Lactose intolerance

Lactose intolerance stems from a diminished capacity to digest lactose, a disaccharide sugar comprising glucose and galactose, prevalent in mammalian milk. This digestive failure is primarily due to insufficient levels of the enzyme lactase-phlorizin hydrolase (LPH), also known as lactase, produced by enterocytes in the brush border of the small intestine. When lactase is deficient, lactose remains undigested in the lumen of the small intestine.

It then travels to the large intestine, where colonic bacteria ferment it, producing short-chain fatty acids and gases like hydrogen, methane, and carbon dioxide. This fermentation process leads to osmotic effects, drawing water into the colon, and the accumulation of gases, manifesting as the characteristic symptoms of abdominal pain, bloating, flatulence, diarrhea, and sometimes nausea. The onset and severity of symptoms are dose-dependent on the amount of lactose consumed and the degree of lactase deficiency.

Lactose intolerance represents the ancestral human condition; the ability to digest lactose into adulthood, known as lactase persistence (LP), is a relatively recent evolutionary adaptation. Following the domestication of dairy animals approximately 10,000 years ago, populations that relied heavily on milk as a food source experienced strong selective pressure favoring individuals with genetic mutations that maintained lactase production beyond infancy. These mutations, primarily in the MCM6 gene upstream of the LCT gene (which encodes lactase), allow for continued transcription of the lactase gene.

LP evolved independently in several distinct human populations, including those in Northern Europe, parts of Africa (e.g., Fulani people), and the Middle East, leading to the current mosaic of global lactase activity. Conversely, the majority of the world's adult population, particularly those of East Asian, African, and Southern European descent, exhibit lactase non-persistence, meaning their lactase production naturally declines after weaning.

Lactase deficiency can be categorized into four main types. Primary lactose intolerance is the most common, characterized by a genetically programmed decline in lactase activity starting in late childhood or early adulthood. Secondary lactose intolerance results from damage to the small intestinal mucosa, which can be caused by various conditions such as infectious gastroenteritis, celiac disease, Crohn's disease, or chemotherapy.

Developmental lactose intolerance may occur in premature infants due to immature intestinal development and often resolves over time. Congenital lactase deficiency is an extremely rare autosomal recessive genetic disorder where individuals are born with little to no lactase production from birth, requiring strict lifelong avoidance of lactose.

Diagnosing lactose intolerance typically involves a combination of symptom assessment and dietary elimination. Objective confirmation can be achieved through tests like the hydrogen breath test, which measures hydrogen gas produced by bacterial fermentation of undigested lactose, or a stool acidity test. It is crucial to differentiate lactose intolerance from milk allergy, which involves an immune system response to milk proteins and can be life-threatening.

Management strategies focus on reducing lactose intake to a level that minimizes symptoms. This can involve limiting dairy consumption, choosing low-lactose dairy products (like aged cheeses or yogurt with live cultures), or using lactase enzyme supplements. The cultural and economic significance of dairy in Western societies, coupled with historical research biases favoring lactase-persistent populations, has sometimes led to the over-medicalization of lactose intolerance as a disorder rather than recognizing it as a normal physiological variation.