There’s no question that modern, processed diets are bad for the gut. Due to the vast majority of farming subsidies going to producers of corn, wheat, and soy, prices for the foods containing these crops (many of which are high in sugar and refined carbohydrates and low in meaningful vitamins, minerals, micronutrients, and fiber) have dropped—making them cheap and readily accessible for consumers.
Fiber—found in vegetables, fruits, and a variety of whole grains—promotes gut microbial biodiversity and feeds beneficial bacteria in the GI tract. In fact, these “good” bugs use fiber food to produce gut-healthy, anti-inflammatory compounds called short-chain fatty acids (SCFAs). But the average American gets just 15 grams of fiber per day, when we should be getting at least 25 to 30 grams. “Part of the problem we’re seeing today could be due to three to four generations of progressively diminished fiber consumption,” says gastroenterologist Will Bulsiewicz, M.D., author of Fiber Fueled, referencing a groundbreaking 2016 study in Nature by Stanford University microbiome researcher Justin Sonnenburg, Ph.D.
For the study, researchers fed mice a low-fiber diet over multiple generations. After one generation, there was a decline in microbial biodiversity in the gut, which was reversible when fiber-rich foods were returned to the diet. But with each subsequent generation, there was a progressive loss of biodiversity that was harder to reverse (and impossible to completely reverse).
“Compared to the Hadza of Tanzania, some of the last remaining communities of hunter-gatherers, people in the U.S. have about 40% less microbial diversity in the gut,” says Bulsiewicz. “This leads us to believe that we’ve essentially lost 40% of what we’re supposed to have as humans. To an extent it’s reversible, but this study shows that we might be in a place where we’re impaired from the get-go.”
Excessive consumption of animal products (particularly in the absence of fiber-rich foods) may also have a negative impact on the gut microbiome. This was illustrated in a 2014 study by Harvard researchers in which they put the same group of people on two drastically different diets—an animal-based diet of foods like bacon, eggs, salami, and pork rinds; and a vegan diet of foods like rice, tomatoes, lentils, squash, and fruit—and measured the effects of each. What they found: On the animal-based diet, there were significant increases in the bile-tolerant gut microbes, which are necessary for breaking down fat but also associated with inflammatory processes.
Additionally, excessive refined carbohydrate and sugar intake has been shown to reduce the microbial biodiversity and feed bad microbes. “Someone who’s eating way too much sugar is going to become a fertile ground for yeast to grow in their gut,” a form of gut imbalance, says Vincent Pedre, M.D., integrative physician and author of Happy Gut.