What’s Going on Inside Your Stomach


When the knife parts the belly of the impala, the innards flow out in a gush of blood and heat, peppering the dusty ground with puffs of gore. The animal, felled moments before by a poison arrow, hangs lifelessly by its horns in the crook of an acacia tree in Tanzania. A Hadza tribesman proceeds to field-dress it in minutes, working with a butcher’s silent efficiency until he reaches the animal’s stomach. Then a final slash opens the white, veiny globe, filled with partially digested grass, and the man and his helpers scoop up the stomach contents and rub it between their hands like a bar of soap. During this perfunctory Hadzan ritual, the men unwittingly inherit millions of organisms critical to their health and survival: the impala’s belly-residing bacteria.

Jeff Leach, founder of the Human Food Project, a nonprofit working to shed light on mankind’s ancestral link to microorganisms, observes the slaughter. He’s recently decamped to East Africa because its savannas are great, flourishing bacterial oases—ecosystems untouched by antibacterial tormentors such as pesticides, processed foods, antibiotics, and hand sanitizers. Leach samples the skin of the carcass, the hands of the hunters, the contents of the gut. With their hands now “clean,” the Hadza men slice the stomach into pieces and toss them into their mouths like popcorn, chewing them raw and crawling with microbes.

“The Hadza are literally connected to the gut of every animal in the landscape,” Leach says. Those microbes—and others picked up from the skin of unwashed vegetables and sipped from streams of unfiltered water—are transferred freely among members of the population like air. “They’re connected to this huge microbial meta-community,” he says, adding that the Hadza, in many ways, are a lot healthier for it. “We’ve lost a lot of that diversity in the Western world—and that diversity is extraordinarily protective against pathogens.”

Even though the hunter-gatherers lack modern medicine and are exposed to disease, they’re seemingly free of heart disease and diabetes. Leach emphasizes that their body mass index refuses to budge, even when the tribe goes on massive caloric binges that can only be described as superhuman. (A kudu carcass, for instance, which can offer up 600 pounds of meat, will vanish in 48 hours, feeding around 20 people.)

“The Hadza hunt the same animals and collect the same plants that humans have for millions of years,” Leach says. “They drink the same water and are covered in the same soil. They’re walking on the bones of Homo erectus and they still live in the microbial environment that gave rise to modern humans.”

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Scientists have long known that bacteria exist within our bodies in abundance. In fact, there are 10 times more bacterial cells in your body than there are human cells, which technically makes you only 10% human. But what researchers didn’t comprehend until recently was the degree to which our bodies don’t really belong to us at all: We are fully functioning, symbiotic organisms, groomed from birth to coexist and thrive with the help of countless microbes that inhabit areas on our bodies that are exposed to the environment, like our skin and mucosal membranes—inside our mouths and noses, lining the long and complicated passage from where food goes in to where it comes out. Typical flora consists of some fungi and other microorganisms, but it’s largely made up of bacteria. Each environment—nasal passages, oral cavity, stomach, gastrointestinal tract—has its own separate and distinct population.

“We’re coming to the conclusion that we’ve really missed out, scientifically, by not appreciating just how important the microbiota, in the broadest terms, are,” says Leach. “We’re only now recognizing that these microorganisms are important in just about everything we do.”

There are a thousand distinct bacterial species living in your gastrointestinal (GI) tract at any given time; you inherited your initial population from your mother when you slid through the birth canal as a newborn. If you were breast-fed, you picked up more then, too. In fact, the foundation of your microbial makeup for the rest of your life is laid within 48 hours of birth. And as you grow, the great mishmash of bacteria in your gut settles into something of a checks-and-balances system: The troublemakers, microbes with names like Staphylococcus and Streptococcus, are typically canceled out by beneficial ones called Bifidobacterium and Lactobacillus. Scientists aren’t sure exactly what any of those bugs do, but they know we have a complex, dynamic relationship with them. Our membranes provide them with nutrients and offer a secure environment. In turn, these tiny dynamos help us digest food, give us more energy, and prompt our immune system to fight pathogens while also actively helping protect us from sickness and poor health.

When your gut’s balance is tipped in the wrong direction (official name: dysbiosis), your more insidious microbes begin to take over, prompting protective but sometimes harmful inflammation that potentially paves the way to disease. “Those that have been specifically linked to microbiota include things like obesity, diabetes—both type-1 and type-2—and even things like neurological disease,” says Rob Knight, Ph.D., a professor in the department of chemistry and biochemistry at the University of Colorado Boulder, and co-founder of the American Gut Project. Other conditions linked to GI tract microbial imbalances include inflammatory bowel disease, multiple sclerosis, cancer, and cardiovascular disease.

So what triggers dysbiosis? Well, your everyday decisions, such as poor diet, overuse of antibiotics, or simply an overly sedentary, lazy lifestyle, according to analysis in the September 2013 edition of Clinical Reviews in Allergy and Immunology. Bruce German, Ph.D., a food chemist and director of the Foods for Health Institute at the University of California, Davis, says that bacteria have fundamentally altered our understanding of weight gain and obesity. “Obesity is a profound disregulation of the normal prioritization of fuel,” he says. In other words, obese people’s GI tracts have a less diverse population of bacteria than those of lean people—thanks, for instance, to the processed fats and carbs in fast food.     

In a study whose findings were published in Science last year, researchers took gut bacteria from human twins—one obese, the other lean—and transplanted them into genetically identical mice with no microbiota of their own. The mice that got the bacteria from the obese twin got 15–17% fatter than the mice with the bacteria from the thin twin.

Knight also helped uncover evidence that microorganisms are directly involved in the digestion of fat. He assisted in discovering that the same types of microbiota in the human gut play a large role in the absorption of lipids, or fats, in the GI tracts of zebra fish. “This was the first evidence of yet another thing that the gut microbes might be doing,” he says.

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