A type of probiotic -- the term for bacteria supplements deemed to be good for health -- boosted the immune system of long-distance runners, according to an Australian study published on Thursday.
Twenty elite male athletes, competing in events ranging from 800m to the marathon, were enrolled in an experiment that coincided with intensive winter training.
The volunteers were given the probiotic Lactobacillus fermentum, in the form of freeze-dried powder in gelatin capsules, or a dummy lookalike capsule called a placebo that contained harmless starch. Neither they nor the researchers knew what the individual capsules contained.
Each runner took the probiotics daily for a four-week stretch, followed by a four-week "washout period" in which they took nothing. They then followed this with the placebo for four weeks, followed by another "washout."
The bug did not change athletic performance but did reduce respiratory illness, according to the researchers, led by David Pyne of the Australian Institute of Sport in Canberra.
Seven of the runners reported having respiratory problems, such as a sore throat, a cough, runny nose, chest congestion and sneezing on days when they took the placebo.
These problems were recorded on a total of 72 days and rated on average 1.7 on a scale of severity, where three was the highest mark.
But when they took the probiotics, only three of the runners reported respiratory problems. The symptoms occurred on only 30 days and rated 1.0 for severity.
In addition, blood samples showed that levels of interferon gamma -- an immune-system molecule that is a key player in fighting viral infection -- doubled during the "probiotic" period.
Lactobacillus fermentum is a lactic acid bacterium that is well established as a treatment for diarrhoea and other gut infections.
Probiotics are increasingly showing up in food products, led by Lactobacilli -- the germ that provides the sour taste in yoghurt and other fermented dairy foods.
The argument made for probiotics is that they strengthen gut flora and boost the immune system against disease.
But the mechanism by which this happens is unclear and the clinical evidence for some claims is sometimes sketchy or absent.
In a study unveiled last month and published online on Thursday by The Lancet, Dutch researchers, warned against using probiotics to treat patients with a dangerous disease of the pancreas called severe acute pancreatitis.
Among 296 volunteers, 24 people among the probiotics group died, a mortality rate of 16 percent, compared with nine (six percent) in the placebo group.
The experiment involved a combination of six probiotics: three strains of Lactobacillus (acidophilus, casei, salivarius), one of Lactococcus (L. lactis) and two of Bifidobacterium (bifidum and infantis).