Lactose Intolerance

Lactose Intolerance

Like cows’ milk allergy, lactose intolerance is also a food hypersensitivity reaction. However unlike allergy, an intolerance does not involve the immune system and is instead caused by a deficiency or absence of an enzyme e.g. lactase which is essential to break down lactose – the sugar mainly found in milk and dairy products. 1, 2

Causes of Lactose Intolerance

There are a few reasons why an individual may be deficient or have low levels of the enzyme lactase and hence there are different forms of lactose intolerance.

  • Congenital Lactose Intolerance

This is an extremely rare condition and is caused by the absence of the enzyme lactase from birth.1

  • Primary Lactose Intolerance
  1. This is a genetically inherited condition associated with a decline of the lactase enzyme occurring gradually after weaning 4, 5. Primary lactose intolerance is uncommon before 2–3 years of age and usually becomes apparent after 5 years of age. 1,3,4 It is also uncommon in white Northern Europeans, who tend to consume more dairy in their diet, whereas it is quite prevalent in some ethnic groups who historically consume very little dairy. 1,3,4 While this form of lactose intolerance is uncommon in the Irish population it is more common in people of Asian or African-Caribbean descent.
  • Secondary Lactose Intolerance
  1. This is the most common form of lactose intolerance that we see in infants in Ireland. However, this is a temporary condition which may occur after an episode of acute gastroenteritis or persistent diarrhoea. Temporary damage to the border of the gut where the enzyme lactase is produced causes temporary depletion or the absence of lactase. This type of lactose intolerance can occur at any age but is more common in infancy. It is usually temporary and once the underlying problem is resolved, lactose can be reintroduced and is tolerated again. 1,3,4

The normal digestion of lactose involves the breakdown of lactose into glucose and galactose, which are more easily absorbed into the bloodstream. 1,3 In the absence of lactase for the above reasons, the undigested lactose is fermented in the gut causing the undesirable symptoms of lactose intolerance. 1,2,6


Symptoms of Lactose Intolerance

Lactose intolerance causes gastrointestinal symptoms such as diarrhoea, pain, gas, cramping and bloating. Lactose intolerance has some similar symptoms to delayed cows’ milk allergy (CMA), so the two conditions are often mistaken for each other. 1,7  As these are separate conditions requiring different treatments, it is very important to differentiate the diagnosis at this point to ensure the correct management of the condition.  Please see the article on cow’s milk allergy for more information.


Managing Lactose Intolerance

There are currently no official UK/Ireland guidelines on the management of lactose intolerance in infants or adults. Depending on the type of lactose intolerance, management can either involve the reduction or removal of lactose in the diet, while ensuring that optimal nutritional intake is achieved.8

  • Managing congenital lactose intolerance

Lactose must be completely removed from the diet and a lactose-free formula must be used for infants. 3,4

  • Managing primary lactose intolerance

Children may be able to consume small amounts of lactose without experiencing symptoms. In cases where lactose may need to be avoided completely, care should be taken to ensure that their diet is still nutritionally balanced. For formula fed infants less than one year, a lactose free formula may be used. 8

  • Managing secondary lactose intolerance

Opinion on the treatment of secondary lactose intolerance varies. The American Academy of Paediatrics suggests that it “generally does not require elimination of lactose from the diet but, rather, treatment of the underlying condition causing the intolerance’’. 1  Some healthcare professionals recommend total lactose avoidance until lactase production resumes after the underlying cause has resolved. 2



  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. Lomer ME et al. Aliment Pharmacol Ther 2007;27:93-103.
  4. Swallow DM. Annu Rev Genet 2003;37:197-219.
  5. Bahna SL. Ann Allergy Asthma Immunol 2002;89(Suppl):56-60
  6. European Food Safety Authority Panel on Dietetic Products, Nutrition and Allergies. EFSA Journal 2010;8(9):1777.
  7. Bahna SL. Ann Allergy Asthma Immunol 2002;89(Suppl):56-60.