'Feeling run-down? Life getting stressful? Take Product XYZ to help your body cope with a hectic lifestyle." We are beginning to see a number of advertisements along these lines. "Product XYZ" could contain several probiotics - so-called friendly bacteria that exist naturally in our bodies and are essential for good health.
If you have bought yogurts, drinks or even tablets on the basis of such advertisements, are you being conned by pseudoscience or is there a scientific basis for taking these dietary supplements?
The most widely studied probiotics are lactic-acid bacteria, such as lactobacillus and bifidobacterium. They enhance intestinal health by impairing the growth of harmful gut bacteria and lining the intestinal wall, to prevent unwanted bugs attaching to it.
Studies performed on children suggest probiotics help to reduce the duration of diarrhoea. Children taking probiotics on an ongoing basis suffer less from viral-type gastroenteritis.
Evidence is also beginning to emerge of the ability of probiotics to influence the functioning of the immune system. Elderly people given bifidobacterium, for example, show enhanced levels of interferon, the naturally occurring antiviral agent.
But what about specific diseases and medical conditions? Lactobacillus has been shown to reduce the severity and duration of infant diarrhoea. The addition of the probiotic to oral hydration solutions benefits children with acute diarrhoea and has helped bring about earlier discharge from hospital.
Probiotics also help reduce the effects of diarrhoea caused by broad-spectrum antibiotics. Other conditions that seem to benefit include ulcerative colitis - in which probiotics may help to maintain disease remission - irritable bowel syndrome and recurrent vaginal infections.
It is important to emphasise that there is still not enough scientific evidence to show that probiotics reduce cholesterol levels in the blood. And despite a suggestion that these agents can inhibit cancer, there are, as yet, no reliable data to back up claims of probiotics preventing colon cancer in humans.
Lactose intolerance is now a recognised cause of tummy symptoms such as flatulence, diarrhoea, abdominal pain and cramping. This is due to poor enzyme function within the gut, and there is some scientific evidence to show that probiotics replenish an enzyme called beta-galactosidase, which helps to restore the body's normal use of the lactose contained in milk and dairy products.
One of the most interesting aspects of potential probiotic therapy is a possible role in the treatment of allergies. Derived from the Greek word allos, meaning "change of the original state", an allergy is an excessive reaction of the immune system against a foreign substance.
These substances do not pose a danger to our health, but the body defends itself vigorously against them, leading to allergic conditions such as rashes, hay fever and food allergies.
A recent Finnish study has supported the hypothesis that specific intestinal bacteria have an important role in the prevention of allergic disease.
Researchers investigated whether administration of Lactobacillus rhamnosus could prevent eczema. They enrolled 159 pregnant women with personal or family histories of hay fever, eczema or asthma.
Each mum-to-be received two capsules of lactobacillus a day for two to four weeks before delivery. A control group was given a placebo, or dummy pill. After they had their babies, the probiotic was continued for another six months. Breastfed children then got lactobacillus with their mothers' milk; bottle-fed babies were given a suspension of the probiotic bacterium.
The group of children was followed up for two years, by which time 132 of them were still enrolled in the study. The results showed a 50 per cent reduction in the number of cases of eczema in the group that had ingested lactobacillus.
Because of their mothers' medical histories, the children had an increased genetic susceptibility to developing allergic disease. That the probiotic exerted a positive effect strongly suggests the normal flora of the gut might have a useful role in the prevention of allergies.
So should doctors prescribe probiotics routinely? Dr Joe Fitzgibbon, a Galway-based specialist in nutrition and allergies, acknowledges that there is a growing body of evidence to suggest probiotics are a useful therapy.
But he says that "further studies are required to clarify their role in relation to specific clinical conditions, and the specific dosage of each probiotic before they could be routinely prescribed".
The latest edition of Dr Fitzgibbon's book, Feeling Tired All The Time, is published by Newleaf this week. It includes a chapter on nutrition and probiotics