What amount of daily lactose intake is tolerable in subjects with diagnosed lactose intolerance?
Among individuals appropriately diagnosed with lactose intolerance, differences in a variety of factors—including lactase activity, gastric emptying rates, fecal bacterial metabolites, colonic mucosal absorptive capacity, and intestinal transit time—can greatly influence their susceptibility to develop intolerance symptoms following the ingestion of foods and beverages containing lactose. Individuals differ in the intensity of symptoms of lactose intolerance due to differences in abdominal pain perception and psychological impact of pain and social discomfort. Determining the amounts of lactose that can be tolerated is an important step in developing evidenced-based dietary recommendations that meet the needs of the individual.
High-quality evidence to address the question is limited as documented by the 28 studies summarized in the systematic evidence review. Studies were variable in terms of the definitions of lactose intolerance, study population selection criteria, how lactose was administered, and the type of assessment methods. The lack of validated measures made quantifying the severity of symptoms difficult to interpret. The majority of studies used a single dose of lactose without food and evaluated short-term responses. Efforts often were not made to mask the taste difference between lactose-free milk and milk containing lactose.
To assess tolerability, only a handful of studies tested the subjects in a double-blinded fashion with increasing amounts of lactose administered throughout the day to determine the daily tolerable load of lactose. Furthermore, the majority of studies examined small numbers of subjects, and no data were reported on the relationships of age, sex, or race/ethnicity. No studies focused exclusively on children; two studies examined adolescents exclusively; and two others included both children and adolescents. Only two studies were conducted on pregnant women; none focused on lactating women.
In the majority of available studies, subjects were classified as malabsorbers or absorbers based on breath hydrogen measurements or a blood glucose test, and symptoms of lactose intolerance were not always required for study entry. A blinded control was rarely employed to define lactose intolerance at study entry; thus it is probable that some individuals would have reported symptoms following ingestion of lactose-free solutions. The majority of studies investigated individuals with proven lactose malabsorption, not diagnosed lactose intolerance. As a result, only recommendations for individuals with proven lactose malabsorption and perceived lactose intolerance can be made with reasonable assurance.
The available evidence suggests that adults and adolescents who have been diagnosed with lactose malabsorption could ingest at least 12 grams of lactose when administered in a single dose (equivalent to the lactose content found in 1 cup of milk) with no or minor symptoms. Individuals with lactose malabsorption can tolerate larger amounts of lactose if ingested with meals and distributed throughout the day. However, 50 grams of lactose (equivalent to the lactose content found in 1 quart of milk) usually induces symptoms in those adults with lactose malabsorption when administered as a single dose without meals. For women with lactose malabsorption, tolerance to dietary lactose may improve during pregnancy but then worsen after delivery. Some data suggest that the routine ingestion of lactose increases the amount of lactose that is tolerable in both adults and adolescents. There is no scientific evidence to identify the tolerable dose of lactose for children with lactose malabsorption.
We stress the importance of additional scientific investigations to provide evidence-based and culturally sensitive recommendations about the amount of daily lactose intake that can be tolerated by lactose-intolerant individuals, with special emphasis on pediatric and adolescent populations and pregnant and lactating women.
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