You carry around 3 to 5 pounds of bacteria in your colon and small intestine, and most, but not all, of these intestinal denizens are helpful. Foods and supplements designed to support the good guys come in two main categories: Probiotics (foods containing live bacteria of the beneficial variety) and prebiotics (containing foods those bacteria like to eat).
Probiotics can banish constipation, diarrhea and bloating, says Dr. Anish Sheth, a gastroenterologist at Yale University School of Medicine in New Haven, Conn. In addition, they can crowd out disease-causing bacteria, such as those found in spoiled food that might cause diarrhea, says Dr. Leonard Smith, a retired general vascular surgeon in Miami.
The good gut bacteria, Sheth says, eat the starches and fibers that we digest, turning them into products we can use. The bad ones produce fewer beneficial products and more undesirable output, such as gas.
People can easily get useful bacteria from fermented foods, such as sauerkraut and miso, Smith says. In foods and supplements designed to be probiotic, lactobacillus and bifidobacterium are the most common additions. Lactobacillus, used to make yogurt, is a normal resident in the small intestine, and it produces vitamins and antimicrobials that prevent nonbeneficial bacteria from growing. Natural bifidobacteria boost the immune system and digestion while inhibiting growth of disease-causing bacteria and yeasts.
Prebiotics are fibers, such as inulin, that the good bacteria in the gut love to eat. Many foods, such as banana, flax and barley, naturally contain inulin and other carbohydrates that gut bugs adore digesting. You can also buy pure inulin to mix into soups or smoothies.
Probiotics and prebiotics are best used in combination, Sheth says. He also advises looking for products that combine several kinds of bacteria. Since scientists are far from understanding which ones are best, he says, using multiple kinds is the best strategy.
Some research studies support probiotic use. For example, researchers from Imperial College in London reported in the British Medical Journal in 2007 that probiotic drinks prevented diarrhea in elderly people in the hospital. The 135 people in the study were taking antibiotics, which can interfere with the normal gut community and allow diarrhea-causing bacteria, such as Clostridium difficilis, to move in. Of those who drank a probiotic yogurt drink, only 12% developed diarrhea, compared with 34% who drank a bacteria-free milkshake.
Other reports claim probiotics can stave off the sniffles in children or prevent allergies. For example, in a study published last year in the journal Pediatrics, researchers based at Danisco reported that probiotics are good for respiratory health. Danisco makes probiotics and prebiotics. The scientists gave probiotic powders, taken with milk, to about 200 children in a child-care center in Jinhua City, China. One hundred others received a bacteria-free beverage. Over the six-month study, the kids taking the probiotics had fewer instances of fever, cough and runny nose than the kids on the inactive product.
However, some scientists remain wary of the claims made by probiotics manufacturers. And since the Food and Drug Administration does not regulate probiotics, proof of efficacy is often sketchy. Last year, Dannon settled a lawsuit with yogurt buyers who asserted its ad campaign made false promises.
"Be skeptical," advised Dr. Michael Starnbach of Harvard Medical School. The bacteria that have inhabited a person's gut for decades will not easily give up their place to newcomers, he said. Prebiotics, he says, seem logical — but there is not much in the way of scientific data on them yet.