CHICAGO — Bugs in baby food? Microbes in your milkshake? Relax, this is not the latest tainted food scare — it's a growing trend in foods designed to boost health, not make you sick.

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.

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 fermented milk drink that contains probiotics.

"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.

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National Institutes of Health: http://nccam.nih.gov/health/probiotics