Trusting Your Gut
Why your stomach is smarter than you think
If you’ve ever had butterflies in your stomach, experienced a gut reaction, or felt fire in the belly, you might have wondered why so many feelings seem to dwell in the gastrointestinal tract. This part of the body — also known as the gut — includes everything from the mouth to the anus, and it’s full of surprises. The gut has its own nervous system, one that’s been called the second brain. It is home to bacteria so important that they may constitute an organ within an organ. And, far from merely digesting our food, it is intimately linked to immunity, inflammation and even mental health.
Taking the tour
The gut is a complex tube that changes shapes and specialties along the way. Digestion starts when food is ground up in the mouth and mixed with enzymes. Once swallowed, food heads down the esophagus, a muscular tube leading to the stomach. A tight muscular ring or valve seals off this entrance to the stomach when food isn’t passing through; if it loosens, gastrointestinal reflux can occur. The stomach is a bag of muscle that adds more enzymes and churns the meal in an acid bath. This kills germs in the food, though some survive it. The next stop is the small intestine, a 20-foot tube where the pancreas and liver add digestive juices and food becomes nutrients.
The small intestine’s main job is to absorb nutrients, and it is fantastically convoluted to allow maximum surface area for that task. As the small intestine’s muscle pushes food along, nutrients pass through the intestinal walls and into the bloodstream to head to wherever they’re needed in the body.
What’s left of your meal is squeezed like toothpaste into the five-foot-long large intestine. Also known as the colon, the large intestine is home to the appendix and to most gut bacteria, though some also live in the small intestine. It ends in the rectum, where stool is stored before it exits via the anus.
Who lives in the biome?
Architecture aside, it’s the residents of the gut — most notably, some 10 trillion bacteria — that make it especially interesting. The large intestine in particular is teeming with a hugely diverse population of bacteria termed the “gut biome.”
The gut biome is an ecosystem, and like all ecosystems, it may or may not be in good health. Its bacteria take up residence in the gut shortly after we’re born, and for the rest of our lives other species join the ecosystem and thrive there or dwindle. These species vary widely from one person to another and from one time of life to another, and some are more desirable than others. Different bacteria send signals that may provoke or ease allergies, asthma or Crohn’s disease, a type of inflammatory bowel disease. One species may reduce the risk of kidney stones, while another may raise the risk of stomach cancer. Normal gut bacteria formulate vitamins B and K for us. Low levels of “good” bacteria have been linked to autism, schizophrenia and anxiety.
Even obesity may be related in part to an unhealthy gut biome: One study transferred gut bacteria from obese mice into lean mice, and the lean mice gained weight. Another transferred microbes from thin humans into pudgy mice, which soon lost weight. Obesity and thinness, in other words, might literally be contagious.
Therefore, we might want to be careful with what we do to our gut biome. Antibiotics can alter the balance of intestinal bacteria in ways we don’t fully understand. Because different bacterial species keep one another in check, killing friendly bacteria with a course of antibiotics can mean that unfriendly species take over. The most notorious example of this phenomenon is infection with Clostridium difficile, better known as C. diff. This species is notorious for causing a serious colon infection after normal flora is eradicated by certain antibiotics. There are antibiotics that kill C. diff, but it often resists those or comes back.
Diarrhea can have a similar effect, flushing healthy species out of the body and leaving the gut vulnerable. (In fact, some scientists think the purpose of the appendix is to hold good bacteria in reserve to “re-seed” the gut after such an illness.)
There’s a surprising new way to reseed the gut, too, and The University of Texas Health Science Center at Houston (UTHealth) is helping to develop it. Gastroenterologist Andrew DuPont, MD, MSPH, is an assistant professor at UTHealth Medical School and director of the university’s Inflammatory Bowel Disease Program. Many of his patients suffer from Crohn’s disease or ulcerative colitis — diseases that cause severe gut inflammation. They are thought to be related to an immune system imbalance, and they may be affected by gut bacteria.
DuPont is helping to launch an innovative therapy at UTHealth that may help sufferers of these diseases as well as C. diff infection: the stool or fecal transplant.
You read that right. Also known as fecal bacteriotherapy, a fecal transplant is simply the transfer of a healthy person’s stool into the colon of a sick one. “Repopulating the intestinal bacteria seems to be very effective,” Dupont says.
Although it’s still in the research phase, the fecal transplant may cure over 90 percent of C. diff-infected patients. Early results on patients with inflammatory bowel disease are promising, too.
Inflammation and the gut
The gut biome plays a role in other diseases as well. In inflammatory bowel disease, genetics collide with environmental factors to disrupt the gut’s usual peaceful coexistence with normal flora. The immune system, rather than viewing these bacteria as the benign tenants they are, instead attacks them. That leads to inflammation, leaky membranes and increased permeability of the gut. Bacteria and the byproducts of the immune attack can slip outside the gut into the bloodstream, provoking full-body inflammation in a vicious cycle.
Gluten, a protein that’s found in wheat and other grains, can also lead to inflammation. It’s off-limits for people with celiac disease, whose bodies attack it and cause gut damage, malabsorption of nutrients and a host of non-gut problems, including anemia and bone loss.
But in celiacs and healthy people alike, gluten carries a component that opens the tight junctions between the cells of the intestine. Researchers don’t yet know how leaky a gut can get before symptoms set in. But leakiness may underlie gluten intolerance, a condition in which there’s no apparent injury to the gut but the person still suffers symptoms like bloating and diarrhea after consuming gluten.
Many people suspect they are gluten intolerant, and trying out a gluten-free diet can bring relief even to people without a definitive diagnosis of celiac disease or gluten intolerance. But gastroenterologists prefer that patients get tested first, says DuPont, since a gluten-free diet can inhibit doctors’ ability to diagnose a problem. If you’re going to avoid bread for the rest of your life, that is, it might be best to be sure it’s really necessary.
A mind of its own
“Bowels weep the tears unshed,” a surgeon told this author once, referring to the fact that patients often experience emotional distress via unexplained belly pain. In fact, there are strong connections between the gastrointestinal system and the mind and emotions. The gut’s vast nervous system is linked to the brain, so psychological stress can increase gut leakiness, alter gut immune responses and affect our perception of abdominal pain. The health of the gut biome may, in turn, affect mental health.
In a study of mice who were given the dangerous bacteria Campylobacter, the mice showed signs of anxiety well before their intestines began to show inflammation from the bug. In another study, mice living in cages near hungry cats showed an increase in intestinal bacteria and decreased contractions in the small intestine compared to mice who weren’t menaced by a kitty.
In humans, psychological stress can trigger symptoms in people with inflammatory bowel disease, while depression can cause constipation in otherwise healthy people. In irritable bowel syndrome, patients without obvious gut disease experience chronic abdominal pain and a change in bowel habits. Though these patients aren’t necessarily depressed, treating them with antidepressants can help. One intriguing recent study found that giving a group of women a four-week course of probiotic supplements (high doses of good bacteria) led to changes in how they processed emotion. The changes were visible on brain MRI.
Feeding the gut
If fat is unhealthy, then why is olive oil okay? Or is it carbs that are unhealthy? Are non-organic vegetables safe? Should I eat more eggs or fewer?
If it seems like contradictory nutritional advice is all around us, that’s because the science of nutrition is young, and because nutritional research is inherently tough to carry out. It’s hard to study individual foods when most diets contain so many — and people usually have trouble recalling what they’ve eaten. It’s also difficult and expensive to feed people only certain foods to see what happens, though some well-funded research does take this approach. Moreover, the effects of food can take years to show up, and can interact with other foods, with genes and with the person’s environment in countless unpredictable ways.
Bottom line: there’s no consensus among gastroenterologists about the ideal diet, says DuPont. He recommends consuming a variety of foods. “People who are going to start any dietary restriction might want to consider meeting with a dietician or nutritionist,” he says, explaining that people on a gluten-free diet may be unaware that they’re eliminating certain vitamins or minerals.
What about probiotics? The jury’s out, says DuPont, because there hasn’t been much research. A placebo effect could be responsible for some of the benefits people observe.
“We have very modest evidence that probiotics do something significant,” he says. “But we don’t know how much people should take, how long they should take them, or even which ones to use.” Fortunately, for most people probiotics are safe.
Then there are “prebiotics”, which aren’t bacteria at all, but rather are food for them. “Prebiotics have definite theoretical advantages,” says DuPont. Onions, garlic and asparagus are among the foods that healthy gut bacteria seem to prefer. These foods contain soluble fiber, a substance we can’t digest but intestinal bacteria can. As they do so, they produce beneficial substances like short-chain fatty acids that nourish the colon. They may even enhance calcium absorption, which could in turn help people at risk for osteoporosis. And the fiber can help prevent constipation, which adds to the bargain.
We still have a lot to learn about the gut and how to keep it and its ecosystem healthy. Science is rapidly advancing our understanding of links between the gut and the rest of the body, and it might not be long before a gut-biome assessment is a common lab test. In the meantime, it seems wise to avoid antibiotics unless they’re really needed, to eat a wide variety of foods and to include prebiotics in our diet. The occasional belly laugh, too, might not go amiss.