I know that's the conventional wisdom, but as an adult of European descent who usually handles lactose/dairy well, I sometimes have periods of reduced tolerance, perhaps due to changes in my gut biome.
And there's evidence even lactose-intolerant ("lactase nonpersistent") demographics can tolerate ever-larger amounts of lactose with a gradual ramp-up of consumption that shifts their gut biomes in a different direction:
There's a chance the conventional view overweights human genetics and underweights biome, which could create a self-fulfilling diagnosis of "lactose intolerance".
That is: a person notices a bad reaction, and is told (influenced by demographic tendencies) "oh, as an adult you'll inevitably be lactose-intolerant". They avoid dairy, further thinning (or completely eliminating) the bacteria that could help from their gut, ensuring a more-noticeable negative reaction upon further consumption.
OTOH, if the right bacterial mix was maintained, or reintroduced then maintained if necessary, many considering themselves lactose-intolerant might not persistently be.
(Separate from the bacteria themselves assisting in lactose digestion, there's even some chance that the positive or negative shock of a wildly-changed biome might epigenetically affect the beyond-childhood activation of the human genome's own lactase genes – a possibility mentioned in the middle of this microbiologist's writeup: https://www.cnn.com/2019/04/17/health/lactose-intolerance-mi... )
My gastroenterologist says that intestinal illness can render you temporarily lactose intolerant for a few weeks after the fact. That might be what's happening to you, or some version of it!
And there's evidence even lactose-intolerant ("lactase nonpersistent") demographics can tolerate ever-larger amounts of lactose with a gradual ramp-up of consumption that shifts their gut biomes in a different direction:
https://pubmed.ncbi.nlm.nih.gov/38159728/
There's a chance the conventional view overweights human genetics and underweights biome, which could create a self-fulfilling diagnosis of "lactose intolerance".
That is: a person notices a bad reaction, and is told (influenced by demographic tendencies) "oh, as an adult you'll inevitably be lactose-intolerant". They avoid dairy, further thinning (or completely eliminating) the bacteria that could help from their gut, ensuring a more-noticeable negative reaction upon further consumption.
OTOH, if the right bacterial mix was maintained, or reintroduced then maintained if necessary, many considering themselves lactose-intolerant might not persistently be.
(Separate from the bacteria themselves assisting in lactose digestion, there's even some chance that the positive or negative shock of a wildly-changed biome might epigenetically affect the beyond-childhood activation of the human genome's own lactase genes – a possibility mentioned in the middle of this microbiologist's writeup: https://www.cnn.com/2019/04/17/health/lactose-intolerance-mi... )