. On the other hand, he said he was unaware of any studies of probiotics as prophylaxis against COVID-19. "It would be really interesting to look into," Mullish said.