The use of dietary lactose as a source of energy requires its prior hydrolysis at the intestinal brush border into glucose and galactose. Indeed, as a disaccharide, intact lactose cannot be actively absorbed. Therefore, the presence of unhydrolysed lactose in plasma or urine has mostly been associated with specific cases such as lactating women or altered gastrointestinal permeability. Nevertheless, lactose has also been detected in the blood and urine of healthy men, indicating that the presence of lactose in the circulation of healthy subjects is not incompatible with normal physiology. The present crossover study monitored, in 14 healthy men, the postprandial appearance of lactose in serum after a single intake of 800 g of milk or yoghurt. Genetic testing for lactase persistence and microbiota profiling of the subjects were also performed in order to investigate their potential contribution to postprandial lactose serum levels. Data revealed that lactose does appear in serum after dairy intake, and with delayed kinetics when compared to the actively absorbed galactose. A notable inter-individual variability was observed and, although lactose levels were inversely correlated with galactose levels, the presence of the lactase persistence allele could not explain this variability. Finally, lactose levels have been associated with the abundance of the Veillonella genus in faecal microbiota. The measurement of systemic lactose following dietary intake could provide information about lactose metabolism and nutrient transport processes under normal or pathological conditions.
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