Understanding lactose intolerance

Similar to Cow’s Milk Allergy (CMA), lactose intolerance falls under the umbrella term ‘food hypersensitivity’. However, unlike CMA, lactose intolerance results in an adverse reaction following ingestion of lactose that does not elicit an immune response1.


Lactose intolerance is caused by a deficiency in the enzyme lactase. Located in the villi of the small intestine, lactase digests the disaccharide lactose (the carbohydrate mainly found in human milk and dairy products), breaking it down into the monosaccharides, glucose and galactose, which are then absorbed into the bloodstream. In the absence of lactase, the undigested lactose is fermented in the gut, resulting in the characteristic symptoms of lactose intolerance1,2,3.

  • Congenital lactose intolerance: This form of lactose intolerance is extremely rare, resulting from a deficiency in the enzyme lactase from birth. It presents in the neonatal period with loose stools from the first feed (either breast milk or infant formula contain lactose). There will be failure to gain weight, poor growth and lifelong symptoms if a diet excluding lactose is not followed4.
  • Primary lactose intolerance: This is a genetically inherited condition associated with a decline of the lactase enzyme occurring gradually after weaning. Primary lactose intolerance is uncommon before 2–3 years of age and usually becomes apparent after 5 years of age. It is more common in certain races, e.g. African, Asian, and American Indians. In Ireland, up to 4% of the total population may be affected4.
  • Secondary Lactose Intolerance: This is the most common form of lactose intolerance that we see in infants in Ireland. This is usually a temporary condition which may occur after an episode of acute gastroenteritis or persistent diarrhoea. Temporary damage to the villi of the small intestine causes temporary depletion or absence of lactase. Once the underlying problem is resolved and the gut heals, lactose can usually be reintroduced and is tolerated again as lactase levels return to normal1,5,6.

Signs and symptoms

Symptoms of lactose intolerance may be mild or severe and include:

  • Diarrhoea
  • Angry burnt red skin around the anus and buttocks
  • Very sore bottom
  • Excess wind and bloating
  • Abdominal cramps

Lactose intolerance can elicit similar symptoms to non IgE-mediated (delayed) cow’s milk protein allergy (CMA), and so the two can commonly be mistaken for one another. It is therefore crucial to differentiate the diagnosis to ensure the correct management is advised. 


Depending on the type of lactose intolerance, management can involve either reduction or removal of lactose from the diet, while ensuring that optimal nutritional intake is achieved.

  • Congenital lactose intolerance: Lactose must be completely removed from the diet and a lactose-free formula must be used for infants5,6. If breastfeeding, this should be continued to be supported. Lactase drops may need to be added to expressed breastmilk to aid lactose digestion. Avoidance of lactose from the maternal diet is not recommended.
  • Primary lactose intolerance: Children may be able to tolerate small amounts of lactose without experiencing symptoms. In those who require complete lactose avoidance, ensuring full nutritional requirements is paramount to their care, paying particular to the child’s calcium and vitamin D intake. For infants <1 year of age, a lactose-free formula may be used7.
  • Secondary lactose intolerance: Parents should be reassured by qualified healthcare professionals that consuming lactose will cause undesirable symptoms however, it will not result in damage to the gut (unlike Coeliac Disease for example). In general, lactose avoidance is not required but rather treatment of the underlying cause of the temporary intolerance1. In some cases, where symptoms are affecting a child’s health, feeding and sleeping routines, to the point where growth, development or quality of life is affected, lactose reduction may be carried out for a period of up to four weeks. This can be achieved by transitioning to a lactose free or low-lactose formula. Lactose must be reintroduced by introducing a standard infant formula once the symptoms resolve or within four weeks, whichever occurs first2,4.

Aptamil Lactose Free First Infant Milk is an infant formula suitable from birth for primary and secondary lactose intolerance in bottle-fed infants.

  1. Heyman MB. Pediatrics 2006;118(3):1279-86.
  2. Wright T, Meyer R. Milk and eggs. In: Skypala I, Venter C (eds). Food Hypersensitivity. Oxford: Wiley-Blackwell, 2009. p.117-35.
  3. European Food Safety Authority Panel on Dietetic Products, Nutrition and Allergies. EFSA Journal 2010;8(9):1777. ? updated on new guidelines
  4. Nutrition Support Reference Pack. HSE. 2016; 65-66
  5. Lomer ME et al. Aliment Pharmacol Ther 2007;27:93-103.
  6. Swallow DM. Annu Rev Genet 2003;37:197-219.
  7. Bahna SL. Ann Allergy Asthma Immunol 2002;89(Suppl):56-60.


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