When I was a child - seventy years plus back, yes I'm eighty - ADD was practically unheard of. I put that down to the fact that our food was plainwith virtually no added sugar. Even our intake of fruit and veggies was fairly restricted because there was a World War between my turning three and turning nine. And even after the war wide varieties of food were hard to get.
That was, of course, in England.
I might add that fat kids were few and far between. Those that were a bit plump were generally kids who had some sort of glandular problem - maybe one kid in fifty. It certainly wasn't through over-eating or lack of exercise.
Sweets were something you might get one or two of in a month. Deserts after a main course were minimal. Maybe once a week.
Kids lived outside, played their games, ran wild. Oh, and walked to school. Sedentiary pursuits were listening to the radio or playing board games in the evenings and mostly in the colder months when the daylight days were short.
Frankly, kids were not only heaps healthier they were tougher, more resilient. I rather suspect that 90% of the ADD that occurs today is because of our current lifestyle.