Lactose Intolerance

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This article was published as a CPD eArtcle for NHD Magazine in March 2017 (see link to the published version at the end of this post).

Lactose is a disaccharide sugar mainly found in dairy products such as milk, cheese and yoghurts. Lactose intolerance can cause uncomfortable and sometimes severe symptoms. This article explores the background, diagnosis and treatment options for the different types of lactose intolerance which can present.

Lactose intolerance is a deficiency of the enzyme lactase; which is produced by cells in the lining of the small intestine in order to break down lactose into the monosaccharides glucose and galactose, which can be readily absorbed into the bloodstream1. When there is a deficiency of lactase present, lactose can build up in the digestive system where it becomes fermented by bacteria and results in the production of various gases, which cause the symptoms associated with lactose intolerance, such as: explosive diarrhoea, bloating, abdominal pains, cramps, flatulence and nausea. Symptoms usually get more severe with larger volumes of lactose and, depending on the initial cause of the intolerance, this can either be temporary or permanent2-3.



Types of Lactose Intolerance

1. Congenital lactase deficiency or hereditary alactasia

This is a rare disorder caused by a mutation in the LCT gene which results in the absence of lactase and hence the inability of affected infants to break down the lactose present in breastmilk or formula, this can lead to severe diarrhoea and subsequent dehydration and weight loss if lactose is not excluded1,4.


2. Primary lactase deficiency

This usually develops after infancy and tends to present between the ages of 5 – 20 years old3. Primary lactase deficiency occurs due to the gradual decrease in the expression of the LCT gene which causes a reduced production of lactase, although roughly 5 – 30% of the original lactase activity usually remains3-4. This process occurs in approximately 70% of all humans throughout their life, resulting in a decreased ability to digest lactose with age1-3. With primary lactase deficiency symptoms of intolerance usually occur within 30 minutes to two hours of ingesting lactose and the severity of symptoms is related to the dose of lactose which is consumed1-2,5-6. The prevalence of primary lactase deficiency varies widely depending on geographical location and is highest in countries where dairy products are not traditionally consumed; such as Asian and African populations where the prevalence can range from 80-100%, compared to a much lower prevalence in Northern Europe (for example 2% in Scandinavia, 5% in the UK, Ireland, Holland and Belgium), however can be as high as 70% in other European areas such as Sicily3, 5-6.


3. Secondary lactase deficiency

This can occur at any age but most commonly presents in infants and young children. This occurs as a result of damage caused to the lining of the small intestine which interferes with it’s ability to produce lactase, which may occur for numerous reasons such as: gastroenteritis, bowel surgery, Crohn’s disease, ulcerative colitis, coeliac disease, undiagnosed cow’s milk protein intolerance, chemotherapy or prolonged used of antibiotics2-3. Secondary lactase deficiency is often temporary and may resolve in 2-4 weeks, but this can also be long-term if caused by a chronic condition2,5.


4. Developmental lactase deficiency

This can occur in premature babies (i.e. those born before the 37th week of pregnancy) due to an underdeveloped small intestine; this is usually temporary and tends to resolve as the infant grows2.


Diagnosis of Lactose Intolerance

The main route of diagnosing lactose intolerance involves trialling a lactose free diet for 2-3 weeks and monitoring whether symptoms improve, then reintroducing lactose into the diet to see whether the symptoms return5.

Other methods of diagnosing lactose intolerance include6-7:

  • Hydrogen breath test: this involves ingesting a lactose solution and measuring the subsequent amount of hydrogen produced by colonic bacteria; as this increases in those with lactose intolerance.
  • Lactose tolerance test: where a blood test is taken after ingesting a lactose solution.
  • Milk tolerance test: where blood sugar levels are checked after drinking a glass of milk; as no increase in blood glucose indicates that lactose has not absorbed in the small intestine.
  • Intestinal biopsy: this can be used to test the amount of lactase produced by the intestinal lining, however this is rarely conducted solely to test for lactose intolerance but may be carried out to test for coeliac disease (which can produce similar symptoms to lactose intolerance).

Lactose intolerance can sometimes be confused with cow’s milk protein intolerance in infants and children;  which is an immune response to the protein in cow’s milk rather than an inability to digest lactose adequately8; therefore it is important to take a detailed history to ensure allergenic symptoms are not misdiagnosed as a lactose intolerance. Similarly, it is important to exclude other conditions which can present with similar symptoms to lactose intolerance such as: irritable bowel syndrome, coeliac disease and bowel cancer6-7.


Treatment of Lactose Intolerance

Congenital lactase deficiency is treated by replacing breastmilk and ordinary infant formula with a lactose free formula, then as the infant grows a lactose free weaning diet followed by lifelong lactose exclusion is indicated3,5; dietetic support is often advisable to ensure nutritional adequacy on this exclusion diet.

As those with primary lactase deficiency maintain some degree of lactase activity, total avoidance of lactose is usually unnecessary with this condition and including some lactose in the diet may actually improve lactose tolerance by interacting with intestinal flora; however the long term restriction of lactose intake is usually needed3. Tolerance levels vary, but even those with a low level of lactase activity can usually consume roughly 12-15g of lactose (i.e. about 250ml of milk) and others may be able to tolerate up to twice this amount3. Products made using fermentation processes such as cheese and yoghurt (especially live yoghurt) are often better tolerated than fresh milk due to the presence of beneficial bacteria, hard cheeses are also generally well tolerated as they contain a very low level of lactose and it is thought that lactose is best tolerated when consumed as part of a meal due to the effect of gut transit time1.3.

The treatment of secondary lactase deficiency includes excluding lactose for 2-4 weeks while the symptoms of intolerance continue, with the gradual reintroduction of lactose containing foods as tolerance returns3. Infants may require a lactose free formula (or a soy formula if over 6 months old) during this period, breast milk can often still be tolerated or in some cases lactase drops may be prescribed to help breastfed babies to absorb the lactose from breastmilk3-4.

Those with a severe lactose intolerance they may need to check the labels of food, drinks and medication (lactose can be used as a bulking agent in medication) to see whether they contain lactose; EU food labeling law currently mandates that milk needs to be clearly highlighted when present in a product and written or verbal information needs to be available for food sold without labeling such as in restaurants, delis and cafes8.

It is important to ensure nutritional adequacy within a lactose reduced diet; especially in terms of calcium, protein, vitamin and mineral intake5,8. Lactose free options are widely available from supermarkets, online shops and health food shops; as outlined below in (table adapted from BDA milk allergy food fact sheet8).


Food Lactose-free Alternative
Milks Lactose-free milk, oat milk, soya milk, flaxseed milk, sesame milk, rice milk (although not advised for those under 5 years old as can contain traces of arsenic), pea milk, coconut milk, quinoa milk, hemp milk, potato milk and nut milks such as almond or hazelnut milk.
Spreads Lactose-free and dairy-free spreads.
Cheese Lactose-free cheese, hard/soft/melting varieties of milk free cheeses based on soya, pea, cashew, almond or rice protein. Also, some cheeses are naturally low in lactose such as: Edam, Cheddar, Emmental and Parmesan.
Yoghurts, custards, desserts Lactose-free, soya, pea, coconut and almond varieties.
Ice creams Lactose-free, soya, rice, coconut, almond and cashew varieties.
Creams Lactose-free, soya, oat rice, coconut and almond varieties.


There is another type of milk available which is called A2 milk, this milk excludes the protein A1 beta-casein and only contains the A2 beta-casein (both are present in normal cow’s milk). There has been some reported health benefits related to avoiding A1 beta-casein, and although A2 milk contains the same level of lactose as ordinary milk, a study by Jianquin et al (2016)9 found that replacing ordinary milk for A2 milk improved some symptoms of lactose intolerance; however more large scale evidence is needed to evaluate these claims10.  



Accurate diagnosis and identifying the correct type of lactose intolerance is vital to avoid unnecessary dietary elimination and also in order to provide the most appropriate dietary advice. For example, those with secondary lactase deficiency usually only need temporary lactose exclusion whereas those with primary lactase deficiency generally need long term lactose reduction, but not total avoidance. There are numerous lactose free products available which can support in the nutritional management of those with lactose intolerance in order to avoid any unnecessary nutritional deficiencies.


Click here for the published version of this eArticle which includes supplementary CPD questions.



  1. NIH (2017) Lactose Intolerance (
  2. NHS Choices: Lactose Intolerance, accessed January 2017 (
  3. Bryony & Thomas (2007) The Manual of Dietetic Practice 4th Edition.
  4. NHS Choices: Lactose Intolerance – Treatment, accessed January 2017 (
  5. Shaw & Lawson (2014) Clinical Paediatric Dietetics 4th edition.
  6. The British Nutrition Foundation: Lactose Intolerance, accessed January 2017 (
  7. NHS Choices: Lactose Intolerance – Diagnosis, accessed January 2017 (
  8. BDA (2014) Food fact sheet: milk allergy (
  9. Jianqin et al. (2016) “Effects of milk containing only A2 beta casein versus milk containing both A1 and A2 beta casein proteins on gastrointestinal physiology, symptoms of discomfort, and cognitive behavior of people with self-reported intolerance to traditional cows’ milk” (
  10. EFSA (2009) “Review of the potential health impact of β-casomorphins and related peptides” (



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