Persons with low lactase levels who are aware of milk intolerance report variations in their tolerance of milk ranging from one half glass to 1 quart. Although low intestinal lactase activity and lactose intolerance as determined by lactose tolerance tests are present in the majority of the world's population, the relevance of tolerance tests using 50 g or 100 g to clinically significant amounts of milk has been questioned. Twenty subjects with assay confirmed low lactase levels were fed 3 to 96 g of lactose to determine a threshold level for symptoms. Seventy five per cent of these subjects had symptoms of flatulence, distention, cramps, loose bowel movements, or diarrhea with 12 g of lactose (equivalent to 8 ounces of milk) or less. To determine the intraluminal events that occur with small amounts of lactose, 7 low lactase level subjects and 3 control subjects underwent intestinal intubation with aspiration ports in the duodenum, jejunum, and ileum. They were fed 24, 12, 6 or 3 g of lactose with polyethylene glycol, a nonabsorbable marker, in a solution similar to the electrolyte composition of milk. In addition to the increased fluid load, the concentrations of sodium in the duodenum were higher in low lactase level subjects than in control subjects (P> 0.001) and potassium concentrations in the duodenum were lower than in control subjects (P> 0.001). Thus, there was objective evidence of fluid and sodium accumulation in the small bowel with these small lactose loads. It is presumed that increased speed of transit and intracolonic fermentation also play a role in symptom production.
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