Lactose, Lactose Maldigestion and Lactose Intolerance: Definitions, Misunderstandings, Mechanisms, and Public Health...

Lactose is the principal carbohydrate in milk, serving a critical role in infant energy metabolism as well as contributing to neurodevelopment, and the establishment of a healthy gut microbiota. Later in life, lactose digestion capacity varies widely across human populations owing to the genetically determined decline of intestinal lactase expression after weaning-a condition termed 'lactase non-persistence'. Approximately two-thirds of the global adult population exhibits reduced lactase activity, yet the clinical consequences of this enzymatic reduction are far less uniform than commonly assumed. This review provides a comprehensive examination of lactose functionality, the physiology of lactose digestion, and the consequences of lactase persistence and non-persistence. We present formal definitions for the spectrum of terms used in lactose research-including lactase persistence, lactase non-persistence, lactose maldigestion, and lactose intolerance-and argue that the critical distinction between lactose maldigestion (a physiological state) and lactose intolerance (a clinical syndrome) is frequently overlooked in both research and public health. Drawing on evidence from double-blind randomised controlled trials , microbiome intervention studies, and global epidemiological data, the evidence indicates that the majority of individuals with lactose maldigestion do not develop clinically meaningful symptoms at moderate intake levels of most dairy products. We further discuss the mechanism of colonic adaptation, whereby gradual lactose exposure selectively enriches Bifidobacterium populations capable of fermenting lactose without excessive gas production resulting in greater lactose tolerance. Finally, we examine the public health implications of these findings, noting that food-based dietary guidelines worldwide-including those in populations with high prevalence of lactase non-persistence-consistently recommend dairy consumption owing to its established associations with reduced risk of type 2 diabetes, colorectal cancer, and improved bone health.