What is lactose intolerance?

Lactose is a natural sugar found in the milk of mammals including cows, sheep, goats and humans. Lactose intolerance is a condition in which your body cannot properly digest lactose. This occurs in people who do not have enough of the enzyme lactase, which breaks down lactose into its easily digested and absorbed components, glucose and galactose. Lactose intolerance can cause abdominal pain, increased flatulence and sometimes diarrhea.


A simple sugar found in many foods (such as fruit) that functions as a major energy source for the body.


Lactose intolerance occurs when you are unable to digest lactose. To break down lactose, your body needs lactase, an enzyme normally found in your stomach. If your body does not produce enough lactase, not all ingested lactose will be broken down. This prompts bacteria in your body to try to break down the excess lactose using a fermentation process. This fermentation process can cause increased bloating and wind. The excess lactose load passes into the large bowel and water is leaked into the bowel which, if excessive, can lead to diarrhea.

In some people, lactose intolerance can be a result of ageing. This occurs as our diet becomes less reliant on milk, meaning less lactase enzyme is needed and produced. An illness or intestinal disease can also cause lactose intolerance, such as gastroenteritis, Crohn's disease and coeliac disease. If these conditions resolve or are effectively treated, your gut will usually resume production of lactase, but this can sometimes take many months.

Babies may suffer from a mild lactose intolerance in their early months, which generally resolves on its own and is not known to cause any long-term harm. In rare cases, some babies are born with a severe genetic lactose intolerance that can result in severe diarrhea from birth. This occurs when both the mother and father pass on a defective form of the lactase gene. A lactose-free formula is required for this lactase deficiency, as both breastmilk and standard formulas contain lactose.

A selection of dairy foods including milk, cheese and yoghurt.A selection of dairy foods that contain different amounts of lactose. 


The process of converting sugars into acids, gases and/or alcohol, usually by yeast and bacteria.

Risk factors

Some risk factors associated with lactose intolerance include:

  • A diet high in lactose;
  • Increasing age;
  • Ethnicity - it is common among Asian, African and Southern European people, though less common in Caucasian people, and;
  • Premature birth - lactase is produced during the third trimester of pregnancy.

Signs and symptoms

The signs and symptoms of lactose intolerance include:

  • Bloating;
  • Abdominal pain;
  • Flatulence (wind), and;
  • Diarrhea.

A person experiencing abdominal pain.Lactose intolerance can cause abdominal pain and bloating. 

Methods for diagnosis

If you are experiencing signs and symptoms of lactose intolerance, your doctor may recommend an elimination diet test. This involves stopping certain foods containing lactose to see if your symptoms are reduced. Other more specific diagnostic tests include:

Hydrogen breath test

This test involves you drinking a liquid that is high in lactose. Breath tests are then taken over time for the presence of hydrogen gas. An increase in the amount of hydrogen gas can mean that lactose is not being broken down by lactase, but instead is being fermented by bacteria to produce hydrogen gas.

Lactose tolerance test

This test involves you drinking a liquid that is high in lactose. Blood tests are then taken at intervals to measure for an increase in glucose level. If your blood glucose level does not increase, it means lactose is not being broken down by lactase and you are probably lactose intolerant.

Stool acidity test

This tests for the presence of lactic acid in your feces (stool), a sign of undigested lactose.


A simple sugar found in many foods (such as fruit) that functions as a major energy source for the body.

Types of treatment

Currently there is no permanent cure for lactose intolerance. To help reduce your symptoms, your doctor or dietitian may recommend reducing intake of lactose-containing foods (typically dairy products). Every person's tolerance level is different, so it is likely you may still be able to have a small amount of lactose in your diet without any symptoms appearing. Some dairy foods such as hard cheeses contain little lactose and may be well tolerated. Having small amount of lactose-containing products spread out over the day rather than in one large portion can improve lactose digestion. There are also many reduced-lactose dairy products available.

Potential complications

Lactose helps your body absorb important minerals, including magnesium and zinc. These are key minerals that help your body form strong, healthy bones. If you are suffering from lactose intolerance, you have an increased risk of developing osteoporosis.

Eliminating or reducing dairy products to manage lactose intolerance can reduce overall calcium intake, which may cause you to have too little calcium in your diet. Inadequate calcium intake can also lead to osteoporosis. There are, however, many foods other than dairy that contain calcium and your doctor or dietitian may recommend these as an alternative source of calcium. Calcium supplements may also be recommended if dietary intake is thought to be inadequate. Your doctor may also recommend you take a vitamin D supplement, as this can also be low in people suffering with lactose intolerance.

If you have a lactose intolerance, it is important to note that ingestion of lactose is not known to cause damage to the bowel; rather, it causes the uncomfortable symptoms as previously mentioned.

Vitamin D

A vitamin that is important for the health of bones and teeth as it promotes absorption of calcium from the diet.


Symptoms of lactose intolerance can usually be effectively managed with dietary measures. Long-term problems are less likely if your diet and nutrient intake is adequately managed. If you are diagnosed with lactose intolerance, your doctor or dietitian may recommend you reduce the amount of lactose in your diet to lower your signs and symptoms. This might involve a reduction in cow's milk and other products containing lactose. There are alternatives available including low-lactose dairy products, soy products and rice milk.


To help prevent lactose intolerance, it is important to identify which foods and drinks are causing your symptoms and reduce your intake. It is a good idea to seek advice from your doctor or dietitian about limiting foods and replacing relevant vitamins and minerals, according to your individual needs.

FAQ Frequently asked questions