If it seems like you're waiting a lifetime for a bowel movement, then you might want to ask your doctor about probiotics.

Mike Kemp/Rubberball Productions/Getty Images

Probiotic Bacteria

While probiotics have long been popular in Europe and Japan, these friendly bacteria are just now making it to North America in food products and dietary supplements. But it's not just a matter of swallowing random bacteria. The two main types of bacteria considered to be probiotics include strains from the Lactobacillus and Bifidobacterium genera (other genera, such as Escherichia, Enterococcus and Saccharomyces have also been designated probiotics, but to a lesser and more questionable extent). Lactobacillus and Bifidobacterium are made up of different strains, each of which has a different health function. To truly understand the potential benefit to your body, you have to research the difference between Lactobacillus acidophilus and Lactobacillus rhamnosus.

In general, though, probiotics are espoused for a variety of ills, everything from constipation to cancer treatment. Scientists believe that probiotics work by repopulating the gut with good bacteria, which can be eliminated along with bad bacteria during a course of antibiotics. Additionally, probiotics are believed to have the power to fight off pathogens and toxins, as well as strengthen the gut fortresses that will eventually have to do battle with the same.

How does this hypothesis of how probiotics work translate into a health benefit? In some cases, the evidence is a bit shaky. Right now, the clearest benefit of probiotics, backed up by scientific study, comes in the field of gastrointestinal conditions, such as antibiotic-associated diarrhea, acute infectious diarrhea (such as traveler's diarrhea) and irritable bowel syndrome. In one study, probiotics reduced the risk of developing diarrhea associated with antibiotics by 52 percent, and they have been shown to cut the risk and duration of infectious diarrhea as well [source: Kligher, Cohrssen]. Probiotics are successful in treating diarrhea caused by rotavirus in children as well.

As for irritable bowel syndrome, probiotics seem to have the neat effect of both relieving you when it won't come out (constipation) as well as when too much is coming out (diarrhea). In the case of Activia, Dannon's studies show that Activia can speed up the amount of time it takes for waste to exit the system by about 40 percent, if a person with irregular bowel movements were to eat one container a day [source: Warner].

­As for the rest of the claims, the evidence is not quite there. While some research has demonstrated effectiveness, more studies are needed to back up claims regarding the effect of probiotics on childhood allergies; urogenital infections including urinary tract infections, bacterial vaginosis and yeast vaginitis; upper respiratory infections; breast and colon cancer; and lactose intolerance.­

If the results aren't completely in, then how can manufacturers already have probiotic products on the market for these conditions? Find out about how the package doesn't always match the product on the next page.