The small and large intestines (gut) do most of the work involved in digesting the 20 tons of food that the average person consumes in a lifetime. This process involves trillions of bacteria -- some of them harmful and others beneficial.

What you may not know: While the gut is most commonly associated with digestion, it's estimated that at least 60% of a person's immune system is located there. "Good" bacteria protect against the growth of harmful bacteria to help prevent infections, such as vaginal yeast infections and urinary tract infections.

Probiotic bacteria (a subset of good bacteria) also secrete substances that act on intestinal muscles and help regulate motility (the intestinal contractions that move food and waste through the intestine at the proper rate).

Because these good bacteria play a key role in preventing infections as well as keeping the digestive system functioning properly, probiotics -- dietary supplements or foods that contain beneficial bacteria or yeasts similar to those found in the human gut -- have become increasingly popular, particularly among people who take antibiotics.

Reason: Antibiotics kill not only harmful microorganisms that cause disease, but also the body's beneficial bacteria, sometimes leading to gas, cramping and such conditions as diarrhea. People who take antibiotics regularly may have permanent reductions in probiotic organisms unless they replenish the body's natural supply. For most people, the occasional use of antibiotics -- such as a 10-day course -- is unlikely to cause lasting problems.

WHEN SUPPLEMENTS HELP

Probiotics are live microorganisms. Two of the most beneficial types of probiotics -- Lactobacilli and Bifida organisms -- thrive in the naturally acidic environments of the stomach and small intestine.

Probiotics are often recommended for digestion (to help reduce such problems as gas, bloating, constipation and diarrhea), but they appear to be equally important -- if not more so -- for the immune system.

Probiotics in the intestine stimulate production of white blood cells known as regulatory T cells, which help fight inflammation associated with such disorders as eczema, seasonal allergies and inflammatory bowel disease (IBD), a condition in which the bowel becomes inflamed, often resulting in abdominal cramps and diarrhea.

Probiotics also help prevent and treat diarrhea related to Clostridium difficile infection.

There are many dozens of species of probiotic organisms. The most reliable probiotic formulations now available in the US are in capsule form. Most probiotic capsules should be refrigerated.

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