Background:The aim of the current study was to evaluate the prevalence of lactose malabsorption (LM) in Galicia (NW Spain) in order to design nutritional intervention and/or public education strategies for high risk groups.Methods:We conducted a study of LM by breath-hydrogen carbohydrate absorption test (BH2test) in 850 healthy subjects. All subjects underwent BH2tests following ingestion of a aqueous solution of 2 g lactose/kg body weight up to a maximum of 50 g. Subjects with LM were retested after ingesting 250 ml of milk and/or 250 ml of yogurt.Results:The frequency of LM in the subjects who ingested 2g lactose/kg body weight was 32.5%. This percentage decreased significantly with a decrease in the quantity of administered lactose and the vehicle was milk or yogurt-only 13.7% was LM after 250 ml of milk and 3.8% after 250 ml of yogurt. Gastrointestinal symptoms also depend on dosage of lactose and vehicle, decreasing from 54.3% after 2g lactose/kg to 18.5% after milk and to 0% after yogurt. The frequency and number of gastrointestinal symptoms were significantly higher (p < 0.001) in LM than in lactose absorption (LA).Conclusions:Lactose malabsorption is prevalent in the population of Galicia. An important number of subjects identified as LM with usual clinical testing become LA when the ingestion of dairy products is limited so that the amount of lactose consumed is similar to that contained in a usual serving. Our results suggest the importance of BH2testing following ingestion of usual consumed amounts of lactose per serving.