Yogurt, the fermented panacea?

Have the health benefits of yogurt been exaggerated? 'Science-ish' investigates.

The Statement: “Fermented milk products such as yogurt and kefir contain friendly probiotic bacteria that may help guard against disease… Yogurt’s active bacterial cultures can help stimulate the immune system, reduce symptoms of antibiotic-associated diarrhea and improve the absorption of lactose in people with mild to moderate lactose intolerance.” (Leslie Beck, The Globe and Mail, 06/23/2011)

Yogurt has been starring in the news lately for its panacea-like ability to do everything from “stimulate the immune system,” as Beck writes, to help aid weight loss, according to reports about a new 20-year Harvard study. Of the latter, the Toronto Star recently stated, “Vegetables and fruits helped keep weight in check, but nuts and yogurt were the most effective. Most of the test subjects ate no-fat or low-fat yogurt.”

But before you go stocking up on the fermented miracle food, know this: research into the health benefits of yogurt and probiotics is patchy and inconsistent.

Let’s start with the question of weight loss. Dr. Brian Haynes, professor of clinical epidemiology and medicine at McMaster University, looked at the Toronto Star article and found the claim that certain foods, such as yogurt, were more effective at keeping weight off to be a “misdemeanor” of sorts. He said, “The study does not test whether one food item has a different effect than another. If you switched, say, yogurt and nuts for vegetables and fruit, you would likely gain weight—especially if you were heavy on the nuts, which have way more calories than low-fat yogurt.”

As with many nutrition studies, it’s important to note the great difficulty (near impossibility?) of drawing correlations between the consumption of a specific food item and a particular health outcome. For example, people who eat low-fat yogurt are likely to have healthy lifestyles or may be trying to control their weight, so their success at the scale may have more to do with a calorie-limiting approach than any special attributes of the dairy products they consume.

Also, Dr. Haynes pointed out that those who reported on the study may have missed this little caveat from the authors: “Yogurt consumption was also associated with less weight gain in all three cohorts. Potential mechanisms for these findings are unclear; intriguing evidence suggests that changes in colonic bacteria might influence weight gain.”

In other words, even the study’s authors can’t explain the association between yogurt and weight.

For Beck’s assertions that the “friendly probiotic bacteria” found in yogurt may “help guard against disease” and “stimulate the immune system,” we called Dr. Gregor Reid, director of the London, Ont.-based Canadian Research & Development Centre for Probiotics.

(Before we go any further, it’s worth defining what probiotics are. According to the WHO, they consist of “live microorganisms which when administered in adequate amounts confer a health benefit on the host.” They can include one or more species of bacteria, including Lactobacillus and Bifidobacterium—both part of normal human intestinal flora—or yeasts such as Saccharomyces.)

Dr. Reid noted that probiotics come in many different forms and have many different health effects—many of which haven’t been adequately studied. These effects are not all intrinsic to yogurt. “People have to be very careful when they generalize about yogurt,” said Dr. Reid, pointing out that few yogurts in Canada actually contain probiotics, despite the claims bandied about in the dairy aisle.

Of Beck’s statement that yogurt’s “active bacterial cultures can help stimulate the immune system,” Reid said, “Yogurt has active bacterial cultures but, in general, they don’t do anything to the immune system because they die in the stomach and bile.” In fact, the European Food Safety Authority (EFSA) reviewed the scientific evidence for health claims made by food companies, and it found that of the hundreds of probiotic strains of bacteria examined, none were shown to improve immunity or gut health.

After these negative rulings, yogurt-maker Danone withdrew its requests for the EFSA to approve claims about its Actimel and Activia yogurts’ immune-system boosting and digestion-aiding effects. On a similar note, in the US last year, the Federal Trade Commission found Danone went too far in promoting the healing power of its yogurt products. The company agreed to pay the government $21 million in a settlement and was prohibited from claiming any “yogurt, dairy drink or probiotic food or drink reduces the likelihood of getting a cold or the flu.”

In 2004, the esteemed Drug and Therapeutics Bulletin analyzed the scientific literature on probiotics and their relationship to gastrointestinal disorders. They found that the evidence about whether probiotics improve gut health was patchy, though there was strong evidence to suggest probiotics help children with acute infectious diarrhea. The authors also stated, however, that more studies were needed to find which strains and doses of probiotics are most effective and whether probiotics help adults with the same affliction.

Science-ish found another systematic review (the highest form of evidence) on the subject of probiotics and diarrhea, which concluded that they can shorten the duration of acute diarrhea and reduce the risk of a prolonged diarrhea episode. So while Beck could have been more specific about the effects of yogurt on diarrhea, there was some evidence to back this claim.

The probiotics question is just one example of the general dubiousness of diet-related health claims that flourish like bacteria in newspapers every day—an inspiration for this blog and something you’ll be reading more about. As one of the patron saints of Science-ish, Ben Goldacre of the Guardian newspaper, writes in his book Bad Science, “Diet has been studied very extensively, and there are some things that we know with a fair degree of certainty: there is reasonably convincing evidence that having a diet rich in fresh fruit and vegetables, with natural sources of dietary fiber, avoiding obesity, moderating one’s intake of alcohol, cutting out cigarettes, and taking physical exercise are protective against such things as cancer and heart disease.” Everything else is probably science-ish.

*Thanks to reader Mariam Morshedi from New York City for requesting a post on health claims about yogurt. “What’s with probiotics?” she asked.

Julia Belluz is the associate editor at The Medical Post. Got a tip? Seen something that’s Science-ish? Message her at julia.belluz@medicalpost.rogers.com or on Twitter @juliaoftoronto