Symptoms of lactose intolerance

Lactose intolerance can cause digestive problems, and symptoms usually start to appear 30 minutes to two hours after eating or drinking foods that contain lactose – a carbohydrate that’s present in milk and other dairy products.

It’s important to remember that individuals who don’t break down lactose efficiently don’t all have lactose intolerance. Digestive problems only occur in about one-third of individuals who can’t absorb lactose.

The most common symptoms of lactose intolerance* include:

  • Diarrhoea (watery and acidic)
  • Stomach cramps and pain
  • Bloating
  • Flatulence (wind/gas)
  • Stomach rumbling and gurgling (borborygmi)
  • In young children, lactose intolerance also presents with weight loss and failure to thrive.

Less common symptoms include:

  • Nausea
  • Vomiting
  • Urgency to go to the toilet
  • Pain in the lower belly
  • Constipation (rare)
  • Systemic symptoms, including headache, fatigue, vertigo, loss of concentration, muscle and joint pain, mouth ulcers, urinary difficulties and cardiac arrhythmia. These symptoms occur in less than 20% of lactose-intolerant individuals.

What causes symptoms?

Unabsorbed lactose produces an “osmotic load”, which causes water and electrolytes to move into the intestines through osmosis. This results in diarrhoea. 

Lactose is also used as a substrate for bacteria in the colon. As the bacteria metabolise (ferment) the lactose, they produce short-chain fatty acids and gasses (hydrogen, carbon dioxide and methane), which results in bloating, flatulence, pain and cramping. 

A possible reason for the systemic symptoms is the production of toxic metabolites during the breakdown of lactose in the colon, which can alter cell signalling (the communication that occurs between and within each cell in our bodies).

It’s important to exclude cow’s milk allergy when these symptoms are present. Note that cow’s milk allergy occurs in up to 20% of individuals with lactose intolerance. 

Factors that affect severity

The severity of lactose intolerance symptoms vary greatly from person to person, and depends on certain factors, including:

  • The amount of lactose in the diet.
  • The severity of the lactase deficiency (mild, moderate or severe reductions in the amount of lactase in the intestines).
  • The lactase activity within the intestines.
  • The type of response to the unabsorbed lactase. It’s unclear why people react differently to unabsorbed lactase, but it relates to differences in the intestinal bacteria (i.e. Lactobacillus and Bifidobacteria can produce lactase and help with the breakdown of the lactose).
  • The sensitivity of the person’s colon to lactose malabsoption and/or its metabolites.
  • The form of food in which the lactose is ingested, i.e. milk, yoghurt or cheese.
  • The amount of fat present with the lactose-containing food (the fat can worsen symptoms).
  • The type of foods eaten with the lactose.

Lactose doses of 15-18g are generally well tolerated within a balanced diet. With intakes greater than 18g, intolerance becomes more common. Quantities over 50g will produce digestive symptoms in most individuals.

Congenital lactose intolerance

This refers to a very rare genetic disorder in which, from birth, no lactase is produced by the small intestine. Congenital lactose intolerance presents with intractable diarrhoea (severe, chronic diarrhoea) as soon as human milk or lactose-containing formula is introduced to the baby.

This type of lactose intolerance must be treated quickly, as continued use of lactose-containing milk will result in weight loss, dehydration, electrolyte losses and metabolic acidosis, which can be life threatening. The diarrhoea is treated within hours by substituting lactose-containing milk with lactose-free formula milk. 

Lactose intolerance vs. galactosaemia vs. cow’s milk allergy

Lactose intolerance is sometimes confused with galactosaemia. This is another rare genetic disorder that is characterised by not being able to properly metabolise galactose – another type of sugar found in milk. This is a serious condition in which permanent damage occurs when galactose is consumed. 

Removing lactose from the diet results in the improvement of some of the symptoms, as lactose is broken down into glucose and galactose. But individuals with this disorder need to remove all sources of galactose from their diet.

Lactose intolerance is also often confused with cow’s milk allergy, which involves the immune system reacting to the protein in milk, leading to injury of the intestines. Milk allergy occurs in infants within the first few months of life and typically resolves within the first few years of life. *Note: These symptoms can be caused by several other gastrointestinal conditions. Their presence doesn’t automatically confirm lactose intolerance.

Reviewed by Kim Hofmann, registered dietitian, BSc Medical (Honours) Nutrition and Dietetics, BSc (Honours) Psychology. August 2018.


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