Lactose intolerance and genetics
Lactose intolerance is a common digestive disorder characterized by the body's inability to effectively break down lactose, the sugar found in milk. This condition can lead to various gastrointestinal symptoms, including nausea, cramps, and abdominal pain. Genetic factors play a significant role in its prevalence, with variations observed across different populations. For instance, lactose intolerance is more frequent among individuals of Middle Eastern, Asian, and African descent, while northern Europeans tend to have lower rates of this condition.
There are three main types of lactase deficiency: inherited, secondary, and primary. Inherited lactase deficiency typically manifests soon after birth, while secondary low-lactase activity may arise from certain medical conditions or surgeries. Primary low-lactase activity can develop as individuals age, affecting a significant percentage of adults. Researchers have identified specific genetic variations associated with lactose intolerance, suggesting that genetics influence how individuals express the lactase enzyme.
Effective management often involves dietary adjustments, including the use of lactose-free products and live-culture yogurts, which can help alleviate symptoms. While some individuals may be genetically predisposed to lactose intolerance, not all will experience symptoms, indicating a complex interplay between genetics and environmental factors.
Lactose intolerance and genetics
DEFINITION Lactose intolerance is a common disorder associated with the digestion of milk sugar. It affects a large portion of the human population and creates unpleasant intestinal effects. Its understanding has led to the commercial availability of alternative products that supplement the lack of dairy products in the diet.
Risk Factors
Lactase deficiency displays remarkable genetic variations. The condition is more prevalent among infants of Middle Eastern, Asian (especially Chinese and Thai), and African descent (such as the Ibo, Yoruba, and other tribes in Nigeria and the Hausa in Sudan). On the other hand, Europeans (especially northern) appear to be statistically less susceptible to the deficiency. Similarly, the Fula tribe in Sudan, which raises the fulani breed of cattle, and the Eastern African Tussi, who own cattle in Rundi, appear to be rarely affected. It is estimated that 10 to 20 percent of American Caucasians and about 75 percent of African Americans are affected.
There are three types of lactase deficiency: inherited deficiency, secondary low-lactase activity, and primary low-lactase activity. In inherited lactase deficiency, the symptoms of intolerance develop very soon after birth, as indicated by the presence of lactose in the urine. Secondary low-lactase activity can be a side effect of peptic ulcer surgery or can occur for a variety of reasons. It may also be present during intestinal diseases such as colitis, gastroenteritis, kwashiorkor, and sprue. Individuals sometimes develop primary low-lactase activity as they get older. A large number of adults, estimated at almost 20 percent, gradually exhibit lactose intolerance, caused by the gradual inability to synthesize an active form of lactase. Susceptible individuals may start developing lactose intolerance as early as four years old.
Etiology and Genetics
Milk is the primary source of nutrition for infants. One pint of cow’s summer milk provides about 90 percent of the calcium, 30 to 40 percent of the riboflavin, 25 to 30 percent of the protein, and 10 to 20 percent of the calories needed daily. Lactose, also known as milk sugar, exists in the milk of humans, cows, and other mammals. About 7.5 percent of human milk consists of lactose, while cow’s milk is about 4.5 percent lactose. This sugar is also one of the few carbohydrates exclusively associated with the animal kingdom; its biosynthesis takes place in the mammary tissue. It is produced commercially from whey, which is obtained as a by-product during the manufacture of cheese. Its so-called alpha form is used as an infant food. Its sweetness is about one-sixth that of sucrose (table sugar).
The metabolism (breaking down) of lactose to glucose and galactose takes place via a specific enzyme called lactase, which is produced by the mucosal cells of the small intestine. Because lactase activity is rate-limiting for lactose absorption, any deficiency in the enzyme is directly reflected in a diminished rate of the sugar absorption. This irregularity should not be confused with intolerance to milk resulting from a sensitivity to milk proteins such as beta-lactoglobulin.
Researchers have found two variations, called single nucleotide polymorphisms (SNPs), in the human genome that are associated with lactose intolerance. Both are near the lactase gene and most likely affect proteins that regulate the expression of the gene.
Symptoms
As a result of lactose intolerance, relatively large quantities of the unhydrolyzed (unbroken) lactose pass into the large intestine, which causes the transfer of water from the interstitial fluid to the lumen by osmosis. At the same time, the intestinal bacteria produce organic acids as well as gases such as carbon dioxide, methane, and hydrogen, which lead to nausea and vomiting. The combined effect also produces cramps and abdominal pains.
Screening and Diagnosis
Definitive diagnosis of the condition is established by an assay for lactase content in the intestinal mucosa. Such a test requires that the individuals drink 50 grams of lactose in 200 milliliters of water. Blood specimens are then taken after 30, 60, and 120 minutes for glucose analysis. An increase of blood glucose by 30 milligrams per deciliter is considered normal, while an increase of 20 to 30 milligrams per deciliter is borderline. A smaller increase indicates lactase deficiency. This test, however, may still show deficiency results with individuals who have a normal lactase activity.
A study of Romanian children published in 2023 showed that a genetic predisposition to lactose intolerance does not necessarily mean that children will develop symptoms. While the children in the study all had congenital lactose intolerance, some were not affected by this and were able to properly digest diary products.
Treatment and Therapy
Patients are recommended a lactose-free diet as well as the consumption of live-culture yogurt, which provides the enzyme beta-galactosidase that attacks the small amounts of lactose that may be in the diet. Beta-galactosidase preparations are also commercially available.
Prevention and Outcomes
The ill effects disappear as long as the diet excludes milk altogether. Often people who exhibit partial lactose intolerance can still consume dairy products, including cheese and yogurt, if the food is processed or partially hydrolyzed. This may be accomplished by heating or partially fermenting milk. Some commercial products, such as Lactaid, are designed for lactose-intolerant people because they include the active form of the lactase enzyme in either liquid or tablet form.
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