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Southern Cross Medical Library

The purpose of the Southern Cross Medical Library is to provide information of a general nature to help you better understand certain medical conditions. Always seek specific medical advice for treatment appropriate to you. This information is not intended to relate specifically to insurance or healthcare services provided by Southern Cross. For more articles go to the Medical Library index page.

Lactose intolerance

Lactose intolerance is the inability to fully digest lactose, which is a sugar that is found in milk and dairy products. The condition isn’t harmful, but symptoms like flatulence and diarrhoea can cause discomfort and embarrassment. There is no cure but you can manage lactose intolerance by reducing consumption of milk or milk products.

Also known as lactose malabsorption, the condition usually develops in early adulthood.

It is advisable to see a doctor if you experience frequent symptoms after consuming dairy foods and you are concerned about possible deficiencies in your diet.  Substances contained in milk-based products, such as calcium, protein and vitamins A, B12 and D - are important parts of a healthy diet. Calcium is required for the formation and maintenance of healthy bones and a lack of it can lead to osteoporosis  - a condition in which bones become weak and are more likely to break. 


Lactose intolerance results from the inadequate production of lactase by the small intestine. Lactase is an enzyme responsible for the digestion of lactose. Lactase breaks down lactose into two simple sugars, glucose and galactose, which are absorbed from the small intestine into the bloodstream.

However, if there is a deficiency of lactase, not all of the lactose in food or drink is digested and absorbed by the small intestine. The undigested lactose passes into the colon where lactose-fermenting bacteria break it down to produce fluid and gas, which cause the signs and symptoms of lactose intolerance.

Lactase deficiency can be the result of genetic factors (primary lactose intolerance) or triggered by illness, injury or surgery to the small intestine (secondary lactose intolerance).  Some antibiotics or tummy bugs may cause temporary lactose intolerance.

Risk factors

The following factors can make someone more prone to lactose intolerance:

  • Older age: Lactose intolerance usually develops in adulthood
  • Ethnicity: including Asian people and (according to some studies) Maori and Pacifica people
  • Premature birth: Because their small intestine hasn’t yet developed lactase-producing cells, infants born prematurely may have reduced lactase levels
  • Diseases that affect the small intestine: For example, bacterial overgrowth, coeliac disease, and inflammatory bowel conditions like Crohn's disease and ulcerative colitis 
  • Certain cancer treatments: Radiation therapy for abdominal cancer and chemotherapy-related intestinal complications. 

Signs and symptoms

The inability to digest lactose produces the following signs and symptoms:

  • Flatulence (farting) 
  • Diarrhoea 
  • Bloated stomach
  • Abdominal cramps and pains
  • Growling or rumbling stomach 
  • Feeling sick (i.e. nausea), and sometimes vomiting.

Symptoms are usually experienced within a few hours of consuming foods or drinks that contain lactose.

The severity of the symptoms and how soon they develop depends on the quantity of lactose consumed. It may also depend on the person. Different people can tolerate different amounts of lactose before they experience symptoms. 


A diagnosis of lactose intolerance is suspected based on a person’s symptoms, eating habits, and their response to reducing the amount of dairy foods and drinks in their diet. Laboratory tests may be used to assist diagnosis:

  • Breath hydrogen test: a quantity of lactose is consumed and then the amount of hydrogen in the breath is measured at regular intervals. If the body doesn't digest the lactose, it will ferment in the colon and release hydrogen, which is absorbed by the intestines and eventually exhaled. Higher than normal amounts of exhaled hydrogen indicate that the lactose is not being fully digested.
  • Lactose tolerance test: A blood test is done two hours after the lactose is consumed to measure the level of glucose in the bloodstream. The absence of a rise in the blood-glucose level indicates that the body isn't properly digesting and absorbing lactose.
  • Stool acidity test: This test is used in infants and children unable to undergo the hydrogen breath test or lactose intolerance test. Fermentation of undigested lactose creates lactic acid and other acids that can be detected in a stool sample.

Other tests may be done to exclude other gastrointestinal conditions, such as irritable bowel syndrome and coeliac disease, which can cause symptoms similar to those of lactose intolerance.

Lactose intolerance is sometimes confused with milk allergy, which is an immune system disorder. Milk allergy is a reaction to proteins in milk and dairy products, ranging from hives, upset stomach and vomiting to life-threatening anaphylactic shock. While lactose intolerance usually appears in later life, milk allergy often appears in the first year of life.


If your lactose intolerance is caused by an underlying health issue (ie secondary lactose intolerance) treatment of that underlying issue may restore lactase levels and reduce symptoms.  Otherwise, there is no treatment that directly boosts the body’s production of lactase so your focus will be on changes that will help you minimise or avoid signs and symptoms:

  • Avoid large servings of milk and other dairy products
  • Avoid foods containing milk solids
  • Include small servings of dairy products with regular meals
  • Consume lactose-free and lactose-reduced dairy products
  • Take lactase tablets when consuming milk or other dairy products
  • Before consuming milk or dairy products, add lactase liquid or powder, which will break down the lactose present in the milk.

Restricting dairy products from your diet may mean not getting enough calcium, which is important for bone health. Non-dairy sources of calcium include:

  • Broccoli and leafy green vegetables
  • Fish with soft bones, e.g. canned salmon or sardines
  • Calcium-fortified products, e.g. soy milk, rice milk, some cereals
  • Oranges
  • Pinto beans
  • Tofu
  • Almonds and Brazil nuts
  • Rhubarb
  • Spinach
  • Calcium supplement products.

It is also important to get sufficient vitamin D, which aids the absorption of dietary calcium. Vitamin D is naturally present in eggs and salmon and is added to some types of milk, yogurt, cereals, and orange juice. Being outside in the sunlight helps the body to make vitamin D. 


With some experimentation it may be possible to identify the dairy products that cause symptoms and to get a feeling for how much of that product can be consumed without producing symptoms. Few people have such severe lactose intolerance that they have to exclude all dairy from their diet.

The following dietary changes can help to minimise symptoms of lactose intolerance:

  • Consume smaller servings of milk or other dairy foods
  • Consume lactose-reduced or lactose-free dairy products
  • Drink milk with meals or other foods, which will slow the digestive process
  • Try different dairy products as not all dairy products contain the same amount of lactose. For example, hard cheeses, such as Swiss or cheddar, have small amounts of lactose. Cultured milk products, such as yogurt, contain bacteria used in the culturing process that naturally produce lactase
  • Use over-the-counter lactase enzyme tablets or drops prior to consuming dairy foods or drinks, which may aid their digestion
  • Take probiotics, which are products containing living microorganisms normally present in your intestines that help maintain a healthy digestive system. They are sometimes used for gastrointestinal conditions, such as diarrhoea and irritable bowel syndrome, but may also help the body to digest lactose.

Milk and milk products may be present in non-dairy foods, including: 

  • Bread and other baked goods, e.g. pancakes, biscuits, and cakes
  • Processed foods, e.g. breakfast cereals, instant potatoes, soups, margarine, salad dressings, and flavoured chips and other snack foods
  • Processed meats, e.g. bacon, sausage, hot dogs, and lunch meats
  • Milk-based meal replacement liquids and powders, smoothies, and protein powders and bars
  • Liquid and powdered coffee creamers and whipped toppings.
The ingredient list on package foods can be checked to see if the product contains lactose. Look for words such as milk, whey, milk by-products, milk solids, and milk powder.

People who cannot tolerate even small amounts of lactose should be aware that it can be present in some prescription and over-the-counter medicines.


Mayo Clinic (2018). Lactose intolerance (Web Page). Rochester, NY: Mayo Foundation for Medical Education and Research. [Accessed: 25/02/19]
NIDDK (2018). Lactose intolerance (Web Page). Bethesda MD: National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). [Accessed: 25/02/19]
NHS (2016). Lactose intolerance (Web Page). Redditch: National Health Service (NHS) 
England. [Accessed: 25/02/19]
O’Toole, M.T. (Ed.) (2013). Lactase deficiency. Mosby’s Dictionary of Medicine, Nursing & Health Professions (9th ed.). St Louis, MI: Elsevier Mosby.
O’Toole, M.T. (Ed.) (2013). Lactose intolerance. Mosby’s Dictionary of Medicine, Nursing & Health Professions (9th ed.). St Louis, MI: Elsevier Mosby.

Created: March 2019
Go to our Medical Library Index Page to find information on other medical conditions.