Marion Nestle, PhD, MPH : A calorie is a measure of energy. It's an amount of heat that you can measure and um, you can burn food in a machine that tells you how much heat it creates.It works exactly the same way in the body. And most people need between 2000 and 3000 calories a day.And an Oreo cookie has 50 calories, you eat two of them, that's a 100. You eat 10 of them that's going to be 500 and a quarter of your calories for the day. So you have to get a feel for it.