Most moms know from instinct, and doctors agree: Breast milk is best for baby for myriad reasons, including the potential to build a strong immune system. Now scientists at Duke University Medical Center confirm that mother’s milk is better than infant formula in protecting infants from infections and illnesses — beginning in the gut.

The findings, published in the August issue of the journal Current Nutrition & Food Science, explain how breast milk, but not infant formula, fosters colonies of beneficial microbiotic flora in a newborn’s intestinal tract that aid nutrient absorption and immune-system development.

In their study, the Duke researchers grew two beneficial strains of E. coli bacteria  — early inhabitants of the gut, not to be confused with the dangerous food poisoning-causing strain — in samples of infant formulas, cow’s milk and breast milk.

Within minutes, the bacteria began multiplying in all of the specimens, but there was an immediate difference in the way the bacteria grew. In the breast milk, bacteria stuck together to form biofilms — thin, adherent layers of bacteria that serve as a shield against pathogens and infections. Bacteria in the infant formula and cow’s milk proliferated wildly, but it grew as individual organisms that did not aggregate to form a protective barrier.

“This study is the first we know of that examines the effects of infant nutrition on the way that bacteria grow, providing insight to the mechanisms underlying the benefits of breast feeding over formula feeding for newborns,” said William Parker, Ph.D., associate professor of surgery at Duke and senior author of the study.

“This study adds even more weight to an already large body of evidence that breast milk is the most nutritious way to feed a baby whenever possible,” said Gabriela M. Maradiaga Panayotti, M.D., co-director of the newborn nursery for Duke Children’s and Duke Primary Care. “We know that babies who receive breast milk have better outcomes in many ways, and mother who breast feed also have improved health outcomes, including decreased risks of cancer. Whenever possible, promoting breast feeding is the absolute best option for mom and baby.”