I have a lot to say about this topic, especially as regards children and 
diet, but as far as the nature or nurture (in this case genetics or diet) 
there is no such thing as either/or.

Some people may have such a poor genetic lot that they'd still have 
problems no matter how good their environment is. Others may have such good 
genetics that it doesn't matter how bad their environment is - they will be 
fine. It's the old story of "my grandpa used to smoke three packs of 
cigarettes a day and he never got lung cancer." He was just lucky. Most of 
us lie somewhere in between the two extremes.

As far as our children go, we aren't in a position to know the genetic hand 
they've been dealt. Therefore we should feed them as well as possible.