Evidence suggests that yogurt may help maintain a healthy gastrointestinal tract, and some companies are introducing products with extra bacteria. But are new products like Activia, which is marketed to regulate the digestive system, all they claim to be?

All yogurts contain the starter cultures Lactobacillus bulgaricus and Streptococcus thermophilus -- without them, you'd just have milk. But several manufacturers of yogurt -- and other products -- are beginning to add extra probiotic strains in the hopes of providing extra health benefits.

Dannon Activia was the first yogurt to market itself as "probiotic," meaning that it has added live bacterial cultures. It contains Bifidobacterium animalis DN-173 010, which Dannon trademarked and markets as Bifidus regularis in the United States. This strain has been shown to survive the trip through the digestive tract and reach the colon intact.

Dannon claims that Activia is "scientifically proven to help with slow intestinal transit when eaten daily for two weeks as part of a balanced diet and healthy lifestyle." In other words, it speeds the path of food through your body and may help with constipation.

Prior to Activia's launch in 2006, studies were conducted on B. animalis DN-173 010, all with a certain amount of involvement by Dannon and often at its research facility in France. In a double-blind, crossover study published in Alimentary Pharmacology & Therapeutics, researchers gave three 4-oz servings a day of either normal yogurt or yogurt with B. animalis DN-173 010 to 36 healthy women. During two 10-day treatment periods, transit time (the time it takes for food to travel from the mouth to elimination) was significantly shorter in the B. animalis group. The improvement was more significant in women who normally had a transit time of 40 hours or more.

In another randomized, controlled study, 267 people with constipation-predominant IBS who ate Activia (two 4-oz servings a day) reported less discomfort and bloating and more frequent bowel movements after three and six weeks than those who ate a yogurt that had been heat-treated to kill all helpful bacteria.

Activia is not the only yogurt to contain added live Bifidobacterium, also known as Bifidus. Stonyfield Farm yogurt, Horizon Organic yogurt, and Yo-Plus from Yoplait, among others, contain this probiotic as well. Although these brands have not been studied in randomized, controlled trials like Activia has, the Bifidobacterium in these products may have similar effects.

Bottom line on probiotic yogurt: More research is needed before we can definitively recommend Activia or another probiotic yogurt for promoting regularity. But while the data continue to come in, for most people, it can't hurt to give a probiotic yogurt a try.