Studies don't show a 100% correlation with genetics. Although it's likely that everything you can imagine has an impact either for or against. This likely even includes parenting and how the child is socialized. Evidence of brain chemical differences is not evidence against that, because when we socialize it means chemicals are moving in certain ways in our brains since socializing is fundamentally a physical process involving sound and light waves coming in and interacting with mechanisms that have evolved in order to be influenced by the various patterns we use in our everyday lives. They have found that counselling for depression even without medication causes chemical changes in the person's brain that are similar to medication (and it works better, going off counseling doesn't lead to as many or as fast of a relapse as going off medication).
Yes, I do think that poor diet can cause ADD/ADHD to develop in children. So much of what we eat today have chemicals in them that it has to be having a negative effect on children. We should ask for more laws to control what is put in our food.
I do not think that a poor diet can cause ADD/ADHD to develop in children. I think that such claims are something that is being claimed by groups with an interest in drugs that are prescribed to treat ADD/ADHD. I think that the disease is something that doesn't even exist.
The conditions of ADD/ADHD are not the result of poor diet. I'm skeptical of most but not all of these diagnoses. What we would call ADD or ADHD is called being a kid in other parts of the world. Guess what society, children don't have a great capacity to pay attention, they never have and never will. Please stop drugging children just for being children.
I don't believe that a poor diet can cause ADD/ADHD to develop and I don't believe that there is any evidence of that happening, but I do believe that poor diet can greatly affect a child's ability to concentrate on what they are doing. This might get misdiagnosed as ADHD.