It’s time to bug out. Bacteria in your gut may be the secret to finally losing weight.

By Laura Beil and Renee Cherry
Updated April 27, 2019
Photo: Susanne Alfredsson / EyeEm / Getty Images

What if you could enjoy a chocolate bar without taking in all its calories? This isn't just wishful thinking. It may already be happening, thanks to the trillions of microbes in your digestive system.

Until recently, the assumption was that the bacteria huddling in your intestine pretty much mind their own business. But now a growing body of research suggests that your internal community of bacteria, known as a microbiota, could be influencing your metabolism and, surprisingly, affecting your weight. Turns out, the gut bacteria-weight-loss connection is a pretty fascinating one. (Here's what you should be eating for a healthy gut.)

What Scientists Know About Gut Bacteria and Weight Loss

For example, having a greater abundance of a recently discovered type of bacteria called Christensenellaceae in your gut is associated with being slim, while having less of the bacteria is linked to being obese, a study in the journal Cell shows. "How much you have is partially determined by genetics," says lead study author Julia Goodrich, a graduate student at Cornell University. The good news is that most of us harbor the bacteria—it was detected in 96 percent of the study samples—and it may be possible to alter our levels of gut bacteria for weight loss.

Christensenellaceae isn't the only gut bacteria that might affect weight loss. A diverse mixture of microbes in the gut seems to be one key to staying slim, says Jeffrey Gordon, M.D., the director of the Center for Genome Sciences and Systems Biology at the Washington University School of Medicine, who was one of the first researchers to link intestinal bacteria and obesity. In fact, research found that lean people have 70 percent more gut bacteria and therefore a more diverse microbiota than that of their overweight peers. Other findings have found that people in the United States, who have a high rate of obesity, have less-diverse gut microbes than people from less developed parts of the world do. The correlation is consistent enough that in a study of twins, "we could predict whether one was lean or obese based solely on their gut microbes," says Rob Knight, Ph.D., a cofounder of the American Gut Project. (Related: Nutritious Juice Shots for Gut Health)

The impact of the gut bacteria-weight loss link isn't known yet, but many researchers believe that your gut microbiota plays a role in processing food and helping to determine how many calories and nutrients your body absorbs. Certain intestinal microbes may also alter your sensitivity to insulin—the hormone that moves sugar out of your blood—so that your body burns fat it would have otherwise stored. Interestingly, it only takes a few days of eating high-fat foods to disturb the balance of good and bad bacteria in the gut, which throws off an important process of breaking down undigested macros.

Your gut bacteria might affect how hungry you are too. One key microbe appears to be Helicobacter pylori, the bacterium that's involved in causing ulcers and stomach cancer. Antibiotic treatments have helped cut H. pylori infection rates in half in recent decades, which is good news for ulcer sufferers-but which could be bad news for our waistlines. H. pylori also dials back the stomach's production of the hunger hormone ghrelin. "When you wake up in the morning and you're hungry, it's because ghrelin is telling you to eat," says Martin Blaser, M.D., a professor of medicine and microbiology at New York University and the author of the book Missing Microbes. "When you eat breakfast, your level of ghrelin usually goes down, but if you don't have Helicobacter in your system, it doesn't." The end result: You could eat more. (Related: 8 Tips to Absorb More Nutrients From Your Food)

The Impact of Antibiotics on Gut Bacteria and Weight Loss

You might not even have to take antibiotics to feel their effects on your gut bacteria. The heavy reliance on antibiotics by the food industry, which routinely uses the drugs in feed to keep livestock healthy, may be fueling the rise of obesity by disrupting the fine balance of our intestinal microbes, some experts believe. "The obesity epidemic really took off in the last 20 years in the U.S. So the question is, what happened then? What was a large segment of the population exposed to that could account for this massive weight gain?" asks Lee Riley, M.D., a professor of epidemiology at the University of California, Berkeley. He points out that that's when the number of large-scale densely packed factory farms expanded, which also increased the use of antibiotics in livestock feed. Today, 80 percent of the antibiotics sold in the United States go toward helping animals remain healthy and gain more weight in crowded conditions. "Counties with the highest prevalence of obesity are those counties with large concentrated animal feeding operations," he says.

Not to mention that antibiotics are often used when they shouldn't be, as when doctors prescribe them for viral infections or because patients demand them. (Sometimes a full course of antibiotics isn't necessary.) The exact repercussion on human health is still being debated, but Dr. Blaser says that the gut bacteria-weight loss link in laboratory studies is pretty clear. "If you put mice on a high-fat diet, they get fat," he says. "If you put them on antibiotics, they get fat. And if you put them on both, they get very fat."

While some of your gut bacteria is determined by genetics, life­style and dietary habits can have a dramatic impact on your mix of beneficial and harmful microbes. A study in the journal Nature found that when people switched from their normal diet to one consisting primarily of meat and cheese, there was an almost immediate increase in Bilophila, a type of bacteria that has been linked to colitis, but that a plant-based diet decreased the levels.

4 Ways to Keep Your Gut Bacteria Healthy

Eat more fiber. It's the number-one thing you can do to better your gut bacteria (and hopefully help with weight loss), says Justin Sonnenburg, Ph.D., an assistant professor of microbiology and immunology at Stanford University. Research suggests that fiber nourishes your microbes, making them diverse and more likely to help keep you at a healthy weight. Avoid the temptation to buy processed foods that have added fiber. Instead, eat vegetables, fruits, and whole grains. Aim for at least two to three servings each of produce and whole grains and 20 to 30 grams of fiber a day, says Mark Moyad, M.D., a urologist and the author of The Supplement Handbook. These foods also provide prebiotics, which are essentially a type of fiber that your gut bacteria flourishes on. Some plants, like sunchokes, garlic, and leeks, are packed with prebiotics. Bananas and whole-wheat breakfast cereals are other good sources. (Related: This Study About Fiber-Rich Carbs Might Make You Rethink the Keto Diet)

Snack smarter. The fact that we consume so much added sugar—more than 22 teaspoons a day for the average person—could actually be starving our gut flora, says Sonnenburg. Bacteria need complex carbohydrates, like legumes and whole grains, in order to thrive. So when you get too many calories from sweets, you're leaving your microbes hungry. They either die or adapt by feeding on the mucus inside your intestine, which, experts hypothesize, could contribute to low-level inflammation, a condition that has been linked to obesity. Instead of grabbing a cookie when your stomach starts growling at 3:00 p.m., reach for a handful of nuts or an apple. Check labels for hidden sugars in foods like pasta sauce and salad dressing. And choose brown rice and whole-grain pasta instead of white.

Pick probiotic foods. If prebiotics are like fertilizer for your microbial garden, probiotics are like seeds. The best way to get them is by regularly eating fermented foods such as yogurt, kefir, sauerkraut, and miso. And about yogurt, that probiotic rock star: A landmark study in the New England Journal of Medicine reported that among all foods studied, yogurt was the one most strongly correlated with weight loss. The average person gained almost a pound a year, but people who regularly ate yogurt actually lost weight. Choose plain Greek yogurt and mix in pomegranate seeds or your favorite berries for a hit of fiber. (Related: 5 Legit Benefits of Probiotics)

Move your body. Your bacteria might benefit from a good workout as much as you do. Exercisers with a normal BMI had more diverse microbes than exercisers with a high BMI, according to an Irish study of male rugby players. They also had higher levels of Akkermansiaceae, a type of bacteria that has been linked to lower obesity rates. So sweat daily to trim your gut—and to boost your gut bacteria.

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