More recently, a double-blind study reported in the journal Environmental Health found that employees who received a daily dose of probiotics missed work less frequently due to respiratory and GI problems than those who received a placebo. In the study group, sick days caused by these health problems could be reduced by 55 percent compared with the placebo group, the Swedish authors noted.

But probiotics aren't just for people with symptoms, adds Baird, who has been a consultant the Dannon Company Inc., a national yogurt producer, headquartered in White Plains, NY.

Depending on the type of bacteria you are consuming, probiotics can help maintain normal GI function, she says. Or they can replace good bacteria wiped out when people take powerful broad-spectrum antibiotics. "Personally," she says, "I think everyone should have some kind of probiotic every day."

Despite clinical evidence suggesting that probiotics have beneficial effects, microbiologists still struggle to understand the underlying science.

"We know that probiotics work with respect to certain GI disorders, such as traveler's diarrhea," says Kathene C. Johnson-Henry, a laboratory manager in the Division of Gastroenterology, Hepatology and Nutrition at The Hospital for Sick Children in Toronto, Canada. "But in terms of the basic science and the reasons behind why they work or how they work, this is what we're trying to establish."

Johnson-Henry and her colleagues recently showed that pretreating rats with probiotics before exposing them to a stressful situation eased gut problems. The study, which appeared in the journal Gut, was designed to elicit some of the same GI-related effects of stress seen in humans, specifically irritable bowel syndrome.