What is Lactose Intolerance?

Lactose intolerance, or intolerance to lactose-containing foods (such as dairy products), affects primarily the gastrointestinal tract, though it can have effects on other systems as a result of nutritional deficits that result from the condition.
Lactose intolerance can be either primary, or secondary to damage to the gastrointestinal tract.


Lactose intoleranceThe prevalence of primary lactose intolerance is estimated to be 7 to 20% for people of Caucasian descent, 65 to 75% for African descent, over 90% in some Asian populations and approximately 70% in Australian Aboriginal populations.
The prevalence of secondary lactose intolerance is difficult to measure.

Risk Factors

The symptoms of lactose intolerance occur when people with a deficiency of lactase ingest foods containing lactose. Lactose intolerance can be divided as primary or secondary, depending on the cause of lactase deficiency.

Primary lactase deficiency can be caused by:

  • Racial or ethnic lactase deficiency, which is the most common (see below).
  • Developmental lactase deficiency, which occurs in premature infants born at 28 to 32 weeks gestation.
  • Congenital lactase deficiency, which is a rare autosomal recessive condition where infants have diarrhoea from birth, and used to be fatal before the introduction of lactose-free formulas. It is most common in the Finnish population.

Racial or ethnic lactase deficiency is a genetically determined reduction of lactase activity and the most common cause of lactose intolerance. In most of the world’s population, a drop in lactase levels occurs at 5 years of age, most prominently in Asian and African populations, and also in Australian Aboriginal populations. In contrast, the majority of the Caucasian population, and in particular in people of Scandinavian background, higher levels of lactase activity persist into adulthood.
In practice, lactase activity is normal in virtually all healthy children of any racial or ethnic group until approximately 5 years of age, and hence lactose intolerance detected in younger children usually indicates an alternative primary cause of lactase deficiency or a secondary cause of lactose intolerance.

Secondary causes of lactose intolerance include:

  • Bacterial overgrowth, which is associated with increased fermentation of lactose in the small bowel.
  • Gastrointestinal infections.
  • Injury to the lining of the gastrointestinal tract, including coeliac disease, inflammatory bowel disease (in particular Crohn’s disease) and drug or radiation induced enteritis.


Lactose intoleranceThe natural history of lactose intolerance varies according to the cause. In the most common form of lactose intolerance, lactase activity falls from around 5 years of age, and symptoms begin to occur after consumption of more than approximately 250ml of milk. The symptoms will vary between people in their severity and the discomfort that they cause. Low levels of lactase are permanent in most populations after this age.
In some secondary causes of lactose intolerance, lactase levels fall transiently. For example, many people find that they become relatively lactose intolerant after gastroenteritis, or a viral tummy infection. This is due to damage to the cells that produce lactase, and once these cells recover and begin to produce lactase again, symptoms of lactose intolerance disappear.
Lactose intolerance is not lethal, and morbidity is low. Sometimes people who are lactose intolerant avoid dairy products to an extent in which their intake of calcium is greatly reduced, and if they do not take calcium supplements or otherwise increase their daily intake of calcium, they may develop osteopenia, or thin bones. Most people with lactose intolerance can tolerate enough milk or other dairy products to get their required daily intake of calcium, but if this is not the case, calcium supplementation is recommended to avoid development of osteopenia, which can lead to osteoporosis (brittle bones).


Characteristic symptoms of lactose intolerance include:

The symptoms of lactose intolerance may closely resemble those of irritable bowel syndrome, and the conditions can be easily confused. Some patients with lactose intolerance also have irritable bowel syndrome, which further adds to the confusion. Some people with irritable bowel syndrome incorrectly believe they are lactose intolerant, which leads to avoidance of dairy products, therefore it is a good idea to consult your doctor before ascribing symptoms to lactose intolerance when they may be due to irritable bowel syndrome.
Infants with lactose intolerance due to developmental or congenital lactase deficiency experience diarrhoea after ingesting milk (including breastmilk) and often fail to thrive or gain weight normally. Premature infants with developmental lactase deficiency may have no symptoms.

How is it Diagnosed

Lactose intoleranceThe most commonly used test to confirm a diagnosis of lactose intolerance is the lactose breath hydrogen test.
A fasting patient (who has eaten no food for the previous 12 hours) is given 25g of lactose. Breath samples are then taken at 30-minute intervals for 3 hours. The amount of hydrogen in the breath is measured, and reflects the degree of lactose intolerance (malabsorption) occurring in the gut.


The prognosis of lactose intolerance is excellent, with many people experiencing improvement of symptoms with dietary restrictions. It is important to ensure adequate calcium intake is maintained, which may require calcium supplementation to prevent osteopenia, or osteoporosis.
For more information, see nutrition and lactose intolerance.


Lactose intoleranceThe treatment of lactose intolerance follows four general principles:

  • Reduced dietary lactose intake;
  • Substitution of an alternative nutrient source to maintain energy and protein intake;
  • Administration of a commercially available enzyme substitute such as Lactaid; and
  • Maintenance of adequate calcium intake, which may require calcium supplementation.

Foods with the highest concentration of lactose by far are milk and icecream, while cheeses and yoghurts generally contain much lower quantities. Most patients find that they can tolerate low levels of lactose in their diet, up to 250ml of milk, which may be enough to maintain calcium intake, however if there is any doubt, calcium supplementation should be considered.
Lactose in milk can be predigested by the addition of a commercially available enzyme substitute, which is refrigerated overnight and leads to digestion of virtually 100% of the lactose. The resultant milk has a sweeter taste than milk containing lactose. It is also important to remember that there may be hidden sources of lactose in other foods (check the label) as well as medicines and other products, which contribute to the daily lactose intake.
In infants with lactose intolerance, lactose free formulas are now available, which are of utmost importance for those with congenital lactase deficiency.
Article kindly reviewed by:
The DAA WA Oncology Interest Group

More information

Dairy For more information on dairy products, including information on nutrition for specific age groups and dairy consumption with certain health conditions, as well as some useful tools, videos, recipes and factsheets, see Dairy.
Nutrition For more information on nutrition, including information on types and composition of food, nutrition and people, conditions related to nutrition, and diets and recipes, as well as some useful videos and tools, see Nutrition.


  1. J. Darnton-Hill, I. Gracey, M. et al. Lactose malabsorption in Australian Aboriginal children. Am J Clin Nutr. 1985; 41: 620-2
  2. Chitkara, D. Montgomery, R. Grand, R. et al. Lactose intolerance. 2005. UpToDate.
  3. Roy, P. Ojeaburu, J. Nwakakwa, V. et al. Lactose intolerance. 3icine. 2003. Available at: http://www.3icine.com/med/topic3429.htm

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