That is true but there's also science that says certain foods process and digest easier and faster and turn into nutrients and others that dont process easily and turn into junk that the body can't utilize. 1200 calories of protien isn't the same as 1200 calories of carbs or 1200 calories of sugar. you'll feel sluggish and not want to work out if you eat 1200 calories of junk food. good for you healthy food like veggies fruits and lean proteins like chicken will leave you feeling energetic and full of energy that doesn't make you crash.People lose weight when the expend more calories than they eat over a period of time.
That is the science behind it.