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Lactose Intolerance

Lactose intolerance is a common gastrointestinal condition attributed to insufficient levels of the enzyme lactase, which is needed to properly digest lactose, a complex sugar found in milk and milk-containing foods. Lactase deficiency may also cause lactose malabsorption, which results in undigested lactose passing to the colon where it is fermented to monosaccharides, free fatty acids and gases. It is the undigested lactose and the resulting breakdown products that cause symptomatology. Thus, lactose intolerance is a syndrome composed of characteristic symptoms, including abdominal symptoms (e.g., pain, cramping, bloating, gas movement), and release of gas (flatulence) and finally bowel-related symptoms (e.g., bowel movement, loose stool, and bowel urgency) following ingestion of lactose, contained in milk or milk-containing products.

The following depicts the lactose intolerance condition:

  1. Inadequate lactase activity in the small intestine results in lactose not being broken down properly, thus the undigested lactose proceeds through to the large intestine
  2. Bacteria in the large intestine ferments the undigested lactose, causing gas production
  3. The fermentation of the undigested lactose leads to gastric symptoms: abdominal pain, gas, cramping, bloating and diarrhea

Symptoms can be painful, embarrassing, and socially difficult to manage to the point that it can significantly impact one’s quality of life. According to the American Academy of Pediatrics Committee on Nutrition (2006), “the symptoms of lactose intolerance can lead to significant discomfort, disruption of the quality of life, and loss of school attendance, leisure and sports activities, and work time, all at a cost to individuals, families and society.” Furthermore, in an Engage Health 2008 study on lactose intolerance, 82% of patients reported that their lactose intolerance affects their daily activities.

Causes of Lactose Intolerance

There are two commonly known factors which can lead to lactose intolerance:

  • Primary causes include one’s heredity. As infants mature, there is a genetically programmed decrease in lactase activity. This decrease in lactase production tends to be stronger in non-dairy consuming societies, for example certain African and Asian populations. Symptoms of lactose intolerance typically become present in children and young adults, but can also develop in older individuals.
  • Secondary causes may include less common factors, such as intestinal parasites, chemotherapy, penicillin reactions, surgery, pregnancy, or dairy avoidance. In rare cases, infants can be born with a genetic disorder that prevents the production of lactase.


Lactose intolerance is often diagnosed by evaluating an individual’s clinical history, which reveals a relationship between lactose ingestion and onset of symptoms. Hydrogen breath tests may also be utilized to diagnose lactose malabsorption and a milk challenge may be used to differentiate between lactose malabsorption and lactose intolerance. Further tests can be conducted to rule out other digestive diseases or conditions, including: stool examination to document the presence of a parasite, blood tests to determine the presence of celiac disease, and intestinal biopsies to determine mucosal problems leading to malabsorption, such as inflammatory bowel disease or ulcerative colitis.

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