IBS Fact : Evidence-based management of irritable bowel syndrome

Lactose intolerance vs irritable bowel syndrome (IBS)

Lactose is a natural sugar present in milk and milk products. Its digestion requires an enzyme called lactase, which is produced in the small intestine. Due to genetic differences, some persons lose the ability to produce this enzyme in sufficient amounts to properly digest the dairy products containing this lactase. Prevalence of lactase deficiency varies enormously geographically. It is most prevalent in Asian and African countries with a frequency as high as 95%. In Northern Europe, prevalence is only about 5%. In lactose intolerant persons, the consumed lactose remains undigested and passes into the colon. Here some of it is fermented by colonic bacteria, which produces large amounts of hydrogen and methane gas. The undigested lactose also attracts water molecules, which prevents them from being properly absorbed into the bloodstream. Together, the excessive gas and retained water result in symptoms such as diarrhoea, cramping, bloating and flatulence. The threshold for lactose intolerance, i.e., the amount that can be ingested without symptoms, varies widely among individuals. In addition, symptoms may occur sometime after the ingestion. Therefore, one may not connect the symptoms to dietary factors or even mistakenly attribute it to IBS. Diagnosis of lactose intolerance is most commonly performed by the lactose hydrogen breath test. This test measures the amount of hydrogen produced in your breath following the consumption of lactose. If the lactose is not digested by your body, it will be fermented by bacteria, leading to production hydrogen which can then be detected in your breath. The lactose hydrogen breath test usually involves taking a specified amount of lactose orally and measuring breath hydrogen levels over the following 3 – 6 h. Other lactose intolerance tests include blood sugar measurements after ingesting lactose, and measurements of stool acidity. Using the above diagnostic tests, studies revealed that lactose malabsorption affects about 24 – 45% of patients with IBS, which is considerably higher than the 4.7 – 5.7% found among non-IBS sufferers. Studies on dietary exclusion support these findings, showing that 19% – 44% of IBS patients improve their symptoms following a lactose-free diet. This indicates a considerable overlap between IBS and lactose intolerance, although the exact cause-effect relationship is still a subject of scientific debate. One possible explanation for the high incidence of lactose intolerance among IBS patients is misdiagnosis of IBS, i.e, patients mistakenly diagnosed with IBS while in fact suffering from lactose intolerance only. Although this might apply to some presumed IBS sufferers, many of them experience only a partial improvement of symptoms after omitting lactose. Thus, despite dietary changes, these persons continue to experience some degree of abdominal complaints suggesting IBS as an independent underlying condition. A second explanation is that true IBS sufferers are much more sensitive to certain dietary components such as lactose. In these persons even a low-grade intolerance that would go unnoticed in healthy persons, and which might be in fact even undetectable by lactose breath testing, produces significant abdominal complaints. This is supported by studies showing that as many as 62% of IBS sufferers who actually test negative for lactose intolerance do experience much improvement of symptoms following a lactose-free diet. This is further complicated by the fact that some persons might be intolerant to milk proteins instead of lactose. These individuals would also experience an improvement of their abdominal complaints despite negative lactose breath test. Interestingly, some persons testing positive for lactose intolerance do not experience symptoms after consumption of significant amounts of milk, presumably due to an adaptation of their intestinal flora to compensate for the lack of lactose digesting enzymes. In summary, dietary intervention, and to a lesser extent, diagnostic tests such as the lactulose breath test might provide important clues to the diagnosis of IBS and to determine whether lactose is a contributing factor relevant for the severity of symptoms. Prolonged exclusion of dairy-containing products should be done with care and preferably under professional assistance as it may remove an important source of nutrients such as calcium and vitamin D. A possible alternative may be the use of commercially available lactose digesting enzymes. These can be ingested to compensate for the lacking endogenous enzymes, improving thereby the digestion of lactose-containing foods. Although such supplementation seems a promising strategy, its efficacy has not been studied in detail and should be considered from a critical perspective. Finally, although lactose intolerance is a common condition and can be mistaken for IBS, other dietary components such as proteins, fats, and carbohydrates, and in particular fibres, may also cause food intolerances and contribute to IBS-like symptoms. Here, proper testing and effective dietary intervention may reveal misdiagnosis of IBS in some persons, and reduce the severity and frequency of true IBS symptoms in others.

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