That is something lots of old timers will tell you. (I'm pretty old so I can say that! It was a common attitude 40 years ago when I got my first chickens) It's right up there with putting a knife under a mattress to cut the pain of labor during childbirth. It is something that has been proven totally wrong by modern nutritional science, even years ago some people disputed it and I learned better in 4H thank goodness!
Corn has lots of carbs, sugar and starch mostly. What happens to any critter that eats lots of sugar and starches? They get full, eat less protein and get chubby and lazy, less healthy, hair (feathers) often gets dull and thin, and generally less healthy. If they get chubby, they suffer more in the heat too, which maybe is a little of the reasoning behind the 'heating them up' theory I'm not sure. It doesn't do that directly, it just is a side symptom of their being too fat.
They will survive on corn if that's all you can afford to feed them, but they won't lay well, and will be fattier if you are using them for meat. It's a yummy treat for them, a couple handfulls for them to scratch around in the bedding of the coop or out in the yard is fine, never free choice like layer feed though. I also mix in some Black Oil Sunflower seeds in scratch or corn too, they love em as much or more than the corn and they're better for them.
I'll step down off my soap box now.... sorry bout the near rant.
