Lactose Intolerance

Updated: Dec 04, 2019
  • Author: Praveen K Roy, MD, MSc; Chief Editor: Burt Cagir, MD, FACS  more...
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Lactose intolerance is a common disorder and is due to the inability to digest lactose into its constituents, glucose and galactose, secondary to low levels of lactase enzyme in the brush border of the duodenum. [1] Lactase deficiency is the most common form of disaccharidase deficiency. Enzyme levels are the highest shortly after birth and decline with aging, despite continued intake of lactose. Within the animal world, nonhuman mammals usually lose the ability to digest lactose as they reach adulthood. Some populations of the human species, including those of Asian, South American, and African descent, have a propensity for developing lactase deficiency. By contrast, races descended from northern Europe or from the northwestern Indian subcontinent are likely to retain the ability to absorb lactose into adulthood. [2]

Symptoms of lactose intolerance include loose stools, abdominal bloating and pain, flatulence, nausea, and borborygmi. [3, 4, 5] A diagnosis or even the suggestion of lactose intolerance leads many people to avoid milk and/or to consume specially prepared food with digestive aids, adding to health care costs.



Lactose, a disaccharide, is present in milk and processed foods. Dietary lactose must be hydrolyzed to a monosaccharide in order to be absorbed by the small intestinal mucosa. A deficiency of intestinal lactase prevents hydrolysis of ingested lactose. The osmotic load of the unabsorbed lactose causes secretion of fluid and electrolytes until osmotic equilibrium is reached. Dilation of the intestine caused by the osmosis induces an acceleration of small intestinal transit, which increases the degree of maldigestion. Within the large intestine, free lactose is fermented by colonic bacteria to yield short-chain fatty acids and hydrogen gas. [6] The combined increase in fecal water, intestinal transit, and generated hydrogen gas accounts for the wide range of gastrointestinal symptoms.

An association between certain single nucleotide polymorphisms (C>T-13910 and G>A-22018) with lactose tolerance in a northeaster Brazilian population has been reported. [7] In Indo-Europe, lactase deficiency is associated with rs4982235 SNP (or -13910C>T), which may predispose affected individuals to lactose intolerance. [8]



Congenital lactose intolerance is inherited as an autosomal recessive trait and is very rare. [9, 10]

Primary lactose intolerance is due to low levels of lactase, which develop after childhood.

Secondary, or acquired, lactase deficiency may develop in a person with a healthy small intestine during episodes of acute illness. This occurs because of mucosal damage or from medications. Some causes of secondary lactase deficiency are as follows:



United States statistics

The prevalence of primary lactose intolerance varies according to race. As many as 25% of the white population (prevalence in those from southern European roots) is estimated to have lactose intolerance, while among black, Native American, and Asian American populations, the prevalence of lactose intolerance is estimated at 75-90%. [2]

International statistics

An estimated 70-75% off the world's population is lactose-deficient. [8, 11] Lactose intolerance is very common among Asian, South American, and African persons.

Lactose intolerance also appears to have a higher prevalence in patients with diarrhea-predominant irritable bowel syndrome (IBS-D) than healthy individuals. [12]

Race-, sex-, and age-related demographics

Persons of all races are affected by lactose intolerance, with a higher prevalence among Asian, African, and South American persons.

Males and females are equally affected by lactose intolerance. However, of adult women who are lactose intolerant, 44% regain the ability to digest lactose during pregnancy. This is probably due to slow intestinal transit and bacterial adaptation during pregnancy.

Few data are available regarding the prevalence of lactose intolerance in children aged 1-5 years; however, primary lactose intolerance in this group is estimated to be 0-17.9%, whereas it is a reported 0-19% for secondary lactose intolerance. [13]

Among adults, the age of presentation of lactose intolerance is 20-40 years. [9, 14]



The prognosis of patients with lactose intolerance is excellent with dietary restrictions. Morbidity/mortality include the following:

  • Lactose intolerance is not lethal.

  • Morbidity is low from lactose intolerance.

  • Osteopenia can be a complication of lactose intolerance. [15, 16, 17] Vitamin D deficiency appears to be associated with the LCT-13910C>T gene variant of lactose intolerance in white populations. [17]