Lactose Intolerance


Lactose intolerance (LI) is the inability to completely digest lactose, the predominant sugar found in milk. If lactose is not absorbed properly, it ferments and this results in bloating, cramps, gas, flatulence, and diarrhoea amongst others. Lactose intolerance affects more than half of the world population. It may also be the cause of baby colic. According to various studies, lactose intolerance accounts to more than half of wrongly diagnosed patients with irritable bowel syndrome (IBS). Lactose intolerance is the most common form of food intolerance; others include carbohydrate, fructose, and gluten. Lactose Intolerance can also be a side effect of gluten Intolerance. Lactose intolerance must not be confused with milk allergy.

What causes lactose intolerance?

Milk contains lactose, a disaccharide made of two sugars joined together. Our body is not able to absorb lactose unless it is broken down into two sugars. So to get around this lactase present in the lining of the small intestine splits lactose so it could be absorbed. The figure on the right shows lactose (top) broken down into two sugars by means of the lactase (centre) into glucose and galactose (bottom) two absorbable sugars. If there is insufficient lactase, or nothing at all, lactose remains in the digestive system. This is fermented by the bacteria living in the large intestine resulting in excess gas, stomach rumbling and bloating. Severe intolerance leads to diarrhoea and other symptoms.

Who can get lactose intolerance?

Lactase activity is normally high in babies, although baby colic may be caused by temporarily lactase deficiency (see baby colic). After the ages of 5-6 lactase production starts decreasing in many individuals but the rate varies from one person to another and also between races. From the table below one can conclude that the majority of people in the world are lactose intolerant.

Lactose Intolerance Around the World
Human Group
% Lactose Intolerance
Caucasians 10-20%
Mediterranean 40-50%
Oriental 90%
American Blacks 75%
African 50%
Aborigines 85%

However there are other people irrelevant of what race or age they are that may develop lactose intolerance:

  • Coeliac disease: this condition causes damage to the lining of the small intestine hence lactase is not produced
  • Small intestine surgery: may become unable to produce lactase anymore
  • Antibiotic treatment: may cause diarrhoea which damages lining of the small intestine temporarily reducing lactase production. Treatment can be accelerated with probiotics containing life bacteria.


When lactose is not digested, it remains in the intestines exerting osmotic effects. Osmotic pressure draws fluids and salts into the gut which moves rapidly towards the large intestines. This increased level of fluid and salts in the colon helps the bacteria to ferment lactose into short-chain fatty acids (SCFA), Carbon Dioxide (CO2), Methane (CH4), Hydrogen (H2) and Hydrogen Sulphide (H2S), the odour associated with flatulence. Hydrogen is one of the gases that cause bloating but due to its small size it escapes through the walls of the intestine into the bloodstream and is expelled through the lungs.

The fermentation of lactose leads to the typical symptoms associated with lactose intolerance. The classical symptoms are abdominal bloating, cramping pain and flatulence. Diarrhoea is a common symptom as well but varies from individual to others, depending on how much lactose they can tolerate and how much lactose they have consumed. Other characteristic symptoms include nausea, gaseousness, rumbling and in some cases, especially in children, vomiting. I believe that the nausea and vomiting are caused by the escaped gas from the fermentation back into the small intestines and stomach.

Symptoms associated with lactose intolerance may be mild or severe depending on the degree of lactase deficiency and the amount of lactose sugar consumed.


  • Abdominal pain - cramps, often localised to the per-umbilical area or lower quadrant
  • Bloating - caused by the formation of gases that expand in the warm conditions of the digestive system
  • Flatulence - caused by the gases produced by the bacteria. H2S is the gas associated with the odour
  • Borborygmi - may be audible to the patient and on physical examination
  • Diarrhoea - stools are usually bulky, frothy and watery
  • Vomiting - particularly adolescents
  • Nausea - may be present, probably due to acidity and gases escaping backwards
  • Weight loss - if kept unattended
  • Malnutrition - especially in babies

How can I avoid the symptoms?

One may work out how much milk/dairy can be tolerated before the symptoms kick in. This can, however, be tricky because most processed food contains hidden lactose. Abstaining from milk and dairy is a better solution but this can almost be impossible.

The best treatment to avoid the symptoms is by supplements of lactase enzymes. There are many commercially available lactase enzymes.

There is also lactase enzyme in the form of liquid. This is handy to turn normal milk into lactose free saving you a lot of money. It is suitable for babies to avoid baby colic and ideal for those who like to drink milk or putting milk with cereals, tea or coffee for the breakfast etc.

How can I test for lactose intolerance?

The most common tests used to measure the absorption of lactose in the digestive system are the lactose tolerance blood test, the hydrogen breath test, and the stool acidity test performed on babies. These tests are performed on an outpatient basis at a hospital, clinic, or doctor's office. However, one can do a Homepage self-test in which after fasting for 12 hours a person drinks a large glass of milk taking nothing else for the next 3-5 hours. If symptoms develop one can be certain he/she has lactose intolerance.