Lactose malabsorption, irritable bowel syndrome and self-reported milk intolerance

The relationship between lactose malabsorption, irritable bowel syndrome and development of intestinal symptoms is unclear, especially when the ingested dose of milk is small. Thus, the role of hydrogen breath testing in the diagnostic work-up of patients with nonspecific intestinal symptoms is still debated.

To establish the relationship between lactose malabsorption, severe self-reported milk intolerance, irritable bowel syndrome and related symptoms.

The prevalence of lactose malabsorption was prospectively assessed by means of a hydrogen breath test in 839 patients (503 with irritable bowel syndrome, based on the Rome criteria, regularly consuming milk, and 336 subjects who identified themself as milk intolerant, after an oral load of 25 g lactose). The test was considered "positive" when a hydrogen peak exceeding 20 ppm over baseline values was observed in two or more samples. Attempts were also made to establish whether the predominant presenting symptom (diarrhoea, constipation, alternating diarrhoea and constipation, pain and gaseousness) might be helpful in predicting the outcome of the breath test.

The prevalence of a positive breath test was comparable in the two groups (337 patients with irritable bowel syndrome (66.9%) vs 240 patients with milk intolerance (71.4%)). The same holds true for the first peak of hydrogen excretion, total hydrogen output and prevalence of symptoms during, and in the four hours after, the test. The predominant presenting symptom was not useful for predicting outcome of the test either in regular milk users or in milk intolerant subjects.

The almost identical results of the lactose breath test of patients with irritable bowel syndrome and subjects with self-reported milk intolerance suggests that the two conditions overlap to such an extent that the clinical approach should be the same. A lactose breath test should always be included in the diagnostic work-up for irritable bowel syndrome, as fermentation of malabsorbed lactose is likely responsible for triggering symptoms. Conversely, lactase deficiency is probably irrelevant in most subjects not affected by irritable bowel syndrome, within a moderate milk consumption.