Friendly bacteria known as probiotics are growing in popularity in the United States, as increasing numbers of people catch on to their benefits to your digestive system.

The food industry, eager to capitalize on the latest health food trends, has introduced more than 150 probiotic and prebiotic (nutrients that feed probiotics) food products in 2007, compared with 100 in 2006 and just 40 in 2005.

You can now buy probiotic pills, yogurts, snack bars, baby formulas and even chocolate, all of which claim to “regulate your digestive health” or “strengthen your body’s defenses.”

The National Institutes of Health has even designated the research of gastrointestinal bacteria and probiotics as a major initiative, and other studies have already been completed or are underway, such as:

• A Canadian study that found fermented milk containing Lactobacillus acidophilus and Lactobacillus caseii could help prevent antibiotic-related diarrhea.

• A study from Finland that found an oat drink containing Bifidobacterium lactis bacteria helped bowel function in nursing home residents.

• A study by researchers in Argentina, who are investigating whether milk fermented with lactic acid bacteria can reduce amounts of cancer-causing substances in your intestine.

Probiotic bacteria are beneficial because they help control the illness-causing bacteria in your intestinal tract. When the bad bacteria proliferate beyond control, illness can arise. High levels of good bacteria keep them in check.

U.S. retail sales of probiotic-containing foods and supplements totaled $764 million in 2005, and are expected to reach $1 billion in 2010, according to BCC Research.

Fermented yogurt drinks, which are full of probiotics, have been used since ancient times to cure intestinal illnesses and promote longevity.