Probiotics, which contain millions of the 'good-guy' bacteria, have largely been billed merely as a way to keep the gut healthy. But the latest evidence suggests that the bacteria populating our intestines not only 'kickstart' the immune system, but function as an extension of it, a kind of 'virtual' organ. New developments in probiotics find them useful for treating all manner of diseases, including cancer.
Your intestines contain billions of living bacteria-10 times the number of cells in your body-and new research suggests that they may be crucial to our immune systems and, thus, our health. In fact, the immune system is not a specific entity in itself, but an interaction between our cells and the 'foreign' intestinal flora.
"About 75 per cent of the immune cells of the body are localized to the gut, and almost all immune cells in the body are conditioned in the gut," says Professor Stig Bengmark of University College London. This conditioning is done by bacteria, 400 of which could fit on the dot of this "i".
As Professor Fergus Shanahan of the National University of Ireland explains it: "Gut bacteria have a collective metabolic activity equal to a virtual organ within an organ".
In addition, these bacteria play a vital role in 'completing' our genes (see box on the right).
Although these dramatic discoveries about gut flora and the immune system are relatively new, the means by which we can take advantage of them-probiotics-have been around for a century.
First discovered by Nobel prizewinner Elie Metchnikoff in 1907, probiotic yoghurts, drinks and capsules are one of the fastest-growing sectors in food retailing: UK sales are increasing by 40 per cent each year. Most people take them to promote general wellbeing, but now there's evidence that they can help serious bowel disorders, allergies, cancer and even autism.
The word 'probiotic' means simply 'for life'-in this case, the bacteria that live in our gut. Our intestines are home to around 500 species, some good and some bad. The bad ones such as Escherichia coli, Salmonella and Clostridium are kept in check by the 'good' bacteria, mainly lactobacilli and bifidobacteria. And the experts are only now realizing that the presence or absence of gut bacteria at birth could be the key to a range of health problems in later life.
We are not born with a fully functioning immune system. What it needs is exposure to bacteria-almost entirely from the mother. "The birth process allows the progressive formation of complex intestinal microflora composed of myriad bacteria," says Belgian pediatrician
Dr Jean-Paul Langhendries. "It is used by the young body to initiate its own immune system" (Arch Pediatr, 30 October 2006; epub ahead of print).
So, two things are necessary: natural birth and breastfeeding. Passing down the birth canal exposes the baby to its mother's bacteria. Some of these may be harmful, but that's what the immune system needs, according to the latest theory. Then, as the infant breastfeeds, beneficial lactobacilli from the breastmilk enter the baby's gut to supplant the disease-causing species. This kickstarts the immune system and firmly establishes it-probably for life.
But, modern medicine unwittingly interferes with this process. Infants born by caesarean section or who are not breastfed have a different range of gut bacteria, which can compromise their immune system (Acta Paediatr Suppl, 2003; 91:48-55). Indeed, caesarean-born children have more asthma and food allergies than naturally birthed babies-problems that continue into adulthood (WDDTY vol 17 no 8).