Lactose handling by the human gut by most people, beyond being breast-fed, has been considered a disorder rather than physiological. A non-human mammalian milk source is novel for the majority. During the first 6 months of life, when neonates and infants are best breast-fed, lactose along with other macronutrients, provides energy, but may have other functions as well. At birth, babies are endowed with their mother's vaginal microbiome, but not if they are born by Caesarean section. How much maternal milk lactose survives the infant's small intestine and is processed by this unique gut microbiome and to what end is still uncertain, but no lactose or galactose appears in the faeces. Once intestinal lactase activity declines in most infants, lactose may enhance innate immunity through the cathelicidin antimicrobial peptide (CAMP), which is best achieved by lactose synergy with other colonic fermentation metabolites such as butyrate. It is of interest whether this lactose function or a variant of it persists. It might not be evident when lactase is persistent, as it is in most people of northern European ancestry. Population genomics indicate that lactase persistence became prevalent only about 3000-1000 BC, the Bronze Age of Eurasia. Gastrointestinal symptoms (GIS) in lactase nonpersisters who consume dairy foods are partly dose dependent and not usually evident with single lactose intakes≤25 g per day. Spreading intake across the day reduces the risk as can various dietary patterns. Nevertheless, individual differences in GIS lactose sensitivity may merit public health and clinical consideration.