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Lactose intolerance

People with lactose intolerance have difficulty digesting the sugar lactose, which is found naturally in milk. This intolerance is due to an absence or reduction in the amount of the enzyme lactase (lactase-phlorizin hydrolase (LPH)).

Symptoms typically include gastrointestinal reactions such as diarrhoea, flatulence, stomach bloating and cramps, but can also include systemic symptoms like headache and loss of concentration following the consumption of food or drink containing lactose. As lactose intolerance doesn't involve the immune system, it's not classed as an allergy (1,2,3,4,5).

If you find this particularly interesting and would like to learn more, you may enjoy reading Issue 7 of ‘The Oatly Way,’ which has a special feature on lactose intolerance (here)

Different types of lactose intolerance (1,2,5):

  • Primary lactase deficiency: This is what is typically known as 'lactose intolerance.' It's the most common type of lactose intolerance and is caused by a deficiency of the enzyme lactase. It affects more than 70% of the world’s population and its incidence varies with ethnicity. It's common in regions where people traditionally consume little milk after weaning, such as Southeast Asia. Symptoms typically present in late adolescence and adulthood. Sufferers can often tolerate some lactose, due to the fact that they have retained some lactase enzyme activity.
  • Secondary lactase deficiency: This condition, which can present at any age, is usually temporary and results from damage to the mucosa of the small bowel, for example following gastroenteritis, chemo or radiotherapy. Once the cause of the damage is resolved or treated, lactose tolerance should resume.
  • Developmental lactase deficiency: This rare condition is observed among preterm infants of less than 34 weeks’ gestation and typically resolves as the gut matures.
  • Congenital lactase deficiency: In the past, infants with this extremely rare autosomal recessive condition would not have survived. Fortunately, today their prospects are favourable following rapid diagnosis and treatment with lactose-free formula. For this type of lactose intolerance, complete and permanent lactose exclusion is necessary.

In normal digestion, lactase in the small intestine breaks down lactose into glucose and galactose. However, in a person with lactose intolerance, the imbalance between the amount of lactose consumed and the capacity to produce lactase (to hydrolyse the lactose) causes distinct symptoms:

  • The presence of the undigested lactose in the intestines carries an osmotic load, drawing in water and electrolytes, increasing transit time and causing loose stools and diarrhoea.
  • Undigested lactose that passes into the colon is fermented by bacteria, which produce fatty acids and gases such as carbon dioxide, hydrogen and methane. These fatty acids and gases cause the well-documented symptoms of flatulence and bloating. When gas production is sufficient to cause stimulation of the intestinal nervous system, abdominal cramps also result (5).

In addition to clinical history, a diagnosis can be made using various different tests that vary both in reliability and in how invasive they are. They include a hydrogen breath test, a lactose tolerance test, stool testing, intestinal biopsy, genetic testing or simple clinical judgement following a period of complete lactose elimination where symptoms resolve and then return upon the reintroduction of dairy foods (1,2,5).

Restriction of lactose can be achieved with lactose-free dairy foods, alternatives to dairy such as Oatly and the addition of a lactase enzyme supplement to products that normally contain lactose (5).

People with lactose intolerance vary as to how much lactose they can tolerate. In some people, a dash of milk in a cup of tea or coffee will cause symptoms, while others can tolerate considerably more lactose. Some dairy products have lower lactose than others; for example, some yogurts and cheeses may be tolerated well by people with lactose intolerance. People should be encouraged to determine their own level of tolerance so they can include dairy up to that level.

Whether restriction is temporary or permanent, the advice given by the healthcare professional should include information on a lactose-restricted diet while also ensuring nutritional adequacy (1,5).



1. Heyman MB (2006). Lactose Intolerance in Infants, Children and Adolescents. Pediatrics;118:1279-1286.

2. Luyt D, et al (2014). BSACI guideline for the diagnosis and management of cow’s milk allergy. Clinical & Experimental Allergy;44:642–672.

3. Matthews SB, et al (2005). Systemic lactose intolerance: a new perspective on an old problem. Post Grad. Med. J.;81: 167-173.

4. NHS choices (2014). Accessed in August 2018.

5. Stanton H (2014). Lactose Intolerance in Infants and Children. Complete Nutrition;14(3):59-62.

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