Should you get a microbiome test?

Companies can tell you the kinds of microbes that live in your gut, but the results may not help you lose weight or fend off disease.

Q: How do I know if my gut microbiome is healthy? Is there a test I can take to see what’s going on?

A growing number of companies offer tests that provide a glimpse into our gut microbiome, the community of trillions of microbes that live in our digestive tract. Scientists increasingly recognize that these microorganisms play a crucial role in our health, influencing everything from how successfully we age or fight off infections to our risks of developing obesity, heart disease and Type 2 diabetes.

Direct-to-consumer microbiome tests require little more than filling out a form online, paying a fee and sending in a stool sample. Two to three weeks later, you’ll get a report that provides an overview of the microorganisms in your gut and whether they’re associated with various diseases and disorders. A few companies claim they can even tell you which foods are best for you to eat based on the composition of your microbiome and other personal data.

But some experts say that while the science looks very promising, the evidence behind such claims is still in its infancy. “I think in five or 10 years we’ll be able to have reliable and valid consumer offerings that health care practitioners could in good faith recommend,” said Amy Loughman, a senior research fellow who leads the microbiome research stream at the Food & Mood Centre at Deakin University in Australia. “But I don’t think we’re at that point right now. I think the promises are greater than what the science can realistically offer.”

A growing number of companies offer tests that provide a glimpse into our gut microbiome, the community of trillions of microbes that live in our digestive tract. (Aileen Son for The New York Times)

There’s no doubt the microbiome plays an important role in our overall health, converting the food we eat into a range of compounds that have benefits throughout our body. The microbes in our gut produce a variety of vitamins; synthesize hormones like serotonin, which influences metabolic health and mood; and ferment the fiber that we eat, turning it into short chain fatty acids that can lower inflammation. The composition of our microbiomes is shaped by our genetics, our environment and the foods, drugs and beverages we consume.

Scientists have identified gut microbes that are linked to insulin resistance, obesity and metabolic diseases. They have also found that having large numbers of a wide variety of bacterial species generally seems to be a good thing. People who have high degrees of microbial diversity in their guts tend to have lower rates of obesity, depression and diabetes, and they are less likely to show early signs of frailty as they get older.

But microbiomes are as unique as fingerprints. Even identical twins do not have identical microbiomes. And with so much variation between individuals, there is no consensus on precisely what constitutes a “healthy” microbiome. Nor is it always clear whether specific strains of bacteria and low levels of microbial diversity promote obesity and metabolic diseases, or vice versa. Some gut microbes appear to be beneficial in some people and detrimental in others.

“The field has progressed a lot, and we are finding out which bacteria are generally on the good and bad guys rosters,” said Justin Sonnenburg, a professor of microbiology and immunology at Stanford University who studies the microbiome. “But bacteria are complex and rapidly evolving organisms, so they’re difficult to pin down in terms of only-ever-good or only-ever-bad.”

Even if you know how a particular strain of bacteria responds to certain foods, it can be difficult to know what will happen when it’s combined with a vast array of other microbes. “The whole is greater than the sum of the parts,” said Sean Gibbons, a microbiome specialist and assistant professor at the Institute for Systems Biology, a nonprofit research organization in Seattle. “Predicting how a community of microbes will respond to a certain input is more difficult than predicting what a single bug will do.”

Despite how much scientists have learned in recent years, there remains a lot that we still don’t know about the thousands of different microbial species that can inhabit the gut.

“The known unknowns of the microbiome are staggering: Approximately 20 percent of bacterial gene sequences have not been identified,” and the function of 40 percent of the estimated 10 million total of bacterial genes remains unknown, Dr. Loughman and a colleague wrote in a recent review paper published in The Lancet Gastroenterology & Hepatology.

Studies have shown that there is no one-size-fits-all diet that has consistent effects on such factors as metabolic health or weight loss for everyone, and the microbiome is part of the reason for this. As a result, some companies are combining microbiome analyses with other data to give people customized diet recommendations.

One large international study of personalized nutrition, called Predict, has followed 1,100 people in the United States and Britain, including hundreds of identical and nonidentical twins. It found that people can have dramatically different metabolic responses to the same foods and that unique factors, such as a person’s genetics, sleep, stress and exercise levels, and the diversity and types of microbes in their guts, all influence how they metabolize food.

This research formed the basis of a company called Zoe, which provides personalized food recommendations. To do that, the company analyzes its customers’ gut microbiomes and collects a wealth of other health data from them. Zoe has its customers wear continuous glucose monitors, and it takes blood samples from them to see how different meals affect the levels of fat and glucose in their circulations. Prices for the company’s programs start at $354, paid in six monthly installments of $59.

In 2015, a group of researchers at the Weizmann Institute of Science in Israel published a study involving 800 participants that also showed that people had wildly different glucose responses, an indicator of diabetes risk, to the same foods. The researchers developed an algorithm using data from the participants’ glucose responses, gut microbiomes, family histories and lifestyles, which allowed them to predict how a person’s glucose levels would respond to different foods. The research gave rise to a company called DayTwo, which provides personalized nutrition advice to people with diabetes to help them manage their condition.

Using the company’s app, customers can see if a meal they’re thinking of having is likely to spike their glucose levels, and they are guided toward food choices that might be better for them, said Eran Segal, a computer scientist at the Weizmann Institute and a co-founder of DayTwo. “We’ll almost never tell you that you can’t eat something,” said Dr. Segal. “But we’re going to tell you that we may change the quantity a bit or change the food combinations.”

DayTwo’s customers are paired with dietitians and provided continuous glucose monitors. A study sponsored by DayTwo and published last month in Diabetes Care showed that people who followed its program for a year had greater improvements in their blood sugar control than a control group. The company currently only offers its program through employers and health plans and would not disclose its prices.

Dr. Loughman at Deakin University said that while the microbiome studies behind companies like Zoe and DayTwo are exciting, more research is needed. She does not necessarily discourage people who want to get their microbiomes sequenced to learn about their health. But she added that people can take simple steps to nourish their gut microbes by eating plenty of fiber-rich plants and fermented foods, which will also benefit their overall health.

“If you have $300 to spare, feel free,” she said. “But would your health be much better than if a doctor just saw your name and said, ‘OK, you’re a 40-year-old person with an underlying health condition — therefore you should eat more vegetables’? It’s debatable.”

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