For 25 years, Jeffrey Roberts, a technology consultant in Toronto, battled frequent diarrhea and abdominal pain. Roberts, who suffers from irritable bowel syndrome, was unable to attend his children's soccer games and often had to cancel or postpone family vacations. "I'd hold my family back because I'd have a lot of discomfort," he says.

But three years ago, he started taking a powdered drink mix that contains eight strains of probiotic bacteria. "It dramatically changed my symptoms," he says.

Roberts is one of many turning to probiotics. In an ad campaign for Dannon, actress Jamie Lee Curtis cozies up on a green sofa, rubs her tummy and says, "First, the bad news: 87% of this country suffers from digestive issues like irregularity."

The good news, Curtis explains, is that she has discovered Activia yogurt, which is laced with probiotics -- live, "friendly" bacteria -- that the company claims are "clinically proven to regulate your digestive system in two weeks."

Activia has also proved itself with consumers. In 2007, its second year on the market, sales grew by 48% to $181.3 million. Indeed, probiotics are turning up in myriad foods: Naked Juice fruit drinks, Lifeway Foods' energy bars, even a probiotics popsicle recently launched in the United Kingdom.

Last year was a banner one for probiotics, with 158 new food products hitting grocery shelves, compared with just four launches five years earlier, according to Datamonitor's Productscan Online, a database of consumer goods.

Companies claim that the daily consumption of probiotics can provide consumers with benefits such as a boost to the immune system and relief from intestinal distress -- and researchers think that certain probiotic strains hold promise in a number of areas.

But how significant these benefits are is a matter of debate. And it can be tough to decipher which products offer verifiable health claims and which are piggybacking on the hype of the booming industry.

A recent lawsuit filed in Los Angeles has questioned Dannon's probiotic health claims made for Activia and DanActive and charged that the company used scientific-sounding language to deceive customers. And studies have reported that some companies misidentify the probiotic strain they contain or deliver inadequate amounts of bacteria.

What's in a strain?

The World Health Organization defines probiotics as "live microorganisms that when administered in adequate amounts confer a health benefit on the host." Most products contain bacteria isolated from milk products, typically species of Lactobacillus or Bifidobacterium.

In marketing probiotics, companies either make health claims based on research on their own product or make references to the wide range of studies conducted with various probiotic strains.

Dannon says that DanActive "helps strengthen your body's defenses." Lifeway Foods cites "myriad health benefits" including "healthy gastrointestinal functions, increased immunity, and [preventing] the development of cancer-causing toxins." Culturelle claims to offer "numerous health benefits by helping to restore and maintain a nourishing level of good bacteria."

A slew of studies has shown that probiotics can, indeed, boost the immune system. A January report in the journal Surgery examined 14 randomized-controlled trials on the use of probiotics in abdominal surgery, liver transplantation and severe trauma. Nine showed a significant decrease in infectious complications. "We are enthusiastic about preoperative probiotics," says Dr. Mark Besselink, an author on the study at the Utrecht University Medical Center in the Netherlands.

There also is evidence that Lactobacillus rhamnosus GG, sold in capsules by Culturelle, boosts the immune system. Toddlers receiving the bacterium have fewer and less severe milk allergies. In adults, it boosts immune response to vaccines and reduces respiratory infections in athletes.

Probiotics also can be used to counteract the side effects of taking antibiotics, which upset the balance of naturally-occurring microbes in the gut, enabling proliferation of a diarrhea-causing bacterium, Clostridium difficile. In a clinical study of 135 patients published last year in the British Medical Journal, antibiotic diarrhea incidence was reduced by 22% in those consuming a probiotic drink that contained a billion live Lactobacillus and Streptococcus bacteria per milliliter.

Indeed, many companies are targeting the digestive health market because that is the area in which probiotics have shown the most promise

Procter & Gamble has recently begun marketing Align, a pill that contains a proprietary strain of Bifidobacterium infantis, online and in several target cities. In a 2005 study, researchers from Procter & Gamble and the University College of Cork in Ireland compared Procter & Gamble's Bifidobacterium with another probiotic, Lactobacillus salivarius, and a placebo. A total of 80 subjects with irritable bowel syndrome consumed probiotics in a milk beverage. During the eight-week study, Bifidobacterium reduced bloating and abdominal pain more than Lactobacillus or placebo.

Dannon's Activia has been designed for people with less severe digestive problems, says Miguel Freitas, director of scientific affairs at Dannon. He says that stress and traveling can often slow down the digestive system, which results in bloating and heaviness.

Activia uses a strain of Bifidobacterium animalis, and the company's clinical studies have shown that it can reduce constipation by up to 40%. The company's 2002 report studied 36 healthy women during four 10-day periods. During two of them, women ate 3 cups of Activia a day and exhibited more frequent bowel movements than during two periods when they ate a fermented dairy product without the probiotic.