Weight and Your Gut Microbiome
The link between gut health and body weight has received a lot of attention. Research has shown, for example, that people with obesity have less diversity in their gut microbiome, and certain specific bacteria have been linked to obesity. In animal studies, transplanting gut microbes from obese mice to “germ-free” mice led those GF mice to gain weight. This suggests excess weight is, in fact, caused by certain microbes, but to date there’s scant evidence that the same is true with humans.
Dr. Krajmalnik-Brown’s group did an experiment in which they had people follow two different diets for 23 days each, with a break in between. Both provided similar amounts of calories and macronutrients each day but via different foods. The study’s typical Western menu featured processed foods — think grape juice, sandwiches made with deli turkey and white bread, and spaghetti with jarred sauce and ground beef. The other menu, what researchers called a “microbiome enhancer diet,” included foods like whole fruit, veggie sandwiches on multigrain buns, and steak with a side of whole wheat spaghetti.
While the study wasn’t designed for weight loss, an interesting thing happened when researchers analyzed participants’ bowel movements.
“We found that when you feed subjects a diet designed to provide more energy to the microbes and not to the [body], our subjects lost a little weight,” Dr. Krajmalnik-Brown said. “It looks like by feeding your microbes, it seems to make people healthier and potentially even lose a little.”
Another possible mechanism involves the same hormone that powers those injectable weight loss drugs. The lower part of your gut makes hormones that tell the entire gut to slow down and also help orchestrate metabolism and appetite. Among them is GLP-1. The drugs use a synthetic version, semaglutide or tirzepatide, to trigger the same effect.
According to Dr. Damman, you can stimulate your gut to make those helpful hormones with the food you eat — by giving your microbes the right fuel.
Eat to Feed Your Microbes
The foods you eat can affect your gut microbiome and so your weight. But don’t go looking for that one perfect ingredient, experts warn.
“Oftentimes we get this micro-focus, is this a good food or a bad food?” warned Katie Chapmon, a registered dietitian whose practice focuses on gut health. “You just want to make sure your microbiome is robust and healthy, so it communicates that your body is running, you’ve got it.”
Instead, try to give your body more of the kinds of food research has shown can feed your microbiome, many of which are plant-based. “Those are the things that are largely taken out during processing,” Dr. Damman said. He calls them the “Four Fs”:
Fiber: When you eat fiber-rich foods like fruits, vegetables, whole grains, nuts, and beans, your body can’t digest the fiber while it’s in the upper parts of your GI tract. It passes through to your lower gut, where healthy bacteria ferment it. That produces short-chain fatty acids, which send signals throughout your body, including ones related to appetite and feeling full.
Phenols: Phenolic compounds are antioxidants that give plant-based foods their color — when you talk about eating the rainbow, you’re talking about phenols. The microbes in your gut feed on them, too. “My goal for a meal is five distinct colors on the plate,” Ms. Chapmon said. “That rounds out the bases for the different polyphenols.”
Fermented foods: You can get a different kind of health benefit by eating food that’s already fermented — like sauerkraut, kimchi, kefir, yogurt, miso, tempeh, and kombucha. Fermentation can make the phenols in foods more accessible to your body. Plus, each mouthful introduces good bacteria into your body, some of which make it down to your gut. The bacteria that are already there feed on these new strains, which helps to increase the diversity of your microbiome.
Healthy fats: Here, it’s not so much about feeding the good bacteria in your microbiome. Dr. Damman says that omega-3 fatty acids, found in fatty fish, canola oil, some nuts, and other foods, decrease inflammation in the lining of your gut. Plus, healthy fat sources like extra-virgin olive oil and avocados are full of phenols.
Eating for gut health isn’t a magic bullet in terms of weight loss. But the benefits of a healthy gut go far beyond shedding a few pounds.
“I think we need to strive for health, not weight loss.” Dr. Krajmalnik-Brown said. “Keep your gut healthy and your microbes healthy, and that should eventually lead to a healthy weight. You’ll make your microbes happy, and your microbes do a lot for your health.”
A version of this article appeared on WebMD.com.