Created: September 15, 2010; Last Update: November 29, 2018; Next update: 2021. Some people have digestive problems after drinking milk or eating dairy products. They only tolerate very small amounts of lactose because their bowel has trouble breaking it down. Lactose is the main carbohydrate in milk produced by cows and other animals. Human breast milk also contains lactose. It is not present in vegetable products like soy milk. Lactose consists of two sugars: glucose and galactose. An enzyme in our small intestine called lactase quickly breaks down the lactose into its two parts. Only after the two sugars have been separated can they be absorbed by our bowel. Babies' digestive systems are designed to survive on breast milk alone. In order to digest the milk, babies produce a large amount of the enzyme lactase. When a child is weaned off breast milk, their digestive system gradually adapts to other foods. Their body then produces considerably less lactase, and can therefore only break down smaller amounts of lactose. This is referred to as primary lactase deficiency. Due to genetic differences, some people's bodies produce much less lactase than others, so they don't tolerate milk as well as other people. The amount of lactase normally produced also varies between ethnic groups. For example, Northern European adults produce more lactase than adults from parts of Asia and Africa, where traditionally only very small amounts of dairy products are eaten. Some people's bodies produce less lactase due to a bowel problem. This is called secondary lactase deficiency. It may be caused by things like gluten intolerance (celiac disease) or chronic inflammations in the membranes lining the small intestine, such as Crohn's disease. When lactose enters the small intestine it is quickly broken down by the enzyme called lactase. Only then can the separate building blocks of the lactose (glucose and galactose) be absorbed by the small intestine. If that happens quickly and completely, only a small amount of lactose or no lactose at all enters the large intestine (colon).  But if large amounts of lactose do enter the colon, they are broken down and converted by bacteria. This increases the production of gases, and more liquid passes into the large intestine. The colon can absorb some, but not all, of these extra gases and liquids. This is what causes flatulence (bloating and “wind”) and diarrhea. The longer the lactose stays in the large intestine, the more it is broken down, which can cause discomfort. Some people may not have much lactase, but their large intestine works more efficiently. Then larger amounts of lactose are eliminated faster and don't cause too much trouble. The amount and type of bacteria in the large intestine (called intestinal flora) might play a role here. What is in the food and the amount of time it stays in the stomach or the small intestine also affects the symptoms. Food that has high amounts of fat in it can slow down your digestion. Then the lactase has more time to break down the lactose, and the symptoms are often less severe as a result. Bloating, feeling full, pain and diarrhea are also symptoms of a condition called irritable bowel syndrome (IBS). IBS is very common too, especially amongst people over the age of 35. In irritable bowel syndrome the large intestine is sensitive and lactose may be less well tolerated. These symptoms might also be caused by sensitivities to foods other than dairy products, like fructose. Emotional stress can play a role too. Finding the cause of a food intolerance is not easy. There are various ways to diagnose lactose intolerance.
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URL of this page: https://medlineplus.gov/genetics/condition/lactose-intolerance/
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. Congenital lactase deficiency, also called congenital alactasia, is a disorder in which infants are unable to break down lactose in breast milk or formula. This form of lactose intolerance results in severe diarrhea. If affected infants are not given a lactose-free infant formula, they may develop severe dehydration and weight loss. Lactose intolerance in adulthood is caused by reduced production of lactase after infancy (lactase nonpersistence). If individuals with lactose intolerance consume lactose-containing dairy products, they may experience abdominal pain, bloating, flatulence, nausea, and diarrhea beginning 30 minutes to 2 hours later. Most people with lactase nonpersistence retain some lactase activity and can include varying amounts of lactose in their diets without experiencing symptoms. Often, affected individuals have difficulty digesting fresh milk but can eat certain dairy products such as cheese or yogurt without discomfort. These foods are made using fermentation processes that break down much of the lactose in milk. Lactose intolerance in infancy resulting from congenital lactase deficiency is a rare disorder. Its incidence is unknown. This condition is most common in Finland, where it affects an estimated 1 in 60,000 newborns. Approximately 65 percent of the human population has a reduced ability to digest lactose after infancy. Lactose intolerance in adulthood is most prevalent in people of East Asian descent, with 70 to 100 percent of people affected in these communities. Lactose intolerance is also very common in people of West African, Arab, Jewish, Greek, and Italian descent. The prevalence of lactose intolerance is lowest in populations with a long history of dependence on unfermented milk products as an important food source. For example, only about 5 percent of people of Northern European descent are lactose intolerant. Lactose intolerance in infants (congenital lactase deficiency) is caused by mutations in the LCT gene. The LCT gene provides instructions for making the lactase enzyme. Mutations that cause congenital lactase deficiency are believed to interfere with the function of lactase, causing affected infants to have a severely impaired ability to digest lactose in breast milk or formula. Lactose intolerance in adulthood is caused by gradually decreasing activity (expression) of the LCT gene after infancy, which occurs in most humans. LCT gene expression is controlled by a DNA sequence called a regulatory element, which is located within a nearby gene called MCM6. Some individuals have inherited changes in this element that lead to sustained lactase production in the small intestine and the ability to digest lactose throughout life. People without these changes have a reduced ability to digest lactose as they get older, resulting in the signs and symptoms of lactose intolerance. The type of lactose intolerance that occurs in infants (congenital lactase deficiency) is inherited in an autosomal recessive pattern The ability to digest lactose into adulthood depends on which variations in the regulatory element within the MCM6 gene individuals have inherited from their parents. The variations that promote continued lactase production are considered autosomal dominant Learn how to cite this page |
What would it be called if someone does not make lactase, the enzyme that breaks down lactose?
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