Lactose intolerance

Lactose is a disaccharide and the main sugar in mammalian milk (e.g. cow's, goats, sheep).  

The enzyme, lactase, located in the brush border of the small intestinal mucosa, is required to break down lactose into its sugar components, glucose and galactose.  In this condition lactase is either absent (alactasia) or deficient (hypolactasia).  If there is a shortage of the enzyme unabsorbed lactose molecules osmotically attract fluid into the bowel lumen, leading to an increased volume and fluidity of the intestinal content.  Unabsorbed lactose passes into the colon where it is fermented and may result in symptoms such as bloating, excess wind, abdominal pain and diarrhoea.  These symptoms are very similar to those of coeliac disease.

Lactose intolerance does not involve a response by the immune system and no antibodies are produced.

Coeliac disease represents a frequent cause of secondary lactose intolerance related to mucosal damage.  A recent study involving people with lactose intolerance found that 24% of subjects had villous atrophy, confirming coeliac disease (1)

Causes of lactose intolerance

Lactose intolerance can be subdivided into three categories:

• Congenital alactasia is a rare condition inherited as an autosomal recessive trait with lifelong complete absence of lactase.
• Primary lactase deficiency is an age-related condition, characterised by a progressive loss of lactase activity leading to a partial absence of lactase.
• Secondary lactose intolerance can follow any gastrointestinal illness where there has been damage to the brush border cells of the small intestine.  Patients newly diagnosed with coeliac disease may suffer with secondary lactose intolerance, due to a deficiency in lactase as a result of gut inflammation.  Lactase is concentrated in the tips of the villi, which are atrophied in coeliac disease. The condition typically resolves when gut healing occurs and lactase is produced in sufficient amounts.  Since the time for the gut to heal is variable it may take a couple of years for lactose production to normalise.

Lactose intolerance should be considered in patients with on-going symptoms known to be adhering to a gluten-free diet. Whilst lactose intolerance is increased in coeliac disease compared to the general population many patients with coeliac disease do not have lactose intolerance because adequate lactase is available in parts of the small bowel that are unaffected.

Diagnosis

The diagnosis of lactose intolerance can be made on the basis of medical history, and is confirmed by the Hydrogen Breath Test.

The hydrogen breath test is non-invasive and has a good sensitivity and specificity (2).

Treatment

Treatment involves avoiding or restricting the intake of lactose.  The degree of lactase deficiency is variable and most individuals can tolerate the lactose in one glass of milk.   It's essential that during the avoidance or restriction of lactose, dietary advice is given to ensure adequate calcium intake from non-dairy sources.  Supplementation with calcium may be required and patients should be assessed and advised on an individual basis.

Lactose reduced milks are now  available. Fermented dairy products, such as yoghurt, buttermilk, and cheese may be tolerated by most patients, because the level of lactose in these products is reduced.

Reading labels

All prepackaged food bought in the UK are covered by EU wide food allergen labelling legislation. Milk is one of the 14 allergens that are covered by this legislation. This means that if milk is used as a deliberate ingredient, it must appear on the ingredients list, regardless of the amount used. An allergy advice box is only a recommendation, and is not compulsory.

References

1 Ojetti V et al. High prevalence of Celiac Disease in Patients with Lactose Intolerance.  Digestion 2005;71: 106-110.

2 Romagnuola J et al. Using breath tests wisely in a gastroenterology practice:  An evidence-based review of indications and pitfalls in interpretation. American Journal of Gastroenterology 2002;98:1113-1126.