How does the adaption to diet/nutrition promote human biological diversity?

Answer 1

One example is that the Chinese have been for so many generations a non-dairy-eating people that mostly they don't nowadays have the means of digesting milk products that the rest of us do.

I doubt if it's been measured but I wouldn't be surprised if the Inuit, Sami and other arctic peoples have longer guts or different enzymes in their guts to cope with a mostly-meat diet than multigenerationally mostly- or wholly- vegetarian peoples such as the Brahmins of India.

Answer 2

Goodness. The only one that comes to mind is physical size. Regular access to plentiful and varied food resources during childhood growth leads to bigger people - within limits, I'd expect.