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If you eat probiotic yogurt to ease irritable bowel symptoms such as bloating and gas – or to ward off colds and flu – you might want to rethink your reason for doing so.

It turns out that many probiotic yogurts sold in grocery stores contain too few good bacteria to offer the health benefits determined in clinical trials.

That's the conclusion of a review of 92 probiotic products sold in major Canadian grocery chains, just published in the journal Nutrients.

This doesn't mean, however, that probiotic yogurts aren't good for you. It simply means they might not offer the health perks you were hoping for.

What are probiotics?

Probiotics are live organisms (e.g., bacteria, fungi, yeasts) that, when consumed in certain amounts, exert health benefits.

They're identified by their genus, species and strain. For example, for the probiotic bacterium Lactobacillus acidophilus NCFM (found in Astro's BiobBest yogurt), the genus is Lactobacillus, the species is acidophilus and the strain is NCFM.

Different species and strains deliver different benefits and their effects may vary from person to person. Because each of us has a unique microbiota, we may respond differently to probiotics.

Once reaching the large intestine, most or all probiotic organisms inhibit the growth of disease-causing bacteria, regulate bowel transit time and help maintain a healthy microbiota. (Microbiota refers to the population of tens of trillions of micro-organisms that reside in your large intestine.)

Other probiotic benefits, though, are found only among certain species. Still other effects, such as regulating immune function and influencing brain health, are strain-specific.

Different probiotics have been shown to prevent or treat a host of health conditions including respiratory tract infections, eczema, traveller's diarrhea, irritable bowel syndrome and ulcerative colitis.

Not all yogurts are probiotic

In Canada, all yogurts are made with Lactobacillus bulgaricus and Streptococcus thermophilus – bacteria not recognized as probiotic by Health Canada. These yogurts are still a good source of protein, calcium, magnesium and zinc, but they don't contain beneficial probiotic cultures.

Some yogurt manufacturers, however, have added pre-approved probiotic strains to enable their products to deliver probiotic health benefits.

To be called a probiotic food, yogurt must contain at least one billion live colony-forming units (e.g., active probiotic cultures) of a recognized probiotic species per serving.

Danone's Activia yogurt, for example, contains the patented probiotic strain B.L. Regularis, which has been shown in clinical trials to lessen abdominal pain and bloating in people with irritable bowel syndrome.

The company's DanActive drinkable yogurt has L. casei Danone DN 114-001, a well-studied strain associated with a reduced incidence and/or duration of colds and flu, as well as fewer asthmatic episodes in children.

How probiotic yogurts measure up

Sounds good so far, but here's the thing. Just because a 100-gram tub of probiotic yogurt contains the strain proven in trials to help fend off viruses, prevent constipation or speed recovery from traveller's diarrhea, for instance, that doesn't mean it will have the dose that's required to do so.

The newly published probiotic product review, conducted by researchers from the University of Toronto's Department of Nutritional Sciences, revealed that many probiotic foods fell short. Some, in fact, contained up to 25 times less than what clinical trials have deemed effective.

For instance, you'd need to eat anywhere from eight to 25 servings of Danone's Activia yogurt to consume the dose of its probiotic strain shown to reduce bloating, gas and improve stool consistency.

And you'd have to consume 20 daily servings of Astro BioBest yogurt to get the number of probiotic bacteria shown to reduce the incidence of fever, cough and runny nose. Meanwhile, Danone's DanActive yogurt could offer these benefits with only two servings a day.

The review also found that one-half serving a day of Yoplait's Yoptimal provides the amount of specific probiotic bacteria proven to decrease cavity-causing bacteria.

Current labelling an obstacle

To be fair, the blame doesn't lie entirely with manufacturers.

Under Health Canada's current labelling regulations, probiotic foods must contain one billion live probiotic cultures per serving to be able to state the general claim "contributes to healthy gut flora." So, there's no pressure to add more.

The Word Health Organization, however, recommends that, where scientific evidence exists, strain-specific probiotic health claims should be allowed which would link a product to a specific health benefit.

If strain-specific claims were allowed in Canada, manufacturers would have an incentive to add the effective dose of probiotic bacteria confirmed in clinical trials.

Should you switch to regular yogurt?

While many probiotic yogurts don't live up to their full potential, they still have health benefits that make them a worthy addition to your diet.

All probiotic yogurts offer the core benefits of inhibiting the growth of unfriendly bacteria, regulating bowel transit time and helping maintain a healthy gut microbial balance. Regular yogurt, which is not labelled probiotic, can't do this.

You might consider, though, adding kefir to your regular diet. This fermented milk product, typically sold as a beverage, is made with a mixture of 10 to 20 different types of bacteria and yeasts.

The University of Toronto review found that kefir products had the greatest variety and often the highest dosages of probiotic strains. Liberte's effervescent kefir, for example, contains 45 billion active probiotic cultures.

That said, the probiotic mixtures in kefir have not been well studied and, therefore, their specific health benefits are unknown.

Yet, there is data that to suggest that mixtures of probiotic cultures are more effective than single strains, possibly because of a greater concentration of probiotics, a broader range of action or synergistic effects of different strains.

Keep in mind, too, that as storage time increases, the probiotic content decreases. While your yogurt or kefir may be perfectly fine to eat for a short time after its best-before date, its probiotic cultures will have declined.

Leslie Beck is a registered dietitian.

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