The fact that most of the discussions I encounter on a daily basis never address this is maddening.
I blame it on our cultural ideals stating that everyone is equal. As Americans, we have severe reservations about claiming people are genetically different.
Seriously though, this is a core part of dietary concerns, it's almost a certainty that a Russian is going to have a slightly different reaction to foods than a European, due to centuries of eating different foods. I think it's only recently we've really started to discover that evolution isn't just survival of the fittest, but a considerably more complex mechanism that has immediate impacts after very few generations to very slight stimuli. Not to mention stuff like gut flora, environment during pregnancy, etc.
It's hard to study diet's effect on health even assuming the effects are the same for everybody. Either it's a population study with all sorts of uncontrolled variables and uncertain data, or it's a controlled experiment which only lasts for a short time. Adding genetic factors makes it even more complicated...though if it increases the effect size enough, maybe it'll help.
This is a very key point. We're just beginning to scratch the surface on how genetics and diet interact.