I think that's why we get so many posts from folks who don't have a clue as to why their britches are getting too tight when they exercise, after all! What I used to here a lot is "muscle weighs more than fat," so they think it's all muscle!
I used to train a girl at the Y who could not understand why she was gaining weight and I told her for a year to keep a food journal. She gained 30 pounds before she took me up on it, and found that she was eating 5000 calories a day!
Don't we all wish it were true! I would love to eat donuts 7 days a week!
"It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt." Mark Twain ;-)