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Lactose intolerance

What is it?

Lactose Intolerance is a condition where the body cannot effectively digest lactose. As such it is different from an allergy, which occurs when the body has an immunological response to a protein component found in the food. Milk allergy is different from lactose intolerance; it is not possible to have a lactose ‘allergy’ as lactose is not a protein. Lactose is the sugar found in all milks; breast, mammalian (cow, goat etc) and formula, and is a major carbohydrate source for infants and toddlers.

Lactose is a ‘disaccharide’ (two sugar unit), made up of two ‘monosaccharides’ (one sugar units) called glucose and galactose. Lactose needs to be broken down into these two ‘monosaccharides’ in order to be properly absorbed and digested by the body. The enzyme responsible for this is called Lactase.   If this is not there at all, or not there in big enough amounts to breakdown all of the lactose, this carbohydrate cannot be used by the body and stays in the small intestine unabsorbed. It travels through the intestine - its presence here causing water to be drawn into the intestine. Additionally, bacteria naturally present in the body ferment this source of sugar, and it is these which produce the symptoms associated with lactose intolerance.


Can vary but include; diarrhoea, bloating, trapped wind, flatulence, vomiting. In an infant can include being unsettled, crying, frequent nappy changes with loose/watery stools (may be green/yellow in colour) and audible bowel sounds and can occur within a few hours of lactose consumption. The ‘general’ nature of these symptoms can make it hard for lactose intolerance to be easily detected.


There are three different ‘types’ of lactose intolerance; hereditary, primary and secondary. A brief description is shown below;

•    Hereditary lactose intolerance: Infant is born with a complete lack of the enzyme lactase. Symptoms are severe and usually occur soon after birth following first milk feeds. A lactose-free substitute has to be used and lactose will need to be avoided for life.
•    Primary lactose intolerance: A genetic condition whereby production of the lactase enzyme declines over time, causing deficiency. Small amounts of lactose may be able to be consumed up to an individual’s tolerance level – but calcium supplementation and low-lactose products may be required long-term. Usually seen in older children and adults.  
•    Secondary lactose intolerance: This is usually a temporary condition whereby the lactase enzyme production is affected – usually due to damage of the intestinal border by illness or surgery. It can be quite common after cases of gastroenteritis in young infants but usually improves after 3-4 weeks.


In infants lactose intolerance is likely to be either hereditary, or secondary. Diagnosing lactose intolerance involves taking a clinical history, growth monitoring and noting symptoms. Usually intolerance is diagnosed by a lack of symptoms following exclusion of the suspected foodstuff – however this may be difficult in milk-avoidance in very young infants. A stool acidity test can be conducted in these instances – if the pH of an infants stool sample is below 5.5 it indicates that acid is being produced by bacterial fermentation of sugar; a sign of lactose intolerance.

Management / Feeding options

A diagnosis of hereditary lactose intolerance will require a lactose-free diet for life, with supplementation of calcium, magnesium and zinc as required to prevent deficiency. A lactose-free formula will be required in infancy, with a lactose-free weaning diet and avoidance for life. This can be an anxious time for parents, however as removal of lactose completely relieves symptoms the initial difficulty of adapting to the diet is often offset by the change in health of their baby.  

Primary lactose intolerance is rare in infants and young children – however if diagnosed then initial avoidance of lactose is usually required to relieve symptoms – followed by eventual introduction of gradually increased quantities of lactose-containing foods such as yoghurts, cheese etc to establish an individual tolerance level. If only small amounts can be tolerated long-term then consideration needs to be given to calcium, zinc and magnesium status to prevent potential deficiency.  

As secondary lactose intolerance is temporary no dietary changes are usually recommended and adherence to usual milk/diet advised. If the effects are extreme, or there has been little symptom relief over 3-4 weeks then a lactose-free diet may be recommended; however nutritional status is usually not affected.  


Thomas, B., Bishop, J. (2007) Manual of Dietetic Practice, Blackwell Publishing, Fourth Edition, pages 458-459.

CORE charity website information, available at: [Accessed 22/09/2011]

Taylor, C. (2006) Lactose Intolerance in infants, Nursing Practice, Vol 102 (17) page 43. Available online at: [Accessed 22/09/2011]