Probiotic bacteria such as Lactobacillus rhamnosus are believed to positively affect the immune response by improving the intestinal microbial balance leading to enhanced antibody production and phagocytic activity of leukocytes. Hatakka and colleagues studied the ability of milk containing probiotic cultures to decrease gastrointestinal and respiratory infections in children attending day care centers.

The study authors recruited children aged one to six years attending 18 day care centers in Finland. Children with allergy to cow's milk, lactose intolerance, reported food allergy, or severe chronic diseases were excluded from the study. After receiving permission from parents, the study group randomly assigned children to receive either their usual milk or probiotic milk containing live cultures of Lactobacillus. The study was double-blinded, and each child was given milk three times daily for five days of each week. The children consumed at least 200 mL of milk on two thirds of the days of the study. During the seven-month study, parents reported all symptoms and diagnosed illnesses in the children, as well as any use of antibiotics and days lost from day care because of illness.

Although the nearly 300 children in each study group were comparable, more children in the control group reported five or more respiratory infections during the previous year. The children drinking probiotic milk were reported to have a 15 percent reduction, compared with control-group children, in days of absence from day care because of illness, a 17 percent reduction in respiratory infections and a 19 percent reduction in use of antibiotics. The differences were reduced when adjusted for age, but the authors conclude that probiotic milk could provide a useful public health intervention. The estimated 10 to 20 percent reductions in infections and absences from day care centers could have significant medical and economic effects.