lactase
Meaning of lactase: Losing it after childhood is the normal human pattern; keeping it into adulthood is the genetic exception that spread with dairy farming.
Definition of lactase
Lactase is a digestive enzyme that splits lactose, the sugar in milk, into the two simpler sugars glucose and galactose so they can be absorbed. It sits on the surface of the cells lining the small intestine rather than being released into the gut cavity. Production is highest in infancy and falls in most people once weaning is complete.
Where does the enzyme work?
Not in the stomach, and not floating free in the gut. Lactase is anchored to the outer edge of the cells lining the small intestine, on the fine projections that give that lining its brush-like surface. Splitting therefore happens at the moment of contact with the wall, and the two simple sugars released are taken up immediately by the same cells. Because the enzyme sits on the tips of those projections, anything that flattens or damages the lining, such as a gut infection or celiac disease, strips it away and causes a temporary shortage that recovers as the lining heals.
Why do most adults stop making it?
The gene switches down after weaning in most mammals, humans included, because milk was never expected in an adult diet. A separate change in the control region near the enzyme gene keeps production running for life, and that change became common in populations with a long history of herding cattle, sheep or camels. Reduced lactase in adulthood is therefore the majority state worldwide, and continued production is the regional exception. Calling the majority state a deficiency is a habit of language rather than a statement about health.
What happens to undigested lactose?
It stays in the gut. Lactose that is not split cannot cross the intestinal wall, so it draws water into the bowel and travels on to the colon, where resident bacteria ferment it into gas and short chain acids. Bloating, cramping, wind and loose stools follow, usually within a couple of hours of a milk-containing meal. Severity depends on how much enzyme remains, how much lactose arrived and what the gut bacteria do with it, which is why two people with the same test result describe very different experiences of lactose intolerance.
Are there different reasons for low lactase?
The label covers four separate situations that differ in cause and in whether they resolve.
| Type | Cause |
|---|---|
| Primary | The normal fall in production after weaning |
| Secondary | Damage to the intestinal lining from infection or disease |
| Congenital | A rare inherited absence present from birth |
| Developmental | Low levels in infants born very early, resolving as the gut matures |
Also known as
Used in a sentence
Lactase activity was measured on the small bowel biopsy specimen.
Good to know
Reduced lactase after weaning is the worldwide majority pattern rather than a disease, and lifelong production is the inherited exception; the congenital form present from birth is rare and unrelated.
Word origin
Where “lactase” comes from
Lactase joins the Latin lac, meaning milk, with the ending -ase that marks an enzyme; the sugar it acts on, lactose, uses the same root with the sugar ending -ose.