Primary versus Secondary Lactose Intolerance and Lactose Overload in Infants

What is Lactose?

Lactose is a disaccharide of glucose and galactose that is most commonly known as ‘milk’ sugar. It is found in breast milk, cow’s milk and all other mammalian milk types. Lactase is the enzyme secreted by the small intestinal brush border cells that digests lactose to its constituent monosaccharides, ready for absorption.

Primary and Secondary Lactose Intolerance

Lactose intolerance is one of the most common and probably best known food intolerance responses(1) and may be better described as lactose malabsorption.  Ingestion of lactose from milk and its products can cause gastrointestinal symptoms in some people, as it may be poorly absorbed in the small intestine and pass to the large intestine where it may be fermented and mediate symptoms. Lactose intolerance symptoms may be the result of either primary or secondary lactase deficiency(2).

Primary lactase deficiency is the inability to produce sufficient lactase to digest the lactose consumed in milk and milk products. This can occur in post-weaned childhood at various ages and is the most common cause of lactose intolerance. It predominately affects people of Asian and Mediterranean backgrounds. Lactase production has also been shown to decline with age, and as such lactose intolerance becomes more common in the elderly(3).

Secondary lactase deficiency occurs when there is small bowel inflammation, which then impacts on lactase production in the small bowel and subsequent lactose digestion. This can occur in untreated Coeliac disease, Crohn’s disease or in acute episodes of gastroenteritis.

Lactose intolerance in infants and young children is very uncommon and can occur either due to congenital lactose intolerance in very rare cases, or if there is secondary lactose intolerance. If the latter is the case, identifying the underlying cause is extremely important. Once treated, the secondary lactose intolerance will resolve.

Lactose Overload in Infants

If an infant is experiencing gastrointestinal upset including diarrhoea on breast milk, lactose overload provides a much more likely cause. Lactose overload can occur with high breast milk volume intakes and changes in breast milk lipid content. Breast milk is lipid-rich initially, but this lipid fraction reduces further into the feed. With this reduction in lipids, gastrointestinal transit increases, providing less time for lactose digestion. This can lead to lactose malabsorption in the small intestine (with increased lactose and fluid content in the large bowel and stool) progressing to lactose fermentation and gas production in the large bowel and then diarrhoea(4).

Unfortunately, possible lactose overload in infancy is often treated by premature weaning to a lactose free formula, when in fact addressing breast feeding practice and behaviour with advice from a lactation consultant can provide adequate relief and maintain breast feeding.

Due to the familiarity of lactose to the general public, it is always the first food component to be considered when gastrointestinal upset arises. However, there are other common food intolerances which may be involved and an experienced paediatric or gastrointestinal dietitian may help.  

In summary, primary lactose intolerance is rare in infants and young children. Suspected reactions to milk in this demographic should lead to:

  1. consideration of lactose overload in the breast fed infant and follow up with a lactation consultant;
  2. assessment for possible secondary lactose intolerance and management of underlying cause; or
  3. assessment of other food intolerance by an experienced paediatric or gastrointestinal dietitian or healthcare professional and subsequent, minimally restrictive, dietary manipulation for either the child or breastfeeding mother.

Management of Primary Lactose Intolerance

The requirement for a strict dairy free diet is not required for the majority of people with lactose intolerance. Dietary challenge studies have shown that most people with lactose intolerance can manage up to 4 grams of lactose in one sitting without gastrointestinal upset(5). Lactose is also considered to have prebiotic potential(6), so even if lactose is absorbed, its fermentation in the large bowel may offer gastrointestinal benefits.  See Table 1 for the lactose content of common dairy products.

Dairy productTypical serveLactose content (grams)
Milk, full cream/skim250ml (1 glass)16.0
Milk, full cream/skim40ml (cup of tea)2.5
Yoghurt200g (1 small tub)10.0
*reduces daily with natural probiotics in yoghurt
Ricotta cheese60g (3 tb)1.2
Ice cream50g (1 scoop)1.6
Cream cheese20g (1 tb)0.6
French cheese, e.g. brie40g (2 tb)0.04
Cheddar cheese40g (2 slices)0.04
Parmesan cheese40g (2 tb)0.0

*Source: Nutrition Australia


  1. Boyce JA, Assa’ad A, Burks AW, et al. Guidelines for the diagnosis and management of food allergy in the United States: report of the NIAID-sponsored expert panel. J Allergy Clin Immunol. 2010;126(6 Suppl):S1-58.
  2. Heyman MB, for the Committee on Nutrition. Lactose intolerance in infants, children, and adolescents. Pediatrics. 2006;118(3):1279-86.  
  3. Barrett JS, Irving PM, Shepherd SJ, et al. Comparison of the prevalence of fructose and lactose malabsorption across chronic intestinal disorders. Aliment Pharmacol and Therapeutics. 2009;30:165(2)-74.
  4. Douglas PS. Diagnosing gastro-oesophageal reflux disease or lactose intolerance in babies who cry a lot in the first few months overlooks feeding problems. J Paediatr Child Health. 2013;49(4):E252-6.
  5. Vesa TH, Korpela RA, Sahi T. Tolerance to small amounts of lactose in lactose maldigesters. Am J Clin Nutr. 1996;64(2):197-201.
  6. Szilagyi A. Redefining lactose as a conditional prebiotic. Canadian J Gastroenterology. 2004;18(3):163-7.

Article provided by:

Dr Jaci Barrett and Dr Sonja Kukuljan
Accredited Practising Dietitians