How does lactose intolerance develop after weaning?

The development of lactose intolerance after weaning is a natural biological process for most mammals, including humans. During infancy, the small intestine produces an abundance of lactase, an enzyme essential for breaking down lactose, the primary sugar found in dairy products.12

Following the weaning period, the production of lactase normally declines gradually in most humans. This reduction in enzyme levels means that the body becomes less efficient at digesting lactose, which then remains undigested in the colon. When this occurs, the lactose undergoes bacterial fermentation, leading to symptoms such as bloating, abdominal pain, and diarrhea.12

While this decline is a common, permanent physiological change, some populations—particularly those of northern and central European descent—possess a genetic mutation that allows for "lactase persistence," enabling them to continue producing the enzyme and digesting milk into adulthood.13