What is lactose intolerance?


Alternative names: milk intolerance, lactose malabsorption, lactase deficiency, hypolactasia

Lactose is the carbohydrate that is present in milk. It’s a disaccharide made up of two sugar molecules: one glucose and one galactose molecule.

Our bodies use an enzyme called lactase, which is produced in the epithelial cells on the villi in the small intestines, to break down lactose into glucose and galactose so that we can absorb it into our bodies and use it for energy. 

When you don’t have enough lactase to break down lactose, you’re lactose malabsorbant. Note that this is not the same as being lactose intolerant (see note below*). Lactose malabsorption refers to the inability to digest and absorb lactose. The lactose moves into the colon (or large intestine), where bacteria ferment it. This can often result in gastrointestinal symptoms such as diarrhoea, bloating, flatulence, cramping and pain.  

If gastrointestinal symptoms are present, you’re said to be lactose intolerant.  These symptoms usually develop within a few hours of consuming foods or drinks that contain lactose. The severity of the symptoms, and when they appear, depends on the amount of lactose that was consumed. 

Although lactose intolerance is very common, most people can manage their condition without having to give up all dairy foods.

Lactose intolerance isn’t harmful, but it can be uncomfortable and may have a negative impact on your quality of life and nutritional status. While there’s no cure for lactose intolerance, it can be managed by keeping an eye on the amount of milk and milk products you consume and/or by introducing lactase into the milk products you enjoy.

Subtypes of lactose intolerance

There are several different types of lactose intolerance:

1. Primary lactose intolerance

This is the most common form and occurs as a result of a genetically programmed loss of lactase. It develops over time and typically begins after age two, when the body begins to produce less lactase.

Onset is generally gradual, with symptoms typically appearing in late adolescence or early adulthood. The frequency of primary lactose intolerance varies, with the highest prevalence among people of African, Asian, Hispanic, Mediterranean and southern European descent.

2. Secondary lactose intolerance

Secondary lactose intolerance occurs because of an injury to the small intestine. The small intestine contains villi on its surface, and lactase is found at the tip of these villi. When there’s an injury to the small intestine, the villi get sloughed off, resulting in limited lactase enzymes. This will result in short-term lactose intolerance. Fortunately, however, the villi recuperate within 2-3 days. 

Injury to the small intestine may be caused by acute gastroenteritis, coeliac disease, Crohn's disease, ulcerative colitis, chemotherapy, intestinal parasites or other environmental factors.

3. Developmental lactose intolerance

Developmental lactose intolerance occurs in babies who are born prematurely. Lactase is deficient in the small intestine until about the 34th week of pregnancy. At this point, lactase activity is only about 30% of what it is in full-term infants. Lactase maturation occurs predominantly in the third trimester.  

Therefore, babies who are born prematurely have lactase deficiency due to incomplete development. This usually improves quickly and lasts only for a short period of time after birth.

4. Congenital lactose intolerance

This refers to a genetic disorder in which no lactase is produced by the small intestine from birth. It’s a very rare cause of lactose intolerance. 

*Note: Lactose intolerance is often used interchangeably with lactose malabsorption. However, if an individual doesn’t absorb lactose properly, it doesn’t necessarily mean that they’ll experience symptoms. Lactose malabsorption is therefore described as the physiological problem that manifests as lactose intolerance.

**Note: Being lactose intolerant isn’t the same as having a milk allergy.

Course and prognosis of lactose intolerance

Primary lactose intolerance refers to a genetically programmed loss of lactase. Symptoms usually begin in late adolescence or early adulthood, when there’s no longer enough lactase available in the small intestine to digest lactose from dairy foods. It’s a chronic condition of variable severity, which requires constant management. 

Secondary lactose intolerance occurs as a result of another illness. The symptoms are acute and will improve once the primary illness has been resolved, and the villi and lactase in the small intestines have regenerated.

In both primary and secondary lactose intolerance, ingesting dairy products will worsen the symptoms. Both the amount of lactase deficiency and the amount of lactose in the dairy products will determine the severity of symptoms. For example, a glass of milk contains about 12g of lactose, whereas 100g of feta has only 0.5g lactose. Most individuals who are lactose intolerant have enough lactase to digest up to 18g of lactose through the day. 

The management of lactose intolerance requires the removal of dairy foods to varying degrees.

Possible complications of lactose intolerance

The main complication of lactose intolerance is nutrient deficiency – specifically calcium deficiency and, to a lesser degree, vitamin D deficiency. 

In most adults, the main source of daily calcium comes from dairy foods. Lactose from these dairy foods also help with the absorption of magnesium and zinc from the small intestines. All these vitamins and minerals are vital to the development of strong, healthy bones and teeth. 

Excluding dairy foods from your diet may, therefore, lead to nutrient deficiencies. These deficiencies can cause complications such as:

  • Osteoporosis – a condition that causes the bones to become thin and weak, resulting in bone loss or brittle bones. This increases the risk of fractures and broken bones. Osteoporosis can also result in poor posture, severe back pain, and fractures of the hip, spine, wrist and ribs, which can be fatal.
  • Osteopenia – or low bone mineral density. If osteopenia isn’t treated, it can develop into osteoporosis.

In addition to healthy bones, calcium also helps the heart muscles and nerves to function properly.  Studies have also shown that calcium lowers blood pressure and the risk of hypertension, as well as possibly reducing the risk of several cancers (e.g. colon and breast cancer).

Malnutrition and weight loss are also possible complications of lactose intolerance. This particularly concerns infants and children. Malnutrition occurs when the food that’s eaten doesn’t provide the body with the nutrients that are essential for healthy functioning. With malnourishment or deficiency of vital nutrients, many other health problems can develop. This includes feeling very tired, wounds taking longer to heal, and developing depression.

Among individuals with lactose intolerance, calcium deficiency is very common. Research has also shown that these individuals have a higher risk for osteoporosis, a lower bone density, and a higher risk for hypertension and colon and breast cancers.

If you’re lactose intolerant and unable to consume dairy foods, it’s very important to find other foods that will assist you in getting your recommended daily amount of calcium. A supplement may be necessary. 


Lactose intolerance is associated with very little morbidity. The prognosis is therefore excellent, with most people experiencing improvement in their symptoms when dairy foods are removed from their diet. 

As long as an adequate calcium intake is maintained, the complications associated with lactose intolerance can be avoided. 

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

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