ADHD has a strong correlation with parenting, or lack thereof. Also, bear in mind that it is a "disease" limited to North American and some of Wester Europe. As far as the Japanese are concerned, it does not exist. Stupid parents = stupider offspring, and (no offense) Americans really don't rank that high in the eyes of the world in terms of education and common knowledge. Sure, you guys do most things best and have lots of the most brilliant minds. But for each genius, there are thousands of Darwinian nightmares. Admittedly, you could argue Canada is catching up with the US in this respect, but then again we have had an American-esque conservative government that has wreaked havoc in our country during it's short time in power.
I blame them!
There is only so much you can do through diet. There are many other factors such as exercise and parenting. Omega 3 deficiencies effect like 100% of people in the world. To offset omega 6 you should be getting a 1:1 ratio of omega 3 to omega 6. North Americans tend to get infinite levels more of omega 6 which causes inflammation and all kinds of other issues.
ADHD medications are ampetamines, or amphetamine analogues and they funtion in all people the same way. Ritalin (Concerta too) is chemically almost identical to Cocaine but it lasts for 3 hours instead of 20 mins, and Dexederine, Adderall, vyvanse are all amphetamine salts. The reason why these medications work is because they would work no matter who took them ADHD or no ADHD. It is not some sort of miracle cure, but they do work to help the kids perform. It doesn't change what is causing these problems.
And since when have (North) Americans ever eaten well? Sausages with hashburns and corn with maple syrup is a balanced diet? Steak potatoes and peas and beans is a balanced mean? North Americans have always eating horribly and the rollercoaster effect on our insulin levels is what is responsible for the diabetes and obesity. Carbs from potatoes or coke are all the same in your blood stream. You are just slightly better off eating potatoes.
Nutritional retardation has been a long standing thing in our continent, but you are right that the level of processed and refined foods makes it all the more easier and rewarding to get simple carbs, transfat, and salt -But hardly the sole factor.
FWIW as a Canadian it blows my mind when I got down to Vermont. In US grocery stores the Cookie section is like the size of the entire vegetable section in a Canadian one and vice versa. The amount of store space dedicated to actual food to prepare is really small. Could be I just don't know where to go though, but it is very striking.