According to experts as many as half of all probiotic health drinks do not contain the healthy bacteria they claim on the label.
A panel of British microbiologists from British warned yesterday that though established brands such as Yakult, Danone, Müller or Nestlé do not try to mislead consumers, as many as 25 products many of which are sold on the web - either do not contain the right bacteria, or contain it in too small quantities.
The experts say consumers should be suspicious unless the probiotic drinks and capsules state they contain lactobacilli or bifidobacterium and stipulate a minimum of 10 million bacteria per bottle.
The panel comprising Glenn Gibson, professor of food microbiology at the University of Reading, Sandra McFarlane, a microbiologist at Dundee University, and Christine Edwards, head of human nutrition at Glasgow University, say consumers need to be aware of these facts.
It seems the "healthy bacteria" account for 10-15% of the bacteria in the gut in adults, but become depleted through poor diets containing too much fatty, low fibre, preservative-laden food, environmental factors, antibiotics and stress.
Probiotic products attempt to redress the balance, and consumers are increasingly buying into the idea of replenishing their supplies of "healthy bacteria" with probiotic yoghurt drinks and yoghurts; a market said to be worth £307m a year.
Probiotics are thought to aid digestion and cut the risk of stomach upsets and some research suggests they help prevent bowel conditions such as ulcerative colitis, protect children against allergies, and may even reduce the risk of colon cancer.
The Food Standards Agency in the UK says that research shows that some probiotics do reach the gut but out of 35 bacterial strains in 12 commercial products, only lactobacillus was sufficiently robust to survive the whole digestive process.
Professor Gibson, who led the research, warns that half of the 50 products available in the UK contained the "wrong" bacteria and were of no benefit but there is no legislation to protect consumers against this.
As an example he notes a powder called Acidophilus that contains an organism called clostridia, makes spores that can be resistant to antibiotics.
The scientists also advise that anyone over 65 should take the recommended products as with age the number of healthy bacteria decrease 1,000-fold to around just 2% of the gut's natural flora.
Probiotic are also advised for anyone taking antibiotics and for people going into hospital in the hope that it might help ward against hospital-acquired infections such as MRSA or clostridium difficile.
Professor Edwards says the ideal would be for people to look at their entire diet, but with only 8% of Britons eating a healthy diet, functional foods such as probiotics may be necessary.