Don't look now, but your gut is teeming with microbes - as many as 100 trillion of them. It may unnerve you to know that you're providing a home to a population of bacteria and other tiny organisms but they're not just squatters; some beneficial bacteria help you digest food and can protect against infection.
Scientists are also discovering that the microbes, which vary in makeup from person to person, may influence a host of physiological processes that at first glance have little to do with the digestive system. The National Institutes of Health in 2007 launched the Human Microbiome Project, which will sequence the genomes of hundreds of different human-dwelling microbes, so the research into these critters will only continue. But we already know, suspect, or theorize they have the ability to:
1. Influence disease in your gut. It's probably not so surprising that the gut microbes affect the organ in which they live; indeed, bacteria in the gut send signals to various stomach and intestinal cells and vice versa, says Emeran Mayer, director of the Center for the Neurobiology of Stress at UCLA.
Microbes are suspected to play a role in inflammatory bowel disease, including Crohn's disease and ulcerative colitis, for example. Research in mice published last year suggested that while one strain of bacteria, H elicobacter hepaticus, can promote IBD when introduced into the digestive tract, a molecule produced by another strain of bacteria, B acter oi des fragilis, may signal immune cells to suppress the disease's characteristic inflammation.
2. Give you the stomach-flu blues. "Most people who have severe gastroenteritis know that it has a major effect on your well-being," says Mayer. But on top of the obvious unpleasantness of their diarrhea and nausea, sufferers often withdraw socially, don't want to do anything, and are generally bummed out.
It's still in the realm of theory, but it's possible that the gut bacteria could be talking to your so-called vagal nervous system, which sends signals from the gut to the brain. If microbes "speak" to any of the thousands of sensory nerve cells that lie within the gut walls, either directly or through nearby cells, it might influence your emotions. (Evidence that supports such a mechanism has been found in rodent experiments.)
"Is our general sense of well being due to this bacterial-to-vagal system?" wonders Mayer. For now, it's speculation, but planned studies with brain imaging may shed some light on the notion.
3. Influence asthma and allergies. Gut bacteria are also thought to influence the immune system, says Les Dethlefsen, a researcher at Stanford University. Too much of one kind of immune response - led by Th1, or T-helper cells 1 - has been associated with autoimmune diseases, while an overactive response led by another kind of immune cell, Th2, can result in allergies. Certain normal gut bacteria are known to influence the balance of the two responses, and some studies have associated changes in the gut bacteria with an increased risk of allergies.
4. Play a role in obesity. The particular mixture of microbes in your gut may be influencing your ability to absorb nutrients from food and, thus, your tendency to gain weight. Research suggests that the obese are home to different microbe populations from the thin, though it's not yet known whether the variations cause the weight differences or whether the same diet or other factors that contribute to obesity also change the bacterial population.
There are hints of a causal role; one recent study by researchers in Arizona suggests that certain microbes that improve nutrient digestion, and are present in about 10 percent of the population, occur almost exclusively in the obese and not in people who are normal weight or who have had gastric bypass surgery.
Despite the accumulating evidence, though, the field is still in its infancy, says John DiBaise, a gastroenterologist at the Mayo Clinic in Arizona and an author of the study. That means it's far too early to try to manipulate your own gut bacteria through probiotics or prebiotics with an eye to losing pounds.
5. Cut the risk of developing painful kidney stones. One microbe, Oxalobacter formigenes, can break down the substance that makes up most kidney stones, calcium oxalate. Last year, researchers from Boston University reported that people with the O. formigenes bug in their guts had a 70 percent lower risk of kidney stones. It's not clear yet whether those without naturally occurring O. formigenes could reduce their risk by ingesting the bacterium, but researchers are looking into it.
6. Affect cancer risk. Microbes living in the gut may play a role in cancer - for good and for bad. H. pylori, for example, is linked to ulcers and stomach cancer but may have a protective role, too, fighting gastroesophageal reflux disease, or GERD, and one potential complication, a type of esophageal cancer. Meredith Hullar, a scientist at the Fred Hutchinson Cancer Center, Seattle, Washington, is part of a team looking at how diet and gut microbes might interact to alter one's cancer risk. Some of the chemicals called isoflavones, found in soy, can be metabolized by certain gut bacteria into a compound called equol that may play a role in cancer prevention, Hullar says. (Equol binds to estrogen receptors in the body and reduces exposure to the actual hormone, lowering the risk of some forms of breast and other cancers.)
"You either have those bacteria or you don't," says Hullar. Cooked broccoli may contain another compound that requires certain bacteria to digest it into cancer-preventing components.
7. Help out someone else. The obvious question is whether it's possible to fight disease or improve bodily function by altering an individual's microbe population, which is thought to stay fairly stable from early childhood to old age.
Research into the potential of both diet and probiotic or prebiotic supplements to manipulate the makeup of the gut microbiome is ongoing; scientists say there's far more to learn about the diversity and function of bacteria. But in extreme cases of illness brought on by a severe, recurrent infection by the bacterium C. difficile, the healthy balance of gut bacteria can be restored by a gross-sounding sort of procedure called a fecal transplant.
A healthy person "donates" his or her normally functioning gut bacteria via filtered stool, which is inserted into the infected person by enema or using a tube snaking down the nose and into the gut.