(Image: Yogurt Parfait, a Creative Commons Attribution (2.0) image from mrdestructicity's photostream)
In the past couple years, there has been a lot of debate (and at least one lawsuit) regarding the effectiveness of yogurt and other foods and supplements that contain "probiotics," or beneficial microorganisms. Various studies have linked the consumption of probiotics to various health benefits, including treatment of gastoenteritis and diarrhea, lowering cholesterol, improving immune function, and reducing inflammation, among other claims. However, most of these studies studied one particular strain of bacteria, so results may not be applicable to all probiotics, and many studies involving humans show correlation with positive health outcomes, but not causation. To date, little is known with scientific certainty about the effectiveness of probiotics, and even less about how they function in the human body.
A recent study has evidence that probiotics may function by altering the expression of genes in your gut bacteria, thereby altering their metabolism of the foods that are present in your gut. After feeding mouse and human subjects probiotics, researchers at Washington University in St. Louis found that, although the composition of gut bacteria were not altered, gene expression of the gut bacteria of the mice were. The effects of the resulting metabolic changes could be observed in the mice's urine.
This study sheds some light on a previously mysterious process. It seems that probiotics may have a real effect on the human digestive system. Hopefully further studies will be able to further elucidate the mechanism of the changes brought about by these organisms and help us to understand which strains may be beneficial and for what purpose.