As members of a highly germ-conscious society, it's easy to get the impression that all bacteria are hazardous to our health. A growing body of scientific evidence suggests certain types of bacteria, known as probiotics, actually have a number of important benefits.

Probiotics are living microorganisms that thrive in the human gut. Numbering in the trillions, the total population of microbes in the adult intestine can weigh as much as 2 to 3 pounds.

Although the human gastrointestinal tract stays busy digesting food and absorbing dietary nutrients, it has another important function. The gut is a major component of the body's immune system, and it plays a critical role in defending us from disease-causing germs.

Infants typically acquire probiotic organisms from their mothers at birth, and the transfer continues during breastfeeding. Throughout life, the probiotic population in the human body is influenced by a number of factors, including genetics, age, diet and certain medications.

Antibiotics can wreak havoc on the friendly microbes. Designed to kill bacteria, the drugs attack not only the disease-causing organisms in the body, but also the beneficial bacteria in the gut.

A weakened probiotic population in the intestinal tract can lead to digestive disturbances, including cramping and diarrhea, two common side effects of antibiotic treatment. Unchecked by protective probiotics, normal fungal inhabitants of the body may flourish, leading to yeast infections of the mouth, skin and reproductive tract.

Fortunately, it's possible to replace and replenish the beneficial bacteria in the intestine. Probiotics can be found in a variety of fermented dairy products, including certain types of yogurt with live and active cultures, and a cultured dairy product called kefir. Probiotic-containing nutritional supplements are available at many pharmacies, health food stores and supermarkets.

The bacteria most commonly used in food and dietary supplements are lactobacilli and bifidobacteria. A single serving of either product typically contains billions of organisms. Some will perish on the journey through the stomach and small intestine, but the survivors will take up residence in the colon.

Scientists aren't entirely sure how probiotics exert their positive health effects, but it's likely that the benevolent bacteria work in several ways to boost immunity and reduce the risk of infection.

Friendly bacteria appear to interfere with the ability of disease-causing organisms to latch onto the lining of the gut. In a process known as competitive exclusion, probiotics can crowd out harmful bacteria in the gastrointestinal tract.

The beneficial bugs are known to facilitate the digestion and absorption of important nutrients from foods, improving overall health. Mounting evidence suggests they also exert significant anti-inflammatory effects on the cells and tissues of the body.

Probiotic-containing foods and supplements have been used successfully in the treatment of a variety of ailments and illnesses, including lactose intolerance, infectious diarrhea and inflammatory bowel disorders. Preliminary research suggests that correcting abnormalities in the gut's microbial population may improve metabolic function, reducing the risk of diabetes and obesity in some individuals.

To date, most studies involving probiotics have focused on the microbes' gastro-protective and immune-enhancing properties.

In an 80-day Swedish study of 262 working adults, those who consumed the probiotic Lactobacillus reuteri required 55 percent fewer sick days than workers who took a placebo. Employees in the probiotic treatment group reported significantly fewer infections of the gastrointestinal system and respiratory tract than workers in the placebo group.

Working adults aren't the only ones who can benefit from probiotics. In a study of children in daycare centers, youngsters who regularly consumed a product containing Lactobacillus reuteri had a 70 percent lower rate of absenteeism compared to kids who did not ingest the friendly bacteria.

The results of a recent study published in the British Journal of Sports Medicine showed that daily consumption of Lactobacillus fermentum dramatically reduced the rate and length of respiratory illness in professional distance runners. Like other forms of stress, intense exercise can weaken the defenses of the immune system, leaving endurance athletes more susceptible to common respiratory illnesses, including colds and flu.

Runners who took probiotics experienced respiratory symptoms for an average of 30 days during the four-month study, compared to 72 days for those taking the placebo pills. Blood samples revealed that probiotic treatment doubled levels of interferon gamma, a virus-fighting substance secreted by the T cells of the immune system.

With the arrival of cold and flu season, a strong immune system is especially important. Replenishing your body's population of beneficial bacteria might keep you a little healthier this winter.

Rallie McAllister is a board-certified family physician, speaker and the author of several books, including "Healthy Lunchbox: The Working Mom's Guide to Keeping You and Your Kids Trim." Her website is www.rallieonhealth.com. To find out more about Rallie McAllister, M.D., and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate Web page at www.creators.com.

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