In Calories: Why eat healthy food? | MyFitnessPal.com

I am wondering if someone can touch on why people talk so much about eating "healthy" calories. I realize there is a reason behind it. But if you have 1500 calories a day, why does it necessarily matter where the calories come from if calories in=calories out. If you eat 1500 calories of macaroni & cheese, a Snicker's bar, and italian sandwich, why does it matter that it wasn't 1500 calories of eg ...Calories