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The notion that certain bacteria – called probiotics – are good for your health is gaining ground. These supposedly helpful bacteria are showing up in foods, drinks and supplements. They're touted to help treat constipation and diarrhea, reduce bloating, fend off colds, or even lower blood pressure and cholesterol.

Now, a new study published in this week's issue of the Journal of the American Medical Association confirms one of their most well-documented uses: preventing diarrhea caused by antibiotics.

As many as 30 per cent of people using antibiotics experience diarrhea as a result of the medication, with symptoms ranging from mild to severe. Antibiotic-associated diarrhea occurs when antibiotics upset the balance of good and bad bacteria in the gut.

Probiotics refer to live organisms (e.g. bacteria and yeast) that, when consumed in certain amounts, exert health benefits. They're normally found in your digestive tract as part of the intestinal flora, a community of more than 400 species of bacteria. Here, probiotic bacteria help inhibit the growth of unfriendly, disease-causing bacteria and stimulate the body's immune response.

It's thought that probiotics prevent antibiotic-associated diarrhea by restoring the balance of friendly bacteria in the intestinal tract. (Antibiotics kill both the good and bad bacteria in the gut.) Antibiotics that most commonly cause diarrhea include Erythromycin, Penicillins, and Tetracyclines.

The new study pooled the results of 63 randomized controlled trials involving 11,811 participants, children and adults being treated with antibiotics. Probiotic use – through capsules or powders – coupled with antibiotic therapy was associated with a 58 per cent lower risk of developing diarrhea, compared with the control group not taking probiotics.

Probiotics are identified by their genus, species and strain. For example, for the probiotic bacterium Lactobacillus rhamnosus GG, the genus is Lactobacillus, the species is rhamnosus, and the strain is GG.

The main types of probiotics in foods and supplements are Lactobacillus and Bifidobacteria.

Most trials in the current report used Lactobacillus rhamnosus and Lactobacillus casei, however very few trials named the strains used.

Knowing the specific bacterium used is important because probiotic bacteria are not created equal; health benefits linked to one strain do not necessarily apply to another. Different strains provide different benefits in the body and effects may vary from person to person.

Research suggests that consuming probiotics regularly may also help prevent allergies and eczema, improve symptoms of lactose intolerance, alleviate bloating and constipation, treat inflammatory bowel disease (e.g. ulcerative colitis, Crohn's disease), lessen cold symptoms, lower cholesterol and blood pressure, and possibly guard against colon cancer.

Many people think of yogurt when they hear the term probiotic. But not all yogurts are considered probiotic foods. In Canada, all yogurts are made with Lactobacillus bulgaricus and Streptococcus thermophilus and typically provide up to 300,000 live bacteria per serving, an amount most experts say is too low to provide health benefits.

Some yogurt manufacturers have added other bacterial strains to increase the probiotic content of their products, ensuring these bacteria reach the intestinal tract where they exert their health benefits. For example, DanActive (Danone) and BioBest Maximmunité (Astro), provide 10 billion live bacteria per serving.

Choosing probiotic foods and supplements is a difficult task, even for me. While these products may contain probiotic bacteria, there's no guarantee they contain the right amount or the right types that are needed to get the health benefits you are looking for.

It's important to look for products backed by scientific research. For example, if you're prone to vaginal yeast infections, not just any probiotic will do. You need to choose a probiotic that contains the specific strains shown to be beneficial for this condition. In this case, those strains are Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 (found in Fem-Dophilus from Jarrow Formulas; available online and in the U.S.).

You need to do your homework: a tall order, I know. How are consumers to know what strains of probiotic have been shown to benefit various health complaints? In my opinion, the onus is on the manufacturer to provide credible product information to consumers and health professionals, so we can help guide our patients to effective products.

If you don't have a specific health complaint and want only to improve the health of your gut, most experts advise choosing a supplement that contains several kinds of bacteria.

Check the website of the company that sells the product. Ideally it should tell you the strain used, the dose per serving and, importantly, the research that backs up the suggested health benefit. Ask for a product brochure at the health food store or pharmacy.

When buying probiotic foods or supplements, read labels. Look for the full probiotic name, which includes the genus, species and then the strain.

Next, look at the dose or number of live organisms per capsule or serving. Probiotic levels are typically given in CFUs, or colony-forming units. Different probiotics are effective at different levels. The suggested serving size or dose should be indicated.

Supplements typically contain between 2 and 6 billion active cells per capsule. Others, like Bio-K+, deliver 12.5 billion active bacteria per capsule and 50 billion per 98-gram serving of fermented milk. VSL#3, a probiotic used to manage ulcerative colitis and irritable bowel syndrome, contains 450 billion live bacteria per packet.

Proper storage conditions should also be listed for supplements. Some studies have revealed that bacteria numbers in probiotic supplements – and foods – drop as their shelf life diminishes. For long-term storage, most probiotic supplements need to be kept refrigerated.

There are ways to increase the amount of friendly bacteria in your intestinal tract without taking probiotics. Eating a fibre-rich diet that includes prebiotics – foods that contain non-digestible carbohydrates – can increase the growth and activity of beneficial bacteria in the gut. Food sources of prebiotics include oatmeal, flaxseed, onions, garlic, leeks, asparagus, chicory root and Jerusalem artichokes.

Probiotics aren't for everyone. Experts advise that people who have a suppressed immune system due to illness or a medical treatment, such as chemotherapy, should speak to their doctor before taking a supplement.

L eslie Beck, a Toronto-based dietitian, is the national director of nutrition for Body Science Centers, medical clinics focusing on healthy aging (