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Read our interview with article coauthor, Jack Gilbert, director of the Microbiome Institute.

Probiotics have been around for centuries. They form part of our ancestral and contemporary diets and have long been associated with positive health benefits. The World Health Organization defines probiotics as “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host.” They are found in many foods, including yogurt, kefir, sauerkraut, and other fermented products. They are also found in breast milk.

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On the other hand, most commercially available probiotics make torrents of health claims that simply have not been supported by research. A pill with “40 billion live organisms” is not going to help your child lose weight or “boost” their immune system. It won’t stop your baby from crying on an airplane, protect your toddler’s teeth from decay, lessen the duration of a cold or flu, or cure acid reflux. It’s a billion-dollar industry with virtually no medical oversight.

There’s no credible evidence that the regular consumption of a probiotic yogurt (whether dairy or non-dairy) will make your child or you any healthier. But this doesn’t stop marketers from suggesting that it’s a delicious panacea. Yogurt with live active cultures is a probiotic in that it can have health benefits, but only very specific health benefits. Yogurt contains living bacteria, such as Lactobacillus delbrueckii and Streptococcus thermophilus. It may also contain other strains of Lactobacillus and Bifidobacteria. Food manufacturers may not like to admit it, but it is difficult to control the types of organisms that grow in these live cultures, despite industry standards set to determine what types of bacteria should be in yogurt.

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A SPOONFUL OF MEDICINE: While the health effects of probiotics, notably yogurt, are generally oversold, some probiotics can improve childhood afflictions, including allergies, eczema, and ear infections.Matthias G. Ziegler / Shutterstock

As far back as the late 19th century, Nobel laureate Elie Metchnikoff was suggesting that regular consumption of fermented cow’s milk could enhance overall health and even delay the onset of age-related neurological decline. Admittedly, Metchnikoff did predict many of the basic tenets of our understanding of the microbiome and human physiology that we accept today. The human microbiome is the community of friendly microbes that populate the human body, as well as a few that, in the wrong context, aren’t so good. This diverse multitude of tiny, invisible creatures helps us out in all kinds of ways, such as digesting food, making vitamins, protecting us from diseases, sculpting our organs, tuning our immune systems, and even shaping our behavior. But despite more than 100 years of research we have yet to find credible support that Metchnikoff’s theories about regular consumption of probiotics were correct.

Many types of probiotics are sold for all sorts of conditions, but very few of them have been vetted in published literature, with appropriately designed trials that would allow us to evaluate whether they actually work. It might not be advisable to give a probiotic to your child unless you need to—and even then you should select one that has been shown to help kids with the condition you are dealing with. Otherwise you’re likely to throw away your money.

On the other hand, you probably can’t avoid encountering them in the supplement aisles of your local pharmacy or grocery story. Over the last 20 to 30 years there has been an explosion in the development of bacterial cultures that are sold as liquids or in the form of pills full of preserved spores. So are these new probiotics any better?

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The answer is, we still don’t know. The lack of clinical trials demonstrating efficacy is the major obstacle to knowing whether they are better than existing formulations that have been shown clinically to help with diarrhea and atopy. Claims that probiotic mixtures will help to support overall health are still woefully unproven.

The idea behind probiotics is seductive. It’s called dysbiosis. The term simply means a microbial imbalance on or inside the body. It has many causes—overuse of antibiotics, lousy diet, contaminated food or water, disease, and so on.

Gut microbes live together harmoniously. They check each other’s growth, and produce food, vitamins, amino acids, and other beneficial compounds for your child’s body to use. They both stimulate and control your child’s immune system and hormonal balance, and even their neurons. But when gut microbes fall out of balance, the community becomes disturbed. This collapse of equilibrium has unpredictable consequences. Some bacteria can overgrow, others starve and disappear, and some will just shut up shop and wait it out, entering a type of dormancy, like a seed waiting for the right conditions to germinate.

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Probiotics is a billion-dollar industry with virtually no medical oversight.

Probiotics are useful when your child’s gut, skin, or other tissues experience dysbiosis. They contain friendly bacteria, many of which are analogous to the bacteria that normally live in the gut, and they can bring balance back to the equation. When the equilibrium of your child’s microbiome is disrupted—when they have dysbiosis—probiotics may help rescue the community, resetting it to their personal norm.

We expect this, though we actually have limited evidence to support the assumption, and we still don’t really understand why it could occur. One theory is that the mass of cells in a probiotic drink or yogurt can activate immune responses that reduce inflammation or alter the way the immune system interacts with the microbiome.

Think of the immune system as a gardener, keeping the bacteria it wants healthy and active and removing the ones that it doesn’t want around. The immune system during dysbiosis is more like a drunk gardener, making very bad choices. A probiotic seems to act to rebalance the immune system and therefore indirectly rebalance the microbiome.

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We have no evidence that probiotics really hang around in your child’s gut. Many commonly available probiotics seem to mostly be excreted, so the only way they could possibly change the body is by interacting with the immune system. Or perhaps they compete for nutrients (albeit briefly) with their existing microbes before being summarily ejected. It’s possible that taking a probiotic early in life—before the microbiome has stabilized to an adult form—could lead to some bacteria becoming part of an infant’s microbiome. But that would not be the sort of probiotic you can buy in the drugstore. It would be the next generation of probiotics we are currently developing. These are based around bacteria that are isolated from humans and selected to fulfill a specific health-promoting role in the developing gut.

Despite the lack of evidence that regular consumption of probiotics provides specific health benefits, many people will tell you they think their children are healthier because of them. Are they acting as a placebo? The answer can be complicated to pin down. Ideally we should be able to measure improvement in some specific aspect of health. But the benefits can also include things that are simply felt or perceived, without being measurable. We have much to learn.

So should you give a probiotic to your child? If you are currently breast-feeding, you are already doing so. There is a wealth of evidence to suggest that if your child has diarrhea, it can be treated with a probiotic. If your child has had an operation, many pediatric surgeons will recommend Greek yogurt for its nutritional and probiotic benefit, especially to reduce the likelihood of bacterial overgrowth in the intestine.

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Probiotics can treat food allergies, which, over the past decade, have risen by as much as 20 percent in developed societies. Cow’s milk allergy is one of the most common food allergies of infancy and early childhood, with an estimated prevalence of 2 to 3 percent worldwide. Symptoms include frequent spitups, vomiting, diarrhea, swollen lips or eyes, runny nose, and persistent distress. In this case, your baby cannot tolerate a protein, called casein, found in cow’s milk.

The immune system can be like a drunk gardener, making very bad choices.

Can probiotics help induce tolerance by exploiting bacteria? A small clinical trial (in which Jack Gilbert’s laboratory participated) aimed to find out. Three groups of infants under 3 months old, all of them allergic to cow’s milk, were fed formula. One received an ordinary brand. Another got formula containing an extensively hydrolyzed form of casein. In such products, the casein protein is broken down into smaller units that your baby can digest. A third group got the hydrolyzed casein plus the probiotic Lactobacillus rhamnosus GG.

Some of the babies who received the probiotic and hydrolyzed formula showed a significant increase in their tolerance to cow’s milk. And others who got the same mixture did not. Why not? To be honest, we are not 100 percent certain. But we found clues by looking at fecal samples of all the babies.

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Those who got better had enriched strains of a type of gut bacteria that ferments carbohydrates in the colon to produce butyrate, a short chain of carbons and hydrogen atoms similar to those found in fats. Butyrate is a useful molecule. It feeds immune cells in the gut that then produce a chemical that dampens inflammation. Thus, more butyrate production in the gut means less inflammation and improved tolerance to a food allergen.

Similarly, a recent study in Australia showed that a commercial probiotic could be effective at reversing peanut allergies. Thirty allergic children got a small daily dose of peanut protein together with increasing doses of the probiotic L. rhamnosus GG, which were eventually equivalent to eating the amount found in a whopping 44 pounds of yogurt every day. After 18 months, 4 in 5 kids could eat peanuts without an allergic reaction. This and other studies have recently led to a change in the American Academy of Pediatrics recommendations on peanuts, suggesting that they be introduced early rather than avoided for the first two years.

Another study found that probiotics can reduce the number and severity of ear infections in young swimmers. Forty-six girls, average age 13, who swam competitively were broken into two groups. One ate 13 ounces of ordinary yogurt a day for eight weeks. The other ate a yogurt naturally enriched with live bacteria for the same amount of time. In the end, while we assume that none of the girls were particularly faster, the girls eating the probiotic yogurt experienced fewer colds and less ear pain.

In a boon to parents everywhere, probiotics have also been shown to reduce ear infections in young children. Sixty-five children with a recent diagnosis of recurrent ear infections caused by the pathogen Streptococcus pyogenes were recruited. Forty-five were treated every day for three months with an oral, slow-release tablet containing the probiotic S. salivarius K12. The kids treated with K12 showed a 90 percent reduction in their episodes of sore throat and a 40 percent reduction in acute ear infections, which was significantly greater than the kids who didn’t consume the probiotic.

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You may have also been reading lately about “prebiotics.” Prebiotics are foods, food ingredients, or additives that influence bacteria living in your child’s gut. They are defined as nondigestible substances that selectively stimulate the growth and activity of one or a few types of bacteria in the colon, with the aim of improving health.

Prebiotics are typically fiber compounds, or oligosaccharides, that your child cannot digest, such as plant cell membranes made of cellulose, which make it all the way down into their lower gut or colon. There they are fermented by resident microbes, which produce other chemicals, some of which can be beneficial, such as short-chain fatty acids. Common prebiotics include galacto-oligosaccharides, oligofructose, and lactulose. A prebiotic that can be consumed by a probiotic may enhance the activity of the probiotic, helping it to colonize successfully.

A recent study showed that a probiotic could be effective at reversing peanut allergies.

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While we are just starting to explore the potential benefits of prebiotics, it seems self-evident that feeding your child’s gut microbes with the right precursors will also enhance gut metabolism. Remember, this is essentially what your baby’s body does with breast milk; human milk oligosaccharides (HMOs) are produced exclusively to feed a growing microbiome. Your child’s microbes produce immunological, neurological, and hormonally active metabolites that play an important role in his health. Feeding microbes the nutrients they require to produce these metabolites can only be good for your child.

The fact that most American and many European diets have abysmally low quantities of dietary fiber may help explain the explosion of immune and behavioral conditions that are so abundant in our societies. Maybe we just aren’t feeding our children’s microbiomes the right kind of foods! They don’t produce the right chemicals and their bodies and even their brains can suffer.

What about adding dietary fiber to your child’s diet? Shouldn’t that help? Companies are constantly developing products such as inulin to use as a food additive. It’s a naturally occurring oligosaccharide produced by many plants, usually extracted from chicory. This allows the food manufacturer to say that the food has beneficial properties because of its increased fiber content. The problem is that many of these products, including inulin, can lose some of their potency during processing. A huge body of literature deals with how these different kinds of additives lack the effects they claim. And in any case, you are likely better off increasing the amounts of whole foods containing naturally occurring prebiotics, including whole grains, legumes, leafy vegetables, and fruits such as raspberries.

Importantly, oligosaccharides are not the only prebiotics. Many other molecules can influence your child’s gut microbes and improve their health. For example, curcumin, the active ingredient in turmeric, is metabolized by gut microbes to produce compounds that reduce inflammation. The cancer drug cyclophosphamide, in the presence of prebiotics, can help the body mount an immune response to promote tumor suppression. And berberine, a bright yellow substance found in Oregon grape, tree turmeric, golden seal, and many other plants can alter gut microbe metabolism to reduce obesity and Type 2 diabetes—but so far only in animals.

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In the future, we expect to be able to support the growth of specific bacteria by adding the right prebiotics, almost like fertilizer for your microbiome. Some companies are suggesting they are doing this already, but the majority of their evidence would not pass muster in the scientific literature.

Jack Gilbert, Ph.D., is a professor of surgery at the University of Chicago and director of the Microbiome Institute. He is a cofounder of the Earth Microbiome Project and American Gut.

Rob Knight, Ph.D., is a professor of pediatrics and computer science and engineering and director of the Center for Microbiome Innovation at the University of California, San Diego. He is a cofounder of the Earth Microbiome Project and American Gut.

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Sandra Blakeslee has worked for The New York Times for nearly 45 years, winning multiple journalism awards.

From Dirt Is Good by Jack Gilbert, Ph.D., Rob Knight, Ph.D., with Sandra Blakeslee. Copyright @2017 by the authors and reprinted by permissions St. Martin’s Press.

Lead photo collage credits: Pinkomelet; Gabor Tinz/ Shutterstock

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