Abstract
The absence of a dedicated transport for disaccharides in the intestine implicates that the metabolic use of dietary lactose relies on its prior hydrolysis at the intestinal brush border. Consequently, lactose in blood or urine has mostly been associated with specific cases in which the gastrointestinal barrier is damaged. On the other hand, lactose appears in the blood of lactating women and has 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. In this cross-over study we have characterised the postprandial kinetics of lactose, and its major constituent, galactose, in the serum of fourteen healthy men who consumed a unique dose of 800 g milk or yogurt. Genetic testing for lactase persistence and microbiota profiling of the subjects were also performed. Data revealed that lactose does appear in serum after dairy intake, although with delayed kinetics compared with galactose. Median serum concentrations of approximately 0·02 mmol/l lactose and approximately 0·2 mmol/l galactose were observed after the ingestion of milk and yogurt respectively. The serum concentrations of lactose were inversely correlated with the concentrations of galactose, and the variability observed between the subjects' responses could not be explained by the presence of the lactase persistence allele. 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.
Highlights
The disaccharide lactose is the most important energy source during the 1st year of human life, providing almost half of the total energy required by infants
In this cross-over study we have characterised the postprandial kinetics of lactose, and its major constituent, galactose, in the serum of fourteen healthy men who consumed a unique dose of 800 g milk or yogurt
Data revealed that lactose does appear in serum after dairy intake, with delayed kinetics compared with galactose
Summary
The disaccharide lactose is the most important energy source during the 1st year of human life, providing almost half of the total energy required by infants. The identification of unhydrolysed lactose in blood (lactosaemia) or urine has been associated with specific cases, such as nursing mothers, newborns[4,5,6], patients with gastrojejunostomy[7] or subjects with abnormal intestinal permeability caused by gastrointestinal disorders, for example coeliac disease[8] and rotaviral gastroenteritis[9] In this context, Northrop et al[10] have proposed urinary lactose, after oral administration of lactose, as a marker to evaluate intestinal integrity in adults. Increases in urinary lactose were observed, at lower levels, in healthy adults after oral intake of lactose solutions and milk[6,11,12,13], suggesting that part of the ingested lactose can pass through an intact gastrointestinal barrier to reach systemic circulation and, subsequently, urine. Samples were obtained from a recently published crossover study with fourteen healthy men in which the metabolic impact of the ingestion of milk and yogurt, two dairy products that differ in lactose concentration, was evaluated by following changes in markers of inflammation, the faecal microbiota[15], as well as the serum metabolome[16]
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