Behind the health claims made by manufacturers of probiotic foods there often lies scanty scientific evidence. Yet, researchers are finding that specific strains of bacteria have substantial therapeutic value when the dosage is right. Sylvia Thompson reports
Scan the dairy produce shelves of any supermarket and you'll find an array of probiotic yogurt drinks and probiotic milks. In some stores, you can even get probiotic butter, cheese and fruit juice. And probiotic ice-creams and sorbets are expected to reach the market soon.
All are promoted as health-enhancing foods with associated claims of preventing colds and flus, relieving diarrhoea in children and restoring the body's immune system following antibiotic medications. (No food product can legally be marketed as preventing, treating or curing specific conditions.)
In the past, people in Ireland would have consumed probiotics by drinking buttermilk or live culture yoghurts. Nowadays most people opt for the heavily marketed probiotic drinks. Maintaining a balanced intestinal flora is the much bandied about fundamental "scientific" basis for taking probiotics. But, what exactly is your intestinal flora and can a probiotic food really keep it balanced?
Probiotics are defined as live micro-organisms (or bacteria) which when consumed in adequate amount confer a health benefit. Just as antibiotics kill bacteria, probiotics are credited with growing them or at least maintaining them.
The intestinal flora is made up of a mix of bacteria present in the intestine or gut. In very general terms, some of this bacteria are disease-causing agents called pathogens (popularly known as bad bacteria) while others are disease-fighting bacteria (popularly known as good bacteria).
Now, here the problems begin: the number and types of bacteria which colonise each individual's intestine vary hugely (the mature gastro-intestinal tract contains some 100,000 billion bacteria, representing more than 400 different species). And whether the specific bacteria culture used in a probiotic food can have an impact on that individual's intestinal flora depends on a large number of other factors.
Consultant gastroenterologist Prof Eamonn Quigley, who is head of University College Cork's medical school and a principal investigator at the Alimentary Pharmabiotic Centre there, has been studying probiotics for eight years.
"It is impossible to make a general comment because there are so many probiotics out there, only some of which have been tested in scientific trials. Before you can make a prediction about a specific probiotic, you need a portfolio of activity in the laboratory and for most probiotics, you don't have that," says Quigley.
For instance, some bacteria marketed as having a beneficial effect on the intestine (ie the large intestine otherwise known as the colon) do not even get there because they are destroyed by acids in the stomach or digestive enzymes in the small intestine. Quigley says: "You have to ask what dose a person is getting and whether that dose has been validated to have an effect. Also, you have to ask what the bacteria is being taken with and whether that improves their activity or not."
Quigley cites a US study which tested a number of probiotic drinks in a microbiology laboratory. "Many products did not contain the bacteria they claimed to or contained other bacteria. There is an enormous quality control issue here," he says.
A similar study by Belgian researchers found that just 20 per cent of 55 probiotic products they tested contained all the organisms that were listed on their labels. Nine of the products had no bacteria in them at all.
Dr Pat O'Mahony, chief specialist in biotechnology at the Food Safety Authority of Ireland, says that one has to maintain a "healthy scepticism" about many of the probiotic products on supermarket shelves. "Competing brands don't always have the evidence to back up their claims. There are some genuine cases out there but probiotic drinks are very popular nowadays and people will go for them."
He points out that there has been no comparative scientific tests carried out on probiotic drinks in the Irish market.
Among the research-producing evidence of the value of probiotics is one Swedish study on Lactobaccillus reuteri, a common bacterium found in the human intestine which is used in probiotic products in Sweden (but not in Ireland). This study found that when factory workers were given a daily probiotic drink containing Lactobaccillus reuteri for 80 days, they were 2.5 times less likely to take time off work for sickness than a group which was given a placebo.
The UCC researchers have discovered that a milk drink containing strains of Bifidus infantis reduced the symptoms of irritable bowel syndrome (IBS) while Lactobacillus salivarius, even though it is a common bacterium species found in the intestine, and a placebo showed no improvement.
"Somewhat to our surprise, we found that the bifidobacterium was very effective and the Lactobacillus basically had no effect and was inseparable from the placebo," says Quigley. The researchers are carrying out more studies to determine both the correct dosage and exactly why this bacterium works to reduce symptoms of IBS such as diarrhoea, constipation and abdominal cramps.
One well-studied bacteria used in some probiotic drinks on the Irish market is Lactobacillus GG (commonly known as LGG). A Finnish study found that children attending day care centres who were given milk containing Lactobacillus GG to drink with all meals over seven months were less likely (by 17 per cent) to suffer from respiratory tract infections with complications.
They also took almost 20 per cent fewer antibiotics than the placebo group who were given ordinary milk. Other studies found that children who were given Lactobacillus GG regularly had a lower incidence of diarrhoea than children receiving a placebo.
Quigley says: "There is also some evidence that certain probiotics can reduce the incidence of allergies in children when given to mothers during pregnancy."
This particular study ties in with other evidence that children born by Caesarean section are more prone to allergies than children born naturally. The theory here is that babies absorb some of the mother's bacteria when passing through the birth canal, thus launching the activity of their immune systems.
Ultimately, probiotics are a good news story in terms of their potential therapeutic value for gastro-intestinal disorders such as inflammatory bowel disease, irritable bowel syndrome, Crohn's disease and ulcerative colitis.
However, precisely because of their potential, commercial interests will continue to generalise from specific scientific evidence to market products which fall below the necessary dosage and type of bacteria required to make a difference.
"It's an area we will have to do more work on in the future," says O'Mahony.
The forthcoming publication of a new consumer leaflet on functional foods (foods which have a health- enhancing effect) will be the first step.