I think that part of the reason for the "obesity epidemic" is that people are BOTH. Over the last 100 years (and especially the last 40), Americans have been told to eat the most remarkable collection of garbage imaginable; no wonder people don't have a normal appetite anymore.
At the end of the 19th century, it was fashionable to have a little "meat on your bones" and it was considered attractive. Not absolutely OBESE, like poor William Banting, but plumpness was definitely a good thing. It made one look healthy. In his early low-carbohydrate book, "Eat and Reduce," Dr. Victor Lindlahr recalled the showgirls of this era being downright ... big. He credits the First War and its aftermath for the social conditions which encouraged us all to shed our furbelows and want to slim down; things haven't significantly changed in that department.
Lindlahr's diet was different from Banting's in that it restricted calories, not just carbohydrate-rich foods. Still, those foods were REAL; nutrient-dense meats, eggs, lots of vegetables and moderate fruits, and he allowed saccharine in one's tea/coffee if desired. If he banned butter, at least he didn't encourage margarine.
The second war did more damage, in my opinion. In Europe there were real food shortages, and the switch from normal choices to make-do's that were actually available is thoroughly understandable. In America the situation was just plain flakey. I STILL don't understand the rationing situation; granted, the food-supply was screwed up by the "dustbowl" thing, but i find it hard to believe that it was THAT damaged. If there was enough food here before, why did it require that much MORE food just because our young men were eating it in a different venue and wearing a uniform?
Whatever the cause (profiteering comes to mind), people were encouraged to consume less meat, eggs, butter and so on, and more "fillers." There were even drives to collect used cooking fats for the war effort. All the "Allies" (as well as the Axis, i'm sure) became used to eating crap -- it may have been an unpleasant change, but it was considered patriotic in America and it was unavoidable in Britain.
At any rate, this seems to be when ordinary Americans started eating garbage on a large scale. Convenience foods began to be popular in the oh-so-modern 1920s, but the advent of television encouraged it beyond anything seen before. The work of the junk-food-manufacturers and their advertisers has only gotten more sophisticated and insidious since. People now believe snacking to be a normal and necessary activity ... with a drink constantly at one's elbow all day.
J Stanton has described and explained only too well how eating foods which provide incomplete nutrition promote obesity and weakness; additions from me are unnecessary. (If you somehow have missed it, go to gnolls.org and look at the left-hand column for links.) In a nutshell -- it IS VERY possible to be overweight and undernourished.
It's all about eating products instead of FOOD. No one food is making the whole world fat -- not even fructose -- but the tendency to eat products seems to lead in that direction.