Yogurt is made by fermenting milk with the “probiotic” bacteria cultures Lactobacillus bulgaricus and Streptococcus thermophilus. Some companies additionally use L. acidophilus, L. casei, L. reuteri, and Bifidobacterium bifidum, among other organisms. Proponents claim that probiotics confer health benefits to the digestive and immune systems by rebalancing the normal intestinal flora. The bacteria also break down the milk sugar, so people with lactose intolerance can usually handle yogurt better than other dairy products.
For the cultures to have any effect, however, they must be alive and present in sufficient numbers. The National Yogurt Association’s “Live and Active Cultures” seal ensures that the yogurt contains at least a minimum number of organisms at the time of manufacture. Plenty of other yogurts without the seal also state that they contain or are made with “live” or “active” cultures (the seal costs a lot of money to obtain), but it’s hard to know how accurate this is. In some cases, yogurts without the seal may have even higher amounts of live, active organisms, as found in testing commissioned by the Cornucopia Institute; in other cases, they may have few, if any. Consumers have no practical way of knowing. Keep in mind also that even the National Yogurt Association’s seal is misleading because it does not guarantee that the organisms are still living at the time you purchase or eat the product. Yogurts that are “heat treated” after culturing must state this on the labels, since this kills the cultures.
In any case, many of the health benefits of yogurt’s probiotics are unsubstantiated or overstated. Some companies even give their own scientific-sounding names (like Bifidus Regularis) to proprietary strains of organisms, wanting you to think that they are somehow superior, despite a lack of evidence to back them. In recent years, class action lawsuits have forced Dannon in particular to stop making misleading claims—that its yogurts relieve bowel irregularity and decrease colds and flus—and pay millions of dollars in restitution to consumers who bought these higher-priced products.