Elderly people should take probiotic supplements, according to scientists.
They said the drinks, yoghurts or capsules could help protect older people against bowel conditions, such as Irritable Bowel Syndrome (IBS).
People over 60 have about 1,000-fold less "friendly" bacteria in their guts compared with other adults.
But experts, speaking at a briefing in London, also warned that some of the smaller brands on sale in the UK were not effective.
The human gut contains different strains of bacteria, some of which are "friendly", such as bifidobacteria or lactobacilli, some that help with food digestion and some that are disease-causing.
Having a balance of friendly bacteria is thought to stop harmful forms taking hold and causing disease.
Probiotics, meaning "for life", are products that contain live strains of bacteria incorporated into yoghurts, fruit juices or freeze-dried powders, which boost levels of the friendly bacteria in the gut.
Drop in bacteria
Glenn Gibson, professor of food microbiology at Reading University, said: "The (scientific) literature has reported about 80 human studies with positive results against bowel conditions like travellers diarrhoea, irritable bowel syndrome and antibiotic-associated diarrhoea."
He said, while probiotic products were useful for the healthy population - helping to prevent bacteria which cause food poisoning, such as E.coli or campylobacter, from taking hold, they would be even more beneficial for older people.
Dr Sandra McFarlane, from the microbiology and gut biology group at the University of Dundee said that as people got older they had reduced levels of friendly bacteria and increased levels of disease-causing bacteria.
She said at about the age of 60 there was a big drop in bacteria levels, and older people had 1,000-fold less friendly bacteria than other younger adults.
They were also more susceptible to gastrointestinal infections and bowel conditions, like IBS, she added.
Professor Gibson added that it was no surprise the 21 people who died in an E.coli outbreak in Lanarkshire, Scotland, in 1996, were all elderly.
The scientists said this population would benefit from a boost in friendly bacteria, and it would help to protect against acute and chronic bowel diseases.
They also said people of all ages on antibiotics would benefit from probiotic products because antibiotics reduce levels of all strains of bacteria in the gut.
Rogue products
However, the researchers also cautioned consumers to check that the products being consumed were effective.
Professor Gibson said of about 50 brands available to buy in the UK, about half did not live up to their claims.
He said while the larger manufacturers such as Nestle, Danone, Seven Seas and Yakult, made products that were effective, other smaller brands did not.
He also said some "bio-yoghurts" on sale were not the same as probiotics, rather they just had bacteria strains that were useful for making yoghurts.
He explained that it was important the product contained the correct strain of live bacteria, such as bifidobacteria or lactobacilli, that they had scientific evidence that the bacteria survived the digestive process, and that there was at least 10 million bacterium in the product.
He added that consumers should check this information with the manufacturer if the label did not specify these details.
Recently introduced EU legislation will mean probiotics manufacturers will have to provide a dossier of scientific evidence for their product, as well as safety information. Professor Gibson said this would help to reduce the number of "spurious" products.
However, Peter Gibson, professor of gastroenterology at Monash University, Melbourne, Australia, added a note of caution, warning the increasing use of probiotic products was not matched by an equivalent amount of research.
"What we still debate is whether probiotics are going to have much impact in improving the health of our community.
"There are only a few specific areas where probiotics have proven benefit so far."
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