Milk After Weaning
Adult-scale dairy consumption varies widely across human populations. Whether milk is comfortable in adulthood largely comes down to one enzyme — lactase — and genetics.
How lactose digestion changes after infancy
Lactose, the main sugar in milk, is broken down by an enzyme called lactase. In many people, lactase production decreases after infancy — which can cause adult lactose intolerance (lactase nonpersistence). MedlinePlus estimates that about 65% of people have a reduced ability to digest lactose after infancy. Others inherit regulatory variants near the LCT gene (in MCM6) that sustain lactase production throughout life.
From infancy to adulthood
Lactase digests lactose
Lactose, the main sugar in milk, is normally broken down by the enzyme lactase.
After infancy
In many people, lactase production decreases after infancy.
Lactose intolerance (for many)
Reduced lactase after infancy can cause adult lactose intolerance. About 65% of people have a reduced ability to digest lactose after infancy (MedlinePlus).
Lactase persistence (in some)
Some people inherit regulatory variants near the LCT gene (in MCM6) that sustain lactase production throughout life.
Variation, and what helps some people
- Lactose intolerance is least common in populations with a long history of dependence on unfermented milk products.
- Some people with reduced lactase still tolerate varying amounts of lactose, and fermented dairy such as cheese or yogurt is comfortable for some.
What this suggests — and what it doesn't
What this suggests
- Adult lactose digestion varies widely across humans.
- Lactase persistence is an inherited adaptation in some populations.
- Dairy tolerance is not a universal human default.
What this does not prove
- Dairy is harmful for everyone.
- Dairy is necessary for everyone.
- People who tolerate dairy should avoid it.
- People with symptoms should self-diagnose instead of getting medical advice.
Sources used
Lactose intolerance
MedlinePlus Genetics, U.S. National Library of Medicine (NIH) · updated 2023
Lactose is broken down by lactase; adult intolerance can follow reduced lactase after infancy; ~65% of people have reduced ability to digest lactose after infancy; some inherit MCM6 variants that sustain lactase production.
On the Evolution of Lactase Persistence in Humans
Annual Review of Genomics and Human Genetics (Ségurel & Bon; PMID 28426286) · 2017
Peer-reviewed review: lactase persistence is an inherited adaptation associated with a history of dairying.