Lactose
The natural sugar in milk (a disaccharide of glucose + galactose), digested by the enzyme lactase, whose deficiency causes lactose intolerance, very common in adulthood.
Lactose is the carbohydrate in mammals’ milk. To digest it, the gut produces lactase, an enzyme that splits it into absorbable glucose and galactose. Without enough lactase, undigested lactose ferments in the colon, causing bloating, wind and diarrhoea.
The original angle (intolerance is the norm, not the exception): a major counter-intuitive fact: worldwide, ~65-70% of adults naturally lose their lactase after childhood. Lactose intolerance is therefore the « normal » condition of adult humanity! It is tolerance (lactase persistence into adulthood) that is the exception, arising from a genetic mutation.
This lactase persistence is a spectacular example of recent evolution: it spread among herding populations (northern Europe, some regions of Africa and the Middle East) ~7,500 years ago, with the domestication of livestock. Those who could digest milk had a survival advantage (a source of calories and clean water). Hence the strong differences between populations: ~90% of northern Europeans tolerate lactose, against ~10% in some Asian populations.
Practical nuances:
- Intolerance is gradual: many people tolerate small amounts (a little milk, mature cheese, yoghurt)
- Aged cheeses (Comté, Parmesan) contain almost no lactose (it is broken down during ageing)
- Yoghurt is better tolerated (the bacteria pre-digest some of the lactose)
- Intolerance can appear secondarily (after a stomach bug, or with dysbiosis) and can be reversible
Not to be confused with milk protein allergy (casein), which is immune-mediated and different.