Probiotics might help your allergies, but we’re still not sure how

We’re still figuring out how the bugs in your gut make you sniffle.

Despite affecting some 50 million Americans, allergies aren’t super well understood. The sparks that ignite your immune system can range from sunlight to onions, and symptoms of an attack are just as varied. For that reason, we’re spending several weeks writing about allergies—what they are, how they manifest, and how we can find relief. This is PopSci’s Allergic Reaction.

For so many people with seasonal allergies, spring means a never ending flow of mucus. Drug store aisles are stock full of pills, nasal sprays, and eye drops, but sometimes that phlegm-busting cocktail just doesn’t cut it.

One aisle over are the ever-so-tempting probiotics, which are constantly touted as the cure-all for today’s modern diseases, despite the lack of evidence. But what can they do for allergies?

While there’s evidence to suggest that the bacteria that live inside our guts play a key role in our immune systems, there’s no evidence that any individual probiotic available on the market can reduce the severity and frequency of seasonal allergies.

One of the most comprehensive reviews to date, published in 2015—which looked at 23 studies assessing the effectiveness of various probiotic strains on seasonal allergies—concluded that the majority found probiotics improved symptoms compared to a placebo. That’s incredible promising. The problem, the researchers noted, was that all of the studies used different strains of bacteria, making it impossible to make any sweeping conclusions. In other words, no one strain stood out. One study would find one kind of bacteria effective against grass pollen and another would find another effective, and yet another study would find both of those strains totally ineffective. While the evidence for probiotics as a broad category having some benefit for allergy symptoms writ large is good, we can’t say which bacteria people should employ to treat what. But even with additional research, it’s unlikely that your antihistamine regimen will ever be replaced completely by probiotic pills.

“I doubt that probiotics will be good enough to replace current allergy medications anytime in the near future,” says Matthew Ciorba, a gastroenterologist and director of the inflammatory bowel disease program at Washington University in Saint Louis. “If anything… these are likely to be an adjunct of therapy.”

Probiotics are living microorganisms that, when ingested, are thought to have a beneficial effect on our health. They are found in fermented foods like yogurt and sauerkraut and now, more recently, in pills sold in drug stores. When we swallow them, they make their way to our colons where they join the teeming ecosystem of billions of other bacteria known collectively as the microbiome. Recently, scientists have found that these microbes play a key role in regulating our bodily functions, including how our immune systems respond to various cells.

The milieu of bacteria that take up residence in our guts can change, depending on a variety of factors like what we eat and the environments we interact with. Certain makeups of bacteria are thought to provide benefits to our health, whereas others are thought to potentially lead the way to certain diseases. The idea behind probiotics is to shift an individual’s gut microbiome to support good health and prevent diseases.

So far, though, scientists have only proved a handful of bacterial strains to be successful in treating certain diseases—mostly gastrointestinal conditions like irritable bowel syndrome and constipation.

As for allergies, a plethora of studies—many of them assessed in the 2015 review—have looked at whether various bacterial strains can relieve some of our seasonal symptoms. For example, a 2013 study in the European Journal of Clinical Nutrition, gave 20 people experiencing seasonal allergic rhinitis (a stuffy nose caused by hay fever) a specific strain of Bifidobacterium lactis, which did help reduce their symptoms compared to a placebo. Another 2005 study also looked at seasonal allergies, but with a different bacterial strain, Lactobacillus acidophilus, and found a similar modest improvement in allergy symptoms.

No bacterial strain has been looked at over and over under the same conditions with a huge population of people; as such, no general recommendations can be made. However, as the researchers note in the 2015 review, any adverse effects from the probiotics themselves should be minor and essentially benign. So while the store-bought probiotics likely won’t provide a major benefit, they haven’t been shown to do any major harm, either.

Will we ever be able to use probiotics to treat allergies? Maybe. But first, in addition to narrowing down exactly what strains provide what benefit, researchers also need to understand the method through which these microbes do their beneficial work. We are still a ways off from figuring that out. Some mouse studies show that they might interfere with the way our T cells (a type immune cell that helps to generate a big immune response to fight off an infection) function.

Other animal studies suggest that the probiotics help to modify parts of the immune system called immunoglobulin E (IgE), potentially reducing their production. When you have an allergy attack, your immune system produces tons of these IgEs as an unnecessary, overactive response to an allergen, like pollen or grass. If we can understand exactly how these bacteria do this, we might be able to tailor the probiotics to the allergic need.

“It’s possible that some day a specific probiotic will be identified which can dampen the IGE mediated immune response that is tied to seasonal allergies,“ says Ciorba.

For now, we are stuck with shelves full of bacteria that might benefit us and almost certainly won’t cause us serious harm.