Q: I don’t have any health issues, but I recently started taking a probiotic supplement because I’ve read that it’s good for me. Is this true? What is it doing for me?

Probiotics have been credited with a wide range of health benefits including gut health, lowering cholesterol, preventing colds and flu and mood balance. Most studies, however, have focused on people with existing health conditions.

If you’re already healthy, taking a probiotic won’t make a notable difference to your health. But that doesn’t mean that they’re totally useless.

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Probiotics are defined as live micro-organisms (e.g., bacteria, yeasts) that, when consumed in the right amounts, deliver health benefits. Most probiotics are similar to the types of bacteria found naturally in our large intestine.

Our gut is home to trillions of microbes, including both “good” and “bad” bacteria. Sometimes, though, the balance between intestinal bacteria can shift in favour of bad bacteria (called dysbiosis) and cause health problems.

Probiotics are identified by their genus, species and strain. For example, for the probiotic bacterium Lactobacillus reuteri RD-14, the genus is Lactobacillus, the species is reuteri, and the strain is RD-14.

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A 2017 Canadian study found that many probiotic yogurts don’t contain enough beneficial bacteria to offer the health benefits determined in clinical trials.

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Probiotic’s specific benefits

In your large intestine, most or all probiotic organisms inhibit the growth of disease-causing bacteria, regulate bowel transit time and help maintain a healthy balance of gut microbes. Other probiotic benefits, though, are found only among certain species. Still other effects are strain-specific.

A few examples: Saccharomyces boulardii (a yeast) has been demonstrated to decrease the risk of antibiotic-associated diarrhea, while Lactobacillus rhamnosus GG helps reduce infection-related diarrhea. Lactobacillus reuteri NCIMB 30242 lowers LDL (bad) cholesterol and Bifidobacterium infantis 35624 eases symptoms of irritable bowel syndrome.

Specific strains have also been shown to improve ulcerative colitis, constipation, eczema, urinary tract infections and vaginal yeast infections.

And preliminary research suggests that certain probiotics can help treat depression and anxiety and control blood sugar in people with diabetes. Probiotics are also being studied for their role in weight control and athletic performance.

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Do probiotics help healthy people?

Until recently, there was limited evidence that taking probiotics on a daily basis delivered benefits to already healthy people. Most experts contended that they didn’t do much, if any, good.

A review of 45 studies published earlier this year in the European Journal of Clinical Nutrition, however, concluded that probiotics (in supplements or yogurt) do, in fact, benefit healthy people.

Probiotics may increase the amount of good bacteria in the gut, which can be reduced if you don’t eat enough fruits, vegetables and whole grains, if you drink too much alcohol, if you’re under stress and/or if you don’t get enough sleep.

The study also found that probiotics can reduce abdominal discomfort caused by irregular bowel movements, increase beneficial bacteria in the vagina (which could help prevent urinary tract infections and bacterial vaginosis) and boost the immune system.

If you choose to take a probiotic to reap these potential benefits, look for a product that provides at least one billion live cells and contains a variety of Lactobacillus and Bifidobacterium species.

Keep in mind, though, that you’ll need to continue taking probiotic supplements for their benefits to last. Once you stop taking them, gut bacteria will return to its original composition within one to three weeks.

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Are there downsides?

In healthy people, probiotics have a good safety record. Mild side effects can include bloating and gas. Probiotics may carry risks, however, in people who are critically ill, who have had surgery or who have weakened immune systems.

According to a review of 384 randomized controlled trials published this summer in the Annals of Internal Medicine, the safety of probiotics isn’t fully known. The researchers found that the reporting of side effects in studies was often missing or inadequate.

Foods versus supplements

Probiotic yogurts must contain at least one billion live units per serving of a recognized probiotic species. A 2017 Canadian study, however, found that many probiotic yogurts don’t contain enough beneficial bacteria to offer the health benefits determined in clinical trials.

Fermented foods such as kefir, kimchi, sauerkraut, miso and kombucha also contain probiotic cultures, but because their cultures aren’t well-defined in terms of strain composition and stability, they can’t be called “probiotics.” (This doesn’t that mean they don’t provide some of the benefits associated with probiotics.)

Unless you have a health condition that could benefit from a probiotic supplement, I recommend that you add prebiotic foods to your daily diet, foods that contain fermentable fibres that, once in your large intestine, stimulate the growth of beneficial bacteria.

Prebiotic foods include asparagus, Jerusalem artichokes, jicama, rye, barley, kefir, leeks, onions, garlic and chicory root. Other sources include white beans, chickpeas, lentils, green peas, cashews, whole grain pumpernickel bread, barley, raw oats and muesli.

Leslie Beck, a Toronto-based private practice dietitian, is Director of Food and Nutrition at Medcan Clinic.