In this study, involving 266 women with constipation, eating yogurt containing active probiotics and fructooligosaccharides (FOS) was found to reduce constipation relative to a milk-based dessert without probiotics. While on the yogurt (Dannon's Activia), people had an average of 6 bowel movements a week vs. 5 a week while on the dessert.
A Consumer Reports test of Activia showed that it has 3 billion Bifidus regularis bacteria per 4-ounce serving. Unfortunately, only 3 million bacteria (1 in a thousand) survived a bath in stomach acid. This is why I recommend using probiotic pearls, where over a billion bacteria per pearl survive the stomach acid (i.e., to get the number of healthy bacteria in 1 pearl to your colon, you'd need to eat around 3 gallons of yogurt!).
"Effect of the ingestion of a symbiotic yogurt on the bowel habits of women with functional constipation," DePaula JA, Carmuega E, et al, Acta Gastroenterol Latinoam, 2008; 38(1): 16-25.
Consumer Reports article, Dannon Activia has cultures that last.
Jacob Teitelbaum, M.D. is one of the world's leading integrative medical authorities on fibromyalgia and chronic fatigue. He is the lead author of four research studies on their treatments, and has published numerous health & wellness books, including the bestseller on fibromyalgia From Fatigued to Fantastic! and The Fatigue and Fibromyalgia Solution. Dr. Teitelbaum is one of the most frequently quoted fibromyalgia experts in the world and appears often as a guest on news and talk shows nationwide including Good Morning America, The Dr. Oz Show, Oprah & Friends, CNN, and Fox News Health.