Probiotics are dietary supplements containing live microorganisms. They're thought to confer health benefits on their users, mainly by changing the makeup of peoples' gut bacteria. But the usefulness of probiotics is more complicated than simply taking a pill.

In the early twentieth century, the Russian scientist Eli Metchnikoff proposed that drinking milk fermented by lactic acid bacteria would decrease the growth of harmful intestinal bacteria such as Clostridia. Clostridia are part of the normal intestinal flora but can cause problems when they overgrow. They don't flourish in acidic environments.

Metchnikoff reasoned that the lactic acid bacteria in the milk would seed the intestine, making it more acidic and suppressing the growth of Clostridia. In 1920, it was found that the main bacterium in Metchnikoff's sour milk (Lactobacillus bulgaricus) couldn't even live in the human intestine. This ended the popularity of Metchnikoff's sour milk diet.

Fiber has been likened to a broom pushing its way through the digestive tract, sweeping it clean.

The state of probiotics remains pretty much the same today: great idea, tough to make it work.

Gail Cresci, an assistant professor of surgery, dietitian and researcher at the Medical College of Georgia works with and studies probiotics and the makeup of people's intestinal flora. She often prescribes probiotics to people recovering from certain types of surgery, where they are clearly useful. They're also useful in treating patients with certain other medical conditions.

But to make them work, you need to choose the right combination of bacteria for each patient's condition. There are about 800 bacterial species with over 7,000 strains living in the average gut. Many commercial probiotics contain only a single species of bacterium. And for it to do any good, it has to survive stomach acid and digestion to get established and growing.

Cresci equates the good bacteria living in your gut to another living being inside of you that helps keep you healthy. But the best way for people to keep them in balance is through healthy eating, not bacterial supplements. Cresci is dubious about the usefulness of probiotics for the general population: "consumers are buying stuff like crazy that is probably not even helping them and could potentially hurt them."

To help maintain a healthy intestinal environment, Cresci recommends a diet high in fiber and protein and low in fat. This means 30% or less of the calories coming from fat, with saturated fat comprising no more than 10% of that. Most of the fat should come from monounsaturated sources such as olive and canola oil. And there should be 25−30 grams of fiber daily, from whole grains, fruits and vegetables, not supplements.

Fiber is one dietary component that's in short supply in many American diets. Sometimes known as roughage, it's the indigestible part of fruits and vegetables. Indigestible for people, but not for their gut bacteria. Fiber has been likened to a broom pushing its way through the digestive tract, sweeping it clean. When it arrives in the colon, bacteria there use it as a food source, converting some of it to compounds known as short chain fatty acids.

Australian researchers have recently shown that these short chain fatty acids interact with immune cells, moderating their activity and lessening their tendency to cause inflammation. Short chain fatty acids have previously been shown to alleviate the symptoms of colitis, an inflammatory gut condition.

The researchers speculate that a typical Western diet, which is low in fiber, may be contributing to an increase in asthma, type 1diabetes and other autoimmune diseases. An overactive immune system plays a part in of all these diseases. While their research doesn't show a definite link, it does show a possible molecular mechanism for how a poor diet can lead to a poorly functioning immune system.

Whether or not the Australians have uncovered a meaningful link, dietary fiber has previously been shown to confer many benefits, including reducing the risk of cardiovascular disease and certain cancers. And it helps keep your gut bacteria happy.

There's no question that maintaining a proper balance of gut bacteria is part of good health. But the best way for most people to do this is through diet, not supplements. Someday probiotics may be more generally useful. Not yet.

Gail Cresci won the 2009 Excellence in Practice Award for Clinical Nutrition of the American Dietetic Association. She presented her latest findings on current and potential clinical uses of probiotics at the association's 2009 Food & Nutrition Conference & Expo in Denver, held October 17−20.

An article detailing the Australian study on fiber, bacteria and the immune system was published in the October 29, 2009 issue of Nature.