[dredmorbius]

Lactose intolerance principally derives from a failure of the human body, not the microbiome, to produce the enzyme lactase which breaks down the sugar lactose.

<https://en.wikipedia.org/wiki/Lactose_intolerance>

The gut biome may play some role, but it's secondary and limited, see:

<https://pmc.ncbi.nlm.nih.gov/articles/PMC8666824/>

All humans (and all mammals) produce lactase as infants and children, but many lose that capacity in adulthood. Several populations (Northern Europeans, some North Africans, and a few elsewhere) inherit a mutation which continues lactase production in adulthood. Many parts of the world, notably east Asia and the Americas (indigenous populations) lack that mutation and adults tolerate unfermented milk products poorly.

Fermented products (cheese, yoghurt, keifer, doogh, buttermilk) tend to have most of the lactate converted in the fermentation process, and tend to be better tolerated.