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Lactose intolerance is the inability to digest lactose, a sugar found in milk. It is due to a deficiency of the enzyme lactase. It is very common in adulthood, either due to a genetic deficiency in the production of lactase (primary lactase deficiency) or can be a transient condition that results after a recent episode of gastroenteritis. Lactose intolerance can result in abdominal bloating, pain and diarrhoea 30 minutes to 2 hours after consuming dairy products. An intestinal biopsy can confirm lactase deficiency.

Lactase levels vary from one patient to the next, and vary over time, therefore lactose intolerance is not usually an absolute condition. Since lactose intolerance poses no further threat to a person’s health, the condition is managed simply by minimising the symptoms. Therefore, management involves the avoidance of dairy products, substituting them for plant-based milks (e.g. soy milk, rice milk, or oat milk).

Secondary lactose intolerance is more common. This can occur temporarily after a bout of gastroenteritis, for example, but often improves after several weeks as the lining of the gut heals.

Diagnosis of lactose intolerance

Various methods may be used to diagnose lactose intolerance, including: 

  • hydrogen breath test – this tests the amount of hydrogen that is breathed out. When lactose is fermented by bacteria in the bowel, instead of being converted by lactase, more hydrogen is produced
  • elimination diet – this involves removing foods that contain lactose to see if the symptoms improve. If the symptoms reappear once the foods are reintroduced, then lactose intolerance is most likely the cause.

Another cheap and simple ‘test’ is to compare whether the person can tolerate lactose-free milk rather than ordinary milk.

Lactose intolerance can be also determined by a duodenal biopsy done at the time of your gastroscopy. Results usually take less than one week. Normal lactase levels are greater than 20 U/g. Lactase levels less than 20 and greater than 10 represent mild intolerance.

Genetic testing for suspected lactose intolerance (“lactase persistence gene” for the common European variant of the lactase promoter LCT-13910C>T). is available but is rarely indicated with the above mentioned diagnostic methods usually adequate.

Symptoms of lactose intolerance

Symptoms of lactose intolerance include:

  • abdominal pain
  • abdominal swelling (bloating)
  • flatulence (excessive wind) 
  • diarrhoea.

It is important not to eliminate dairy foods completely from your diet if lactose intolerance is suspected, as dairy foods are rich sources of nutrients. Some dairy products (such as hard and mature cheeses) contain no lactose, and others (such as cream, butter, cottage cheese and ricotta) contain very little. Many people with lactose intolerance can tolerate small amounts of lactose with minimal symptoms. There is no damage caused by having lactose-containing foods when lactose-intolerant, except that it will cause the above mentioned symptoms.