Millions of us drink yoghurts with 'friendly bacteria' in the belief they are doing us some good. But are they really? Clare Rudebeck reports

Back in 1908, Elie Metchnikoff, a Russian microbiologist, observed that the Bulgarian peasantry tended to live to a ripe old age. After investigation, he concluded that the secret of their longevity was the yoghurt they drank. His theory led to a vogue for drinking soured milk in the belief that it would prolong life.

Back in 1908, Elie Metchnikoff, a Russian microbiologist, observed that the Bulgarian peasantry tended to live to a ripe old age. After investigation, he concluded that the secret of their longevity was the yoghurt they drank. His theory led to a vogue for drinking soured milk in the belief that it would prolong life.

Eighty years later, we are once again in the grip of "probiotic" mania. In the last year, sales of probiotic drinks have risen by more than 50 per cent - the UK market is now worth £150m. Advertisements for probiotics tell us we will "feel the difference in two weeks" and extol the virtues of "friendly bacteria". But what exactly are they? We seem to have very little idea. Earlier this month, Tesco reported that probiotic drinks were their fastest-growing dairy food product. Customer feedback revealed that people were buying the drinks in a bid to combat hangovers and the after-effects of eating spicy foods.

However, none of the eight probiotic yoghurt drinks on sale in the UK claims to be either a hangover cure or a quick-fix for indigestion. And there is no evidence that they would help either affliction. Like the Edwardians who swore by a bowl of yoghurt in the morning, we seem to be in the dark about what products such as Yakult, Actimel and Vitality do for us.

The packaging for most probiotic drinks clearly states the type of bacteria contained inside. The "friendly bacteria" in Yakult are lactobacilli casei shirota. In Actimel, they are lactobacilli casei imunitass. Members of the lactobacilli group, which are found in the human gut, help to fight off harmful bacteria. The idea behind drinking this "friendly bacteria" is that it will top up the beneficial bacteria already there.

"Whenever you eat, you introduce harmful bacteria into your system and so your intestine needs to be very good at keeping them at bay," says Jeremy Hamilton-Miller, the Emeritus Professor at the department of medical microbiology at the Royal Free Hospital in London. "Your body kills these invaders with stomach acid and, if that doesn't work, it does it with so-called friendly bacteria."

Professor Hamilton-Miller explains that if the bacteria in your gut - the gut flora - become unbalanced, this can lead to diarrhoea and constipation - and, in the more extreme cases, to cancer and inflammatory bowel disease. The theory is that by taking extra "friendly bacteria", you can help to ward off these illnesses. However, this has not been proven to work.

Studies show that particular types of probiotic bacteria can protect against diarrhoea. There is also evidence that some probiotic bacteria can reduce cholesterol levels and strengthen the immune system. Experiments on animals have suggested that lactobacilli probiotics can have anti-cancer properties. But the evidence is far from conclusive.

"Probiotic bacteria have been shown to have anti-cancer properties, to regress tumours, but that does not mean that every probiotic on the shelf will do that," says Dr Simon Cutting, head of the Centre for Biomedical Sciences at Royal Holloway, London.

To work out whether a probiotic yoghurt drink can benefit your health, Dr Cutting says that you need to know both the particular probiotic bacteria (the strain) and the dose that it contains. "Everyone wants to know what E numbers are in their food, but for some reason they don't want to know exactly what kind of bacteria they are drinking, and in what quantities."

The dose is important because the stomach will kill off much of the probiotic bacteria swallowed. "A drink needs to contain at least a billion bacteria otherwise it is unlikely to be effective," says Dr Cutting.

Indeed, the Food Standards Agency has commissioned research into whether popular brands of probiotic yoghurt drinks do make it past the stomach and into the intestine. It did so because it is concerned at "an almost total absence of comparative data on the survival of probiotics in the human gut."

So, where do the most popular brands, Yakult and Actimel, fit into this complicated picture? Do they contain enough bacteria to be effective? And do the particular strains of probiotic bacteria that they contain have any proven health benefits?

Yakult, which means yoghurt in Esperanto, was developed in Japan more than 70 years ago. In 1930, Dr Minoru Shirota, a researcher at the faculty of medicine at Kyoto University, isolated a strain of probiotic bacteria, which was named lactobacillus casei shirota in his honour. Shirota then developed the drink, Yakult. It is now drunk by an estimated 25 million people every day.

Yakult clearly identifies the exact strain of probiotic bacteria that its product contains. And each dose contains 6.5bn bacteria - well over the recommended one billion. These two facts were confirmed by an independent study by Professor Hamilton-Miller at the Royal Free Hospital.

However, Yakult, in common with all other probiotic drinks, does not claim that its product can treat any diseases. In fact, it is prevented from doing so by UK and EU law, which bans the use of medicinal claims for foods.

As a result, its TV advertising shows a woman strangely attracted to a geek who knows all about the benefits of "friendly bacteria." There is no mention of whether this friendly bacteria will do you any good. "Yakult is a food product, not a medicine," says Professor Colette Shortt, a visiting lecturer at the University of Ulster and the director of science at Yakult. "We are not here to prevent or treat or cure any illness. What we promise is that Yakult positively influences the balance of beneficial bacteria in the gut."

It is a very modest claim. However, Professor Shortt points out that Yakult has recently published a study in a Canadian dietetics journal that shows that consuming Yakult eases constipation. She says that it has done no research into whether it can ease hangover or indigestion.

Actimel's current TV advertisement shows a boy waiting for his over-active mother to stop playing on a swing. The slogan is, "Try it and see if you feel the difference." On packs of Actimel, consumers are told they will "feel the difference in two weeks".

Danone will not confirm what this "difference" is, but points out that its customers must have felt it, whatever it is. In September and October last year, the company offered people their money back if they did not feel different in a fortnight. Nine and a half million purchases were made in that time - and only 45 people out of that number requested reimbursement.

Like Yakult, Danone clearly labels the exact strain used in its product: lactobacillus casei imunitass. And it guarantees a dose of 10 billion per serving. These claims were independently verified by Professor Hamilton-Miller.

The consensus is that probiotics do help to reduce the incidence of diarrhoea. However, not in the way that most people expect. "People think that if they take Yakult they won't get diarrhoea. It doesn't work like that," says Dr Cutting. "You would have to take one dose a day religiously. And if you did that, you would have less diarrhoea than someone who didn't take it over the course of your lives."

Other health benefits, such as the ability of some types of probiotics to reduce cholesterol, strengthen the immune system and reduce tumours, have not been proven. As research continues into what these friendly bacteria can do for us, the most important message for the consumer is to buy a good quality probiotic that clearly identifies the strain it contains, and which guarantees the dose it delivers.