These products contain probiotics, or "friendly" bacteria similar to those found in the human digestive system.
There are supplement pills, yogurts, smoothies, snack bars and cereals, even baby formula and chocolate. Sold by major names like Dannon and Kraft, they're spreading like germs on grocery store shelves and in supermarket dairy cases.
And they come with vague health claims of "regulating your digestive health" or "strengthening your body's defenses."
Experts say probiotics are generally safe, and in some cases might be helpful. More research is needed, and it's a hot new area, reflecting a growing understanding of the role that naturally occurring intestinal bacteria play in health. This week, the National Institutes of Health is hosting a conference where top scientists will discuss recent advances.
In the meantime, the market is ahead of the science. It's all part of a burgeoning effort to capitalize on an obsession with health foods. Probiotics are already popular in Europe, Asia and South America.
And there are "prebiotics," too, which contain fiber and other nutrients that feed probiotic bacteria.
So far this year, more than 150 probiotic and prebiotic commercial food products have been introduced in the U.S., compared with about 100 last year and just 40 in 2005, said Tom Vierhile of Datamonitor, a market research firm.
"It is definitely a growing trend," Vierhile said.
Holly Maloney, a nutrition instructor at Chicago's Kendall College, eats new probiotic nutrition bars that claim to help digestion and the immune system. She's also a longtime fan of yogurt and kefir, a probiotic-containing fermented milk drink.
"It just makes me feel good," Maloney, 32, said of the products. "If I have a few days where I don't have it, I don't feel right."
While many probiotic products haven't been put to a rigorous scientific test, there is emerging evidence that in huge amounts, some kinds of "friendly" bacteria can be helpful.
Small studies have suggested that certain probiotics might help treat or prevent some types of gastroenteritis, diarrhea and allergic skin reactions, and the bugs are being investigated for many other ailments.
The NIH has declared the study of gastrointestinal bacteria and probiotics a major research initiative. The agency's upcoming meeting will highlight current science so it can identify research gaps and determine the direction of future research, said Crystal McDade-Ngutter, who heads an NIH working group on the topic.
"The fact that there are a number of health implications and a lack of understanding associated with the use of pre- and probiotics makes this a very interesting subject to study," she said.
The bugs are being put under the microscope around the globe.
- A Canadian study published last month suggested that fermented milk containing Lactobacillus acidophilus and Lactobacillus caseii could help prevent antibiotic-related diarrhea.
- A study from Finland published this year found that an oat drink containing Bifidobacterium lactis bacteria helped bowel function in nursing home residents.
- Scientists in Argentina are investigating whether milk fermented with lactic acid bacteria might reduce amounts of cancer-causing substances in the intestine.
- University of California at Los Angeles researchers are looking at whether probiotic supplements can treat allergy-induced skin rashes in babies.
- Israeli scientists are studying whether these supplements can improve complications in liver disease.
Other 2007 products include: Kraft Foods Inc.'s LiveActive prebiotic cottage cheese and probiotic cheddar cheese; Nestle's probiotic Good Start Natural Cultures baby formula; Beech-Nut Nutrition Corp.'s Good Evening prebiotic baby food; and the Swiss firm Barry Callebaut's probiotic chocolate.
University of Michigan researcher Gary Huffnagle calls probiotics "a new essential food group" in his new book, "The Probiotics Revolution."
The concept, however, is not new.
Yogurt, made from milk fermented by bacteria, dates back centuries and has been said to have cured a 16th century French king's intestinal illness and to explain longevity in rural Bulgaria.
But there's an emerging shift in how scientists view probiotic bacteria and their role in health.
Millions of good bacteria live in the intestinal tract, helping keep bad, illness-causing bacteria at bay. Scientists increasingly believe that illness arises when that balance gets out of whack and bad bugs start to take over.
This overgrowth has been implicated in many common digestive problems including inflammatory bowel disease and irritable bowel syndrome, said Dr. Sri Komanduri, a gastrointestinal specialist at Chicago's Rush University Medical Center.
This line of thinking "has prompted not only the medical industry and obviously the food industry to try to create things to shift the balance back toward that good bacteria," he said.
Komanduri prescribes medical-strength probiotic pills containing 450 billion live lactic acid bacteria for inflammatory bowel disease and bacterial overgrowth in the gut.
But he doesn't recommend them for patients without specific complaints, and doesn't recommend probiotic foods because he said there's no evidence that they are as effective.
Patients who use them and report benefits are likely experiencing a placebo effect, Komanduri said.
Commercial products containing probiotics fall under Food and Drug Administration regulations. They are not supposed to make drug-like claims about curing or treating specific illnesses, said FDA spokeswoman Kimberly Rawlings.
"As long as they don't cross the line," they can come pretty close, she said.
Huffnagle advised consumers to be wary of probiotic-containing products that don't specify how much or what type of bacteria.
"If a company says something is probiotic, the question is, how much, and what kind," he said.
Evidence suggests the bugs need to be alive and ingested in huge amounts, generally between 5 billion and 10 billion daily, he said.
While some products claim to have more, he said it's uncertain whether more is better.
For more information: National Institutes of Health: Probiotics