A calorie is a calorie in the sense that an inch is an inch and so on, but the caloric yield from foods can differ. 4 kcal per gram of carbs for example is an average. Take Dextrose for example, it actually yield 3.4 kcal per gram.. not a big difference from 4 yet if your in a hospital etting over time, it could. As far as foods go, a calorie is a measure of the potential heat or energy that the food contains but their are a variety of factors that determine whether the 'calories' will be made available for energy production or fat storage. Eat less, workout, but don't eat crap.