Lactose intolerance

Lactose intolerance is the inability to digest lactose, a sugar found in dairy, caused by a deficiency in the enzyme lactase.

Overview

Lactose is broken down by the enzyme lactase, which is produced in the small intestine. When lactase is absent or insufficient, individuals experience symptoms such as abdominal pain, bloating, flatulence, nausea, and diarrhea after consuming dairy products.

Types

• Congenital Lactase Deficiency: A rare, genetic disorder where infants cannot break down lactose in breast milk or formula, leading to severe diarrhea and dehydration.
• Lactase Nonpersistence: The most common form, where lactase production naturally decreases after infancy. Many people with this form can still tolerate small amounts of lactose or fermented dairy products like yogurt and cheese.

Causes

• Congenital Deficiency: Caused by variants in the LCT gene, which provides instructions for making the lactase enzyme.
• Lactase Nonpersistence: Caused by a gradual decrease in LCT gene expression after infancy. This process is controlled by a regulatory element within the MCM6 gene. Individuals who inherit specific variants in this regulatory element maintain lactase production into adulthood.

Inheritance

• Congenital Lactase Deficiency: Inherited in an autosomal recessive pattern; both copies of the LCT gene must have a variant.
• Lactase Persistence: The ability to digest lactose into adulthood is inherited in an autosomal dominant pattern, meaning one copy of the variant regulatory element is sufficient to sustain lactase production.