What health benefits, exactly, is Activia yogurt supposed to offer?

In 1976, when yogurt was fast becoming a brave new trend in the American diet, televisions lit up with a Dannon advertisement that showed 125-year-old Soviet Georgians slurping down the stuff. The notion of yogurt as fountain of youth wasn't a new one: Nobelist Russian biologist Ilya Ilyich Mechnikov believed—but never managed to prove—that yogurt was the reason for the remarkable longevity of a tribe of Bulgarian peasants. Thirty years after the Georgian ad, Dannon launched a new yogurt in the United States with a perhaps more pedestrian promise that claimed to be scientifically "proven." By pitching itself as the cure to vaguely defined tummy troubles, Dannon's Activia has become a billion-dollar global brand.

Activia is the leader in the probiotics craze—a fad for microrganisms you can now find in a new Kashi cereal, "wellness" bars, a host of competing yogurts, and the "functional food" industry, products consumed not for traditional nutrition or pleasure but for presumed medicinal qualities. Probiotic yogurts recently have been alleged to ward off everything from hay fever to secondary infection. But an appeal to digestive health seems to be aimed specifically at women, of whom half report ailments of the midsection and nearly 100 percent, one can surmise, would prefer a slimming of the midsection. By targeting the female stomach, Activia sales topped $130 million in 2006, a very unusual success for a new food product's first year. The following year, sales increased by 50 percent. * As a very funny video on Current TV puts it, open the refrigerator of any woman over 40, push aside that rotisserie chicken, and there you'll see a tidy row of green plastic yogurt containers.

Those green containers are loaded with live organisms, called Bifidobacterium animalis in the microbiology world, that were renamed and trademarked Bifidus regularis by Dannon. Probiotics are what's known as "beneficial bacteria," which, if carried in the intestinal tract in significant amounts, are thought to ease some digestive concerns. Which concerns Dannon intends Activia to address are left intentionally vague. In their $100 million ad campaign, when Jamie Lee Curtis rubs her tummy and refers to occasional irregularity, you may think she's just being polite. After all, nobody wants to be explicit about the specifics of waste elimination when pitching food.

But is Ms. Curtis referring to excessive or inadequate, um, passage? Dannon issued a press release about a poll on digestion in conjunction with the product's U.S. launch that tried to ascertain America's most "backed up" cities—"Orlando is at a standstill, and we're not talking traffic"—suggesting that constipation is the scourge that dare not speak its name. And, indeed, the Activia Web site says that the product "helps to regulate your slow intestinal transit."

There are two curious aspects to this clam. First of all, Dannon has to be careful about using the C-word. The FDA classifies constipation as a disease, and any product that claims to treat a disease must carry an FDA-approved health claim. Activia does not. But what's stranger is that probiotics are usually used to treat diarrhea—that's why they are often prescribed to travelers journeying to don't-drink-the-water destinations.

The studies Dannon cites on its Web site point toward Irritable Bowel Syndrome, or IBS, which to some has become a catchall for digestive issues, in part because it clinically encompasses both constipation and diarrhea. Up to 75 percent of people diagnosed with IBS are women (though, as the Department of Health and Human Services says on its Web site, it hasn't been proved that IBS affects women in greater numbers than men; "it may be that women are more likely to talk to their doctors about their symptoms"). With this new female trouble has come a vast marketing demographic eagerly filling their shopping baskets with the hope of vanilla-flavored cures.

As Consumerlab.com's Tod Cooperman points out, even the study that Dannon's Web site cites as validation says the clinical relevance is open to discussion. "It just hasn't been shown that that's a clinical benefit," says Cooperman, whose group has studied the effect of probiotics. Four additional studies concur. Larry Bergstrom, staff consultant at the Mayo Clinic, says that isn't necessarily a reason to dismiss the possibility that Activia can help alleviate IBS. He has found that people describe feeling worse after the first week eating Activia, and then better after the second week. (That's presumably why the product launched with a "two week challenge.") That's hardly a clinical study, to be sure. But he says that it's tough to assess IBS empirically, since studies (so far) can only assess symptoms, and symptoms are harder to read conclusively.

If you dig a little, Dannon seems aware that the studies it adduces to support Activia's effectiveness are inconclusive. After all, the company funded a study by the American Society of Microbiologists that concluded, "[A]t present, the quality of probiotics available to consumers in food products around the world is unreliable." (Dannon's spokesman Michael Neuwirth told me, "It's not a scientific study.") The ASM report plays a prominent role in a class-action suit a California woman has brought against Dannon; the suit demands refunds for everyone who has paid for the yogurt, which is sold at a 30 percent inflated price for its "proven" medical promises.

This isn't the first time the company has been accused of deceptive advertising. Last year, the U.K. Advertising Standards Authority told Danone (as the French-owned company is called in Europe) to pull ads for their probiotic yogurt Actimel that say, "Actimel is scientifically proven, and you can see that proof for yourself on our Web site." The Actimel Web site reads: "The disclosure of these studies is restricted to Health Care Professionals not the general public." And in 2006, the same agency put the kibosh on Actimel ads that implied the yogurt could help prevent children from catching bacterial infections.

Though Activia's ad copy in the United States refers to "occasional irregularity," its animated imagery of a bronzed, washboard-flat, tiny-waisted stomach with a canary-yellow arrow pointing down (er, out) suggests a different promise, says Jean Kilbourne, author of Killing Us Softly: Advertising's Image of Women. "I see that as a weight loss implication," she tells me. "It's meant to evoke the idea, 'This is the kind of tummy you can end up with.' The arrow is code for 'This will go right through you.' It's a dieting subtheme that plays on the whole idea of women being much more focused to do whatever it takes to make our bodies feel thin."