Many of the bacteria that naturally live in the human gut aid in digestion and protect the intestinal lining.
These 'friendly bacteria' or probiotics, may also stimulate the immune system and may help to keep potentially harmful microorganisms in check, both through competition for a limited supply of nutrients and possibly by producing substances that inhibit the growth of other organisms.
Probiotics in History and Practice
Health claims for foods such as fermented milk date back to biblical times. With the discovery of microorganisms in the late 19th and early 20th centuries, microbiologists came to realize that the range of intestinal microflora (tiny organisms present in the gut) of healthy people was different to those who were ill. Giving the ill people some of the bacterial cultures they were missing was found to be helpful in treating symptoms such as diarrhoea and these effects were attributed to alterations in the balance of intestinal flora.
The term probiotic was first used in 1965 to refer to substances secreted by one organism that stimulated the growth of another. Over time its meaning was broadened to encompass any product that contains live microorganisms that alter the microflora of a host animal and are beneficial for health.
Probiotics in Practice
Disturbances in the intestines can reduce the population of some probiotic bacteria and cause the overgrowth of others, along with inflammation, infection and digestive disturbances. In some cases these consequences may be prevented by or alleviated by taking probiotics from outside sources. Probiotics are found in foods including cultured milk products such as yoghurt, and can be taken as supplements in capsules, tablets, powders or liquids.
Probiotic organisms feed on undigestible carbohydrates called prebiotics, stimulating growth and activity of beneficial bacteria on the intestinal flora. Prebiotics include complex sugars and foods such as raw milk, bananas, garlic and miso soup. Prebiotics stimulate the growth of probiotics and suppress the growth of potentially harmful organisms. Supplements that combine prebiotics and probiotics are called symbiotics.
Probiotics as Treatments
Many different species of bacteria have been found to have probiotic properties, including acidophilus, lacrobacillus bulgaricus, bifidobacteria and lacrobacilli and the yeast accharomyces.
In addition, prebiotic mixtures of fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS) stimulates the growth of intestinal bifidobacteria and lactobacilli and reduce the growth of potentially harmful microorganisms.
Does the Evidence for Probiotics Stack Up?
Despite the shortage of proper, in-depth studies, there is some evidence that probiotics can help people with gut disorders and inflammatory diseases and possibly allergies.
Probiotics for Allergies
According to a study from the academy of sciences of the Czech Republic published in 2003 in International Archives of Allergy and Immunology, premature infants given a probiotic strain to colonize the intestine after birth had a significantly reduced incidence of repeated infections and allergies 10 years later. Another group of full-term infants were shown to have a lower incidence of allergies 20 years later.
More recently a Finnish team of researchers led by Emma Marschan at the University of Helsinki conducted an experiment by treating pregnant women with either probiotics, or a placebo. Probiotic children were found to be 30% less likely to develop an itchy skin conditions such as eczema, which is often an early manifestation of allergies.
Roger Katz, an allergist at the University of California at Los Angeles, School of Medicine, says the results are good news for people who know allergies run in the family.
"Even if someone has the genes for allergies, these results suggest that people can take action to at least reduce the chances of developing early allergic conditions like eczema," he says.