Probiotics Might Not Do Much. Focus on Fiber Instead.

Probiotics are the latest miracle drug. Swallow a pill full of good bacteria and reap the (alleged) benefits, including improved digestion, a stronger immune system, and reduced risk of anxiety, depression, and even cancer. But recent research has found that supplementing your diet with probiotics doesn’t necessarily mean bacteria will take up residence in your gut. The benefits of a healthy microbiome are indisputable, but the best way to foster good bacteria might not be through eating them.

According to a study published in September, the effectiveness of probiotics can depend on an individual’s resident microbiome. In participants with a “resistant” gut microbiome—about half of the group studied—the bacteria from probiotics wound up in the toilet bowl. In fact, increasing evidence suggests that instead of eating new bugs, we should focus on feeding the ones we already have. Their favorite food? Vegetables.

“Probiotics are a can of worms,” says Jens Walter, a microbial ecologist at the University of Alberta. Walter says that while there is some evidence supporting the use of certain strains in specific circumstances, such as in treating irritable bowel syndrome, it’s hard for a consumer to make an informed decision when facing down an aisle full of supplements. “These bacteria have decades to adapt to us and adapt to the conditions in our gut,” he says. “If you throw in a probiotic, they’re not going to change a lot.”

According to Walter, what you feed the bacteria already inhabiting your body is more important than introducing new strains. The good bugs thrive on certain types of dietary fiber, dubbed prebiotics, found in fruits, vegetables, nuts, seeds, and whole grains.

“Prebiotics are thought to be beneficial in reducing the risk of developing colon cancer, resisting the colonization of harmful bacteria, increasing mineral absorption in the large intestine, and may help with weight loss,” says Emma McCrudden, a sports dietitian at the University of British Columbia. But, she says, much of this research was done in a laboratory setting or with animals—research on human subjects is still underway.

Dietary fibers pass undigested through the small intestine until they hit the colon, where bacteria start to ferment them. There, the fiber is converted into chemicals associated with dozens of health benefits, such as lowering inflammation and steeling the gut lining—important for protecting bacteria from leaching from the intestines into the bloodstream, where they can do some damage. So, eating more prebiotic fibers can help the population of beneficial bacteria grow.

Some particular prebiotic fibers have been of special interest to scientists. One, called inulin, can be found in high concentrations in onions, asparagus, and garlic. In one study of 30 women, inulin supplements significantly changed gut microbiota, leading to increases in strains thought to be beneficial. Prebiotic-fed mice also have similar boosts to those bacteria.

One highly cited study from 2014 showed how quickly the microbiome changes in response to diet. After just four days, participants who switched to either plant- or animal product–based diets experienced a significant change in the number and type of bacteria present. Those who ate meat and other animal products saw a rise in protein-fermenting strains associated with health conditions like inflammatory bowel disease. Eating more plants, on the other hand, increases the activity of fiber-fermenting bacteria, lowering the gut’s pH and reducing protein fermentation, Walter says. Basically, the dietary fiber offsets the impact of eating meat and keeps the balance of bacteria in check. “If you’re really keen on eating a steak, it’s better to eat it with big salad,” Walter says.

While scientists are excited about these findings, there are still a lot of unanswered questions about prebiotics. Hundreds of compounds fall under the umbrella of “dietary fiber,” and many could be important to gut health, Walter says. While the details remain murky, fiber has long been known to benefit overall health. “Most people around the world only get half the recommended amounts of dietary fiber,” says Joanne Slavin, a professor of food science at the University of Minnesota who studies prebiotics. “There’s no reason not to recommend more fiber.”

If you’re eating enough fiber, you’re probably getting sufficient prebiotics, McCrudden says. You may also get the added benefit of better microbes. Today, Americans eat an average of 15 grams of fiber per day. But according to the American Dietetic Association, you should eat 14 grams of fiber for every 1,000 calories—nearly twice as much as the average American consumes. Scientists think this is a big loss for our microbe friends. “We are essentially starving our microbiome,” Walter says.

Everyone, including athletes, can benefit from the potential gut-health benefits of a fiber-rich diet. A healthy microbiome is linked to a stronger immune system and improved digestion and nutrient absorption, McCrudden says, and these effects can in turn lead to better performance. No matter what, there’s no harm in eating more vegetables.

Outside Magazine: Fitness

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