Several years ago, the National Yogurt Association petitioned the U.S. Food and Drug Administration (FDA) to change the standard of identity for yogurt to require the starter cultures to be viable in the finished product. The petition now is part of an FDA proposal.

Most stakeholders have indicated support for the change — after all, the survival and growth of live probiotic bacteria are essential if the consumer is to realize any health benefits, right?

Maybe — and maybe not. A study published in Gastroenterology would seem to cast a small shadow of doubt onthe importance of probiotic viability.

In the study, researchers used gamma radiation to deactivate probiotic bacteria before administering probiotics to a small group of mice suffering from "experimentally induced" colitis. Both the "dead" and viable probiotic bacteria improved the colitis symptoms. Previous studies using heat-deactivated bacteria showed no improvement, but heat treatment destroys the bacteria's cellular structure while gamma radiation leaves it intact. The researchers concluded the protective effects were tied to the probiotics' DNA, not to their ability to colonize the colon.

"There is still some debate about whether the cellular components and/or metabolites in probiotics are the active ingredients," says Dennis Gordon, Ph.D., who recently retired from his position as professor and chairman of North Dakota State University's Department of Cereal Science. "There is more evidence that viable probiotics stimulate the immune system compared to non-viable probiotics."

"The experiment should be looked at as a 'first attempt,'" says Gordon, "and not a definitive statement that DNA is the factor within the cells (probiotics) that initiate the immune response."

The study does provide insight into the way probiotics might act, says Mary Ellen Sanders, Ph.D., of Dairy and Food Culture Technologies, Centennial, Colo. "We've always known they have the potential to produce antimicrobial substances, that their mere presence in the gastrointestinal tract can improve gut barrier function."

It would be wrong, however, to make any sweeping conclusions based on this one small animal study. In fact, "it is likely that probiotics act via many different mechanisms," says Sanders.