Skip to Main Content

Enzyme Deficient Diseases: Lactose Intolerance

Year 11 Biology


Lactose is a naturally occurring sugar found in dairy foods such as cow’s milk and yoghurt. It is normally broken down or digested by an enzyme in our body called lactase, and ends up in the bloodstream as glucose (another term for sugar).


Lactose intolerance related to primary or secondary lactase deficiency is characterized by abdominal pain and distension, borborygmi, flatus, and diarrhea induced by lactose in dairy products. The biological mechanism and lactose malabsorption is established and several investigations are available, including genetic, endoscopic and physiological test.


The signs and symptoms of adverse food reactions (AFRs) in children derive from several mechanisms. These mechanisms can be triggered by different components of the same food. Immune-mediated reactions (i.e., food allergy, celiac disease) are elicited by food proteins, whereas the vast majority of non-immune-mediated AFRs derive from carbohydrate intolerances 

Science Behind Lactose Intolerance

At birth, nearly all human beings, as well as most mammals, are able to digest lactose, a disaccharide present in their mothers’ milk. This is a critical ability, since newborns obtain a large amount of their energy from digesting the lactose in milk. During digestion, lactose is first split into 2 monosaccharides, glucose and galactose, by the lactase enzyme in the small intestine.


Lactose intolerance is simply the body’s inability to digest lactose. Lactose is a disaccharide, or a molecule made up of two types of sugars, in this case, glucose and galactose. Most people have an enzyme (a protein molecule that breaks down other molecules) lactase in their digestive systems. This enzyme breaks down lactose into glucose and galactose. The body can then absorb and use these sugars for energy. When someone has a lactase deficiency, lactose cannot be digested and remains in the digestive system causing a wide range of unwanted symptoms, including abdominal pain, gas, diarrhea, nausea, and a bloated stomach.

Lactose Intolerance is a huge issue for many people, but why does lactose intolerance exist, and why do we have lactose intolerance? Fixing lactose intolerance is possible, but it's not super easy. Ethnicity does play a role in your lactose intolerance!

Lactose intolerance genetics




In humans, the activities of lactase and most of the other digestive hydrolases are maximal at birth. The majority of the world's human population experiences a decline in production of the digestive enzyme lactase-phlorizin hydrolase during maturation, with the age of onset ranging from the toddler years to young adulthood

Nutrition Australia

People with lactose intolerance experience symptoms such as diarrhoea, abdominal pain, flatulence and/or bloating after consuming lactose. Lactose intolerance is mostly a deficiency, not an absence of the enzyme lactase. These symptoms occur because the body does not fully digest lactose, a type of sugar that is naturally present in milk and other dairy foods.


 Lactose intolerance refers to symptoms related to the consumption of lactose-containing dairy foods, which are the most common source for this disaccharide. While four causes are described, the most common is the genetically-determined adult onset lactose maldigestion due to loss of intestinal lactase governed by control of the gene by a 14,000 kb promoter region on chromosome 2.


Globally, ∼70% of adults are deficient in intestinal lactase, the enzyme required for the digestion of lactose. In these individuals, the consumption of lactose-containing milk and dairy products can lead to the development of various gastrointestinal (GI) symptoms.


Purpose of Review To evaluate the clinical and nutritional significance of genetically determined lactase nonpersistence and potential lactose and milk intolerance in 65– 70% of the world’s adult population.

Genetics Home Reference


Lactose intolerance is an impaired ability to digest lactose, a sugar found in milk and other dairy products. Lactose is normally broken down by an enzyme called lactase, which is produced by cells in the lining of the small intestine.