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Lactose intolerance

Key points

  • Lactose intolerance is the most commonly diagnosed adverse reaction to cow's milk among adolescents and adults.
  • Symptoms include flatulence, bloating, diarrhoea and abdominal pain.
  • Many individuals with lactose intolerance can still tolerate cheese and yoghurt as well as about 200ml milk if consumed with a meal.

Introduction

Lactose or milk sugar is a disaccharide consisting of glucose and galactose. It is found only in the milk of mammals and is the main carbohydrate found in milk and other dairy products. Lactose intolerance is the most commonly diagnosed adverse reaction to cow’s milk among adolescents and adults. The main symptoms of lactose intolerance include flatulence, bloating, diarrhoea and abdominal pain. The symptoms are caused by undigested lactose passing from the small intestine into the colon. In the colon the bacteria normally present ferment unabsorbed lactose producing short chain fatty acids and gases (carbon dioxide, hydrogen and methane). Gas production may result in flatulence, bloating and distension pain. Unabsorbed lactose also has an osmotic effect in the gastrointestinal tract, drawing fluid into the lumen and causing diarrhoea.

The terminology used in describing lactose intolerance is defined below:

Lactose is the main carbohydrate in milk products, and comprises a disaccharide consisting of glucose and galactose.

Lactase is an enzyme located in the small intestine that hydrolyses lactose to its components: glucose and galactose.

Lactase deficiency or Lactase non-persistence is a decreased activity of lactase in the small intestine.

Lactose maldigestion occurs as a result of lactase deficiency or non-persistence. Lactose cannot be fully hydrolysed and absorbed into the portal circulation from the small intestine but passes into the colon.

Lactose intolerance comprises adverse gastrointestinal symptoms caused by lactose maldigestion.



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