Lactase: the enzyme that breaks down lactose in milk

Biological role and life-stage regulation

Most mammals produce abundant lactase as offspring to digest their mother's milk (mother's milk). In many species the level of enzyme activity falls after weaning, which reduces the ability to digest lactose as adults. Humans show more diversity: some populations retain high lactase activity into adulthood (lactase persistence), while others commonly experience reduced activity after childhood.

Structure, genetics and evolution

LPH is a membrane-bound glycoprotein synthesized by enterocytes. Regulation of its expression is developmentally programmed; in humans, genetic variants in regulatory DNA regions can maintain expression and confer lactase persistence. This trait is considered an example of recent human evolution associated with dairy farming and pastoral cultures.

Deficiency, symptoms and management

When lactase activity is low, undigested lactose can be fermented by gut bacteria, producing gas, bloating and loose stools. This condition is commonly called lactose intolerance and should be distinguished from immune-mediated milk allergy. Secondary lactase deficiency can follow intestinal infections or diseases that damage the gut lining. As a simple example of adult mammals that often tolerate lactose poorly, giving milk to an adult cat can cause discomfort and diarrhea.

Practical uses and examples

• Lactase is added to milk to make lactose-free dairy by pre-digesting lactose so people with low activity can consume it comfortably.
• Commercial lactase preparations and over-the-counter enzyme tablets help individuals manage symptoms when consuming lactose-containing foods.
• Industries use lactase to modify sweetness, improve solubility and reduce crystallization in certain dairy products.

Notable distinctions

Key distinctions include lactase deficiency (reduced enzyme activity) versus milk allergy (immune reaction), and primary developmental decline versus secondary acquired loss. Understanding these differences guides diagnosis and dietary recommendations. Research into lactase regulation continues to inform nutrition, public health and evolutionary biology.