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Are probiotics actually worth taking? Is there any science suggesting that they're effective?

Perhaps you've seen over-the-top ads for probiotics — live yeasts and bacteria that you can take for a rainbow of wonderful effects: "Strengthen your natural defenses!" "Burn fat and flab!" "Improve gastrointestinal function!"

The first thing to know about probiotics is that most of these ads are wildly exaggerated. As UpToDate, the guide to medical research for physicians, put it dryly: "Enthusiasm for probiotics has outpaced the scientific evidence."

But that doesn't mean probiotics are useless. There's actually decent evidence that they can be effective to treat specific ailments. And it's quite possible that scientists could find more promising uses for probiotics in the future. We just have to look carefully at the evidence and not buy into the hype.

Let's start by understanding what probiotics are supposed to do. Every human being is colonized by millions of microorganisms in his or her body, including bacteria, viruses, and yeast. Scientists once thought these microbes were mostly harmful. But nowadays, the consensus has shifted. Our microbial colonies — or "microbiome" — actually help us survive and thrive. That's where the phrase "friendly bacteria" comes from. And sometimes if we kill off too many of these friendly bacteria (say, because we're taking antibiotics), it can have adverse effects.

That's where probiotics come in. These are essentially live microbes you can buy in supplement, powder, fermented milk, or yogurt form that consist of yeast or bacteria (particularly lactobacillus acidophilus). The idea is that when you consume probiotics, they enhance or restore a proper balance in our microbiomes — bringing back that friendly bacteria and boosting our health.

This isn't a crazy idea. The catch is that right now, scientists still haven't identified what a "good" or "bad" microbiome actually looks like. (It doesn't help that all people's microbiomes are different and often changing, depending on things like diet.) So they can't be sure how probiotics actually work, which probiotics to take, and at what dosage.

At this point, all scientists can say is that probiotics do seem to help treat certain ailments — even if they don't yet understand how that happens.

One systematic review, published in JAMA in 2012, found that probiotics can reduce the risk of diarrhea associated with antibiotic use. There's also compelling evidence that probiotics can help treat infectious diarrhea in children and adults, modestly shortening the duration of illness. In constipated people, it seems probiotics can boost the frequency of bowel movements.

But these findings are still pretty circumscribed. Many of the other popular claims about probiotics — that they treat irritable bowel syndrome, prevent allergies, or help with weight loss — remain unproven. This recent review debunked the notion that probiotics add diversity to the gut. The researchers rounded up the findings of seven randomized control trials, and found no evidence that probiotics had an effect on the microbioata composition in feces from healthy humans.

"We don't know which probiotics are the most useful"

What's more, there are important caveats about the research base generall: The bulk of probiotic studies are small and use different doses and types of supplements, so extrapolating from this evidence is really difficult. Sydne Jennifer Newberry, a nutritionist and evidence reviewer at RAND Corporation who has studied probiotics, puts it this way: "We don't know which probiotics are the most useful. We don't know if it depends on which antibiotics people are taking. And the ones you buy off the shelf have no quality control."

That last point is important. Just because some probiotics can treat ailments doesn't mean that the probiotics in your grocery store will be effective. In the United States, probiotics are regulated as food or supplements, not pharmaceuticals, so none of the wild claims on their packaging need to be backed up by evidence. (For more on just how unregulated the supplement industry is, see here.) Many of the probiotics sold in stores have never even been studied in humans.

As Pieter Cohen, a professor at Harvard Medical School who studies supplements, told me: "The problem is that if anything is sold as a dietary supplement, who knows what's actually in it." He added, "There is likely a role for probiotics, but until the laws are reformed so we're assured what's on store shelves, we can't trust that what's on the label is found in the bottle."

Yogurt is a perfect example. While many yogurts are advertised as a good source of probiotics, the reality is rather murky. Some yogurts may not have many probiotics at all, since the bacteria often don't survive the pasteurization process. And since food is regulated much more lightly than drugs, you also can't really trust the label. Activia does not have to make sure, for instance, that every carton of yogurt is equally chock-full of the microorganisms it claims to contain on its packaging. (Making things worse, the Federal Trade Commission has caught yogurt manufacturers making misleading claims in the past.)

So it's perhaps unsurprising that when the European Food Safety Authority put food companies' health claims about probiotics under a microscope, it rejected nearly every one, finding none of the bacterial strains examined were proven to do the things they promised, like boosting immunity or gut health. As a result, according to Nature, "The EU stated that after December 14, 2012, food and nutritional supplements companies will no longer be allowed to communicate health benefits for their products on account of probiotic content."

So what does this all add up to?

"Probiotics probably do no harm except to your pocketbook"

The available data suggests that probiotics are generally safe to take (although you should consult your doctor before taking them — especially if you've got an immune disorder). "There's probably no harm except to your pocketbook," says Newberry.

But if you do decide to try them, consider seeking out probiotics that have at least been studied in humans. This guide to probiotic supplements, recommended to me by Gregor Reid at the University of Western Ontario, is helpful, listing the products that actually have evidence behind them.

In the future, as researchers gather more evidence and deepen their understanding of the microbiome, we may find more uses for probiotics. Perhaps one day they could prove a promising treatment. For now, though, don't expect any miracles.

Send your questions to Julia via the submission form or @juliaoftoronto on Twitter. Read more about Dear Julia here.