Lactose intolerance

Lactose intolerance is a condition where a person has a reduced ability to digest lactose. Lactose is a disaccharide, which is a type of sugar found in milk and dairy products. This condition is not a milk allergy. A milk allergy is an immune response to milk proteins like whey. Instead, lactose intolerance is a sensitivity caused by a deficiency of the enzyme lactase. This enzyme is produced in the lining of the duodenum, which is part of the small intestine. When there is not enough lactase, the body cannot properly process dairy sugars.

The digestive process fails when lactase is missing. Normally, lactase breaks the lactose molecule into two smaller sugars: glucose and galactose. These smaller sugars can then pass through the wall of the small intestine into the bloodstream. Without enough lactase, the lactose remains intact. It travels through the small intestine and enters the colon. Once in the colon, bacteria begin to metabolize the sugar through a process called fermentation. This fermentation produces large amounts of gas, including hydrogen, carbon dioxide, and methane. These gases cause symptoms like bloating, flatulence, and abdominal cramps. Additionally, the unabsorbed sugars increase the osmotic pressure in the colon. This causes water to flow into the bowels, resulting in diarrhea. Symptoms usually appear between thirty minutes and two hours after eating.

There are four distinct types of lactose intolerance. Primary hypolactasia is a genetic form where lactase production declines as a person grows up. This is the most common type and is linked to the absence of a lactase persistence allele. Secondary hypolactasia, or acquired lactase deficiency, is caused by an injury to the small intestine. This injury might result from infections, celiac disease, or inflammatory bowel disease. Developmental lactose intolerance can occur in premature babies but often improves quickly. Finally, primary congenital alactasia is an extremely rare genetic disorder. In this case, a person has little to no lactase from birth due to mutations in the LCT gene. About 40 cases have been reported worldwide, mostly in Finland.

History shows that lactose intolerance was once the ancestral state for all humans. Most mammals lose the ability to digest lactose after they are weaned from milk. However, humans began domesticating dairy animals roughly 10,000 years ago. This change likely led to the evolution of lactase persistence. This is a condition where the LCT gene remains active into adulthood. This persistence is caused by mutations in a nearby gene called MCM6. The MCM6 gene helps control whether the LCT gene is turned on or off. Analysis of 94 ancient skeletons suggests this mutation spread through Europe about 4,300 years ago. This allowed certain populations to gain nutrition from dairy animals.

The prevalence of lactose malabsorption varies significantly across the globe. A 2016 study of 60,000 participants showed that roughly 65% of adults worldwide are affected. In East Asia, the rate of lactose intolerance is as high as 90%. In contrast, Northern Europe has much higher levels of lactose tolerance. Other regional data shows 70% malabsorption in the Middle East and 66% in Northern Africa. In the United States, intolerance is more common among Asian Americans, African Americans, Mexican Americans, and Native Americans. These numbers show how much human biology has changed based on local environments and diets.

Different cultures have found clever ways to manage low levels of lactose tolerance. Many societies developed traditional foods that contain much less lactose than fresh milk. For example, in Mongolia, people drink kumis. Kumis is a fermented milk product that contains little to no lactose. This allows people to receive dairy nutrition without the discomfort of intolerance. People can also manage their symptoms by reducing dairy intake. Some choose to take lactase supplements to help break down the sugar. The amount of supplement needed usually depends on how much lactose is being consumed.

Understanding lactose intolerance requires looking at how genes and environments interact. The LCT and MCM6 genes are both located on chromosome 2 at the locus 2q21. While many thought a single mutation caused tolerance in different regions, scientists now know that lactase persistence evolved independently multiple times. Different alleles have developed at least three times in East African populations. This shows how human biology adapts to the food available in a specific culture. By studying these genetic patterns, researchers can better understand the history of human nutrition and health.