Does anyone know why some people can seem to power down things like wheat and dairy without any negative effects?

I have a few mates who can eat nothing but pizza, chips, cereal and beer and be fine. Whereas I feel like crap if I eat stuff like that. I also have Swizz family who eat basically bread, cheese and wine and are really healthy. What's going on??

Is it to do with gut health and stuff like that? or is it more that their ancestors 'evolved' to digest these things better.