Lactose Intolerance

Lactose intolerance is a reduced ability to digest lactose, the main sugar found in milk. Most children and babies produce lactase enzyme, which splits the disaccharide lactose into its digestible component sugars (glucose and galactose), in their small intestine. However, in most of the global population, the production of lactase declines steeply in childhood, and around 70% of the world's adult population is lactose intolerant to some degree, due to reduced or no production of the lactase enzyme in adulthood.^[1][2]

People with lactose intolerance experience digestive symptoms due to intestinal bacteria consuming the undigested lactose. Symptoms vary widely from person to person, and can include flatulence and bloating (from the excess gas produced by intestinal bacteria), borborygmus (a delightful word that refers to tummy rumbling from gas and liquid sloshing around), diarrhea (caused by osmotic pressure due to undigested lactose), abdominal pain, and nausea. Symptom onset is usually 30 minutes to 2 hours after ingesting lactose.^[2]

The mainstays of lactose intolerance diagnosis in adults are dietary history and the lactose breath test (LBT).^[2] Infants suspected of having congenital lactase deficiency (CLD) who respond well to elimination of dietary lactose should have genetic testing for mutations in the LCT gene.^[2]

Low to no production of lactase enzyme is actually the norm in adults; it’s not a disorder, and doesn’t require medical treatment per se. Lactose intolerance which occurs secondary to an illness is usually self-limiting; when the underlying condition is resolved, lactase production usually resumes without further treatment. In all cases, the digestive symptoms of lactose intolerance can be managed in one of three ways: by reducing dietary lactose until symptoms no longer occur; by supplementing with lactase enzyme; or by supplementing with probiotics.^[2]

The principal supplement for lactose intolerance is lactase enzyme, taken orally with lactose-containing foods. Supplementing with lactase enzyme has been shown to be clinically effective at reducing or eliminating the digestive symptoms of lactose intolerance.^{[3][4][5][6][2]} Probiotics have also been studied for lactose intolerance. Per a 2019 review, the probiotics that have been studied include Bifidobacterium longum, Bifidobacterium animalis, Lactobacillus bulgaricus, Lactobacillus reuteri, Lactobacillus acidophilus, Lactobacillus rhamnosus, Saccharomyces boulardii, and Streptococcus thermophilus. Of these, B. animalis had the best evidence for reducing symptoms of lactose intolerance.^[7] A promising, but less-studied and proprietary, probiotic is a genetically modified strain of L. acidophilus that produces mammalian lactase. However, no research on this probiotic strain has been published in a peer-reviewed journal, and clinical trials are still ongoing.^[8]

Dairy foods don’t all contain the same amount of lactose. Many people with lactose intolerance can consume moderate amounts of lactose (up to the equivalent of one cup of milk) without experiencing digestive symptoms,^[9] making it possible to enjoy lower-lactose dairy foods on a low-lactose diet.^[2] Medications and processed non-dairy foods may also have lactose added during processing–for example, Pop Rocks candy and Pringles Salt & Vinegar chips both contain lactose,^[10][11] despite having no obvious connection to dairy–so people who experience symptoms with smaller amounts of lactose may need to check labels carefully.

Dietary changes and supplementary lactase enzyme are the best-studied tools for symptom management. Probiotics have also been studied for lactose intolerance; see “Have any supplements been studied for lactose intolerance?” above.

There are three types of lactose intolerance: primary, secondary, and congenital.^[2]

• Primary lactose intolerance is the most common. This is the normal drop in intestinal lactase production that occurs for most people between ages two and five.
• Secondary lactose intolerance is a sudden decrease in lactase production due to damage to the small intestine. This is usually a transient condition resulting from conditions like infection, celiac disease, Crohn’s disease, food allergy, or small intestinal bacterial overgrowth (SIBO).^[2]
• Congenital lactase deficiency (CLD) refers to an inability to produce lactase from birth. It’s an autosomal-recessive disorder, caused by a mutation in the LCT gene, and it is extremely rare.^[12]