CreditAriel Davis

Our bodies house an extraordinarily vast collection of bacteria and other microorganisms so many, in fact, that scientists now estimate there may be just as many bacteria in our bodies as human cells.

They thrive on the skin, where they might help protect it from infection, and in areas like the intestinal tract, where they help break down food and absorb nutrients. Given the sheer volume, its no wonder scientists are discovering that these microbes play an important role in health and disease.

Research on the microbiome, that collection of microorganisms, is still in early stages, but it has become increasingly common to hear references to the microbiome in the news and advertising. Last week, Bill Gates wrote an essay for The Telegraph suggesting that correcting an out of whack microbiome by taking next-generation probiotic supplements, for example, or eating certain gut-healthy foods could eventually help prevent some of the worlds most common ills, such as malnutrition, obesity and perhaps even inflammatory conditions including asthma, allergies and some autoimmune disorders.

And Dove has been advertising an infant soap with prebiotic moisture, as well as a lotion claiming that it nourishes babys delicate microbiome. Mother Dirt, a skin-care company, also has said that its biome-friendly cleansers, shampoos, moisturizers and more help restore harmony in your skins microbiome.

If youre still a bit confused about what all of this means and who isnt? heres a short introduction to a complicated subject: the role of the microbiome.

The microbiome is a community of bacteria, fungi, viruses and other microbes that live inside your body and on its surface. Just as in a community of people, youll find both good and bad actors: Some of the microbes, like the gut bacteria that help you digest food, are beneficial, while others, like certain viruses, can be dangerous.

Everyones microbiome differs, depending on your age, gender, diet and immune system. And the types of microbes on one part of the body may be different from those on another.

Imagine the microbiome as basically like a rainforest, said Jonathan A. Eisen, Ph.D., an evolutionary biologist who directs the Microbiome Special Research Program at the University of California, Davis. On any given spot on the skin, he said, there might be hundreds to thousands of different species of microbes, which can come in hundreds of different strains. With E. coli, you cant just say you have E. coli on your skin, because some E. coli make vitamins and others kill you, said Dr. Eisen. So this is incredibly complicated.

Even something as simple as showering can affect the composition of your skins microbiome, he said, depending on the temperature of the water, the products you used and the length of time you spent in the water.

Scientists know that babies receive helpful microbes from their mothers, who pass them on to their children through the birth canal and breast milk, but the specific benefits derived from those microbes are not fully understood.

While theres still a lot that we dont know about the microbiome, scientists say its clear that it plays an important role in health and disease.

It prevents overgrowth by harmful microbes, thats one important function, said Dr. Gregory A. Storch, M.D., a professor of pediatrics at Washington University in St. Louis. Another is that its important in digestion and breaking down food.

The microbiome has also been shown to help the immune system learn the difference between good and bad bacteria, he added.

The gut microbiome plays a role in several gastrointestinal diseases, according to the American Microbiome Institute, and may even influence behavior. Accumulating evidence also suggests that the microbiome could influence asthma, obesity, heart disease, cancer therapies and even autism, but the research is far from conclusive.

In short, yes. The microbiome has become a buzzword that vaguely signifies good bacteria, so some advertisers have promoted their products as being gentle on the bodys microbes. But without studies showing that these products are truly beneficial to the helpful microbes, experts have said that consumers should be wary.

Its become incredibly hot as a scientific area, but also the realm of snake oil, Dr. Eisen said.

With that in mind, Dr. Eisen created an Overselling the Microbiome Award on his blog, which exposes questionable research as well as companies that are touting unproven claims about the microbiome to sell products. I could probably give out one of them a day if I had time, Dr. Eisen said.

When Dove started running ads for microbiome-friendly lotion and bath soap, for instance, Dr. Eisen began dissecting the claims on his blog, one by one, including the idea that Dove was nurturing the microbiome.

Anindya Dasgupta, Ph.D., a scientist at Unilever R&D, which developed a microbiome-friendly line of Baby Dove products, said its infant cleansers and lotions contained nutrients that were identical to those found naturally in skin and help to replenish skin-natural nutrients as well as a prebiotic moisturizer, which serves as an excellent nutrient source for good bacteria, helping to keep the microbiome nourished.

Unilever did not immediately respond to questions about whether the company had conducted studies that supported its claims, nor did the company explain which prebiotics or nutrients were included in its products.

The challenge in this is that some of the bacteria on our skin can be good and some can be bad, and just nourishing bacteria generally does not guarantee that you are nourishing the right ones, Dr. Eisen said.

When it comes to dietary supplements that say they support the microbiome, such as probiotics, its unclear just how helpful they are, said Dr. Martin J. Blaser, M.D., the director of the Center for Advanced Biotechnology and Medicine at Rutgers University and author of the book Missing Microbes.

Theyre almost completely untested for the kinds of things that people are taking them for, Dr. Blaser said.

Recent studies, for example, have found that the probiotic Lactobacillus offered no benefit when treating stomach virus in children.

Even though we know that the microbiome is very important in human health, we cant assume that probiotics and other treatments that try to manipulate or even protect the microbiome will be effective, Dr. Storch said.

In addition, the Food and Drug Administration does not review, approve or strictly regulate dietary supplements or personal care products such as lotions and soaps. And it doesnt approve claims that appear on product labels.

Avoiding the overuse of antibiotics and antimicrobials is one of the best ways to help preserve the microbiome, Dr. Blaser said.

When people are exposed to antibiotics, it disturbs the beneficial microbes, which can lead to antibiotic resistance and create new illnesses.

One of the most clear examples is when antibiotics allow the bacteria Clostridium difficile to proliferate in the gut, which can damage intestinal cells and lead to inflammation in the colon.

Number one: Minimize the damage, Dr. Blaser said. We have to move from a philosophy of, This might not help you but it wont hurt, to, Is this antibiotic necessary?

Given the wide variations in how pediatricians prescribe antibiotics, Dr. Blaser advised parents to have a conversation with their childs doctor about whether antibiotics are truly necessary for certain conditions. Some ear infections and colds, for example, are caused by viruses and will resolve on their own.

Its the germaphobia that encourages killing all types of bacteria that can do more harm than good.

People shouldnt have a fetish about trying to maintain a sterile environment, Dr. Storch said. In some cases I think people are treating their own anxiety rather than any real disease threat.

While Dr. Eisen also recommended avoiding medically unnecessary antibiotics, he warned against going too far in the other direction by, say, encouraging your child to lick the subway poles to strengthen their immune system, something he refers to as microbiomania. Pathogens are real, he added, and they are transmitted by contact between people or between people and surfaces, so youre not going to protect yourself by licking everything.

Wash your hands often, dont touch things that many other people have touched, he advised. And that alone, just that, will help prevent the spread of infectious disease beyond any antimicrobial that they put in toothpaste or clothing or wherever.

Christina Caron is a parenting reporter at The New York Times.

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Why Do Parents Keep Hearing About the Microbiome? - NYT Parenting