Your question assumes that every calorie is identical and the body metabolism is totally compartmentalized. "Metabolism" means all the chemical reactions within a body. Not just the reactions that turn food into fuel and fuel into motion, but all of them. There are thousands of chemical reactions within the body that are happening in a continuum and they affect each other.
Here is an example: alcohol contains approximately 7kc/g. If you drink 1oz of (200 proof) alcohol, you are consuming nearly 200 calories. But alcohol suppresses the kidney chemical function that creates concentrate urine. So even if you are dehydrated, you are peeing like a racehorse when you imbibe too much. That reduces you "weight" temporarily because you are dehydrated. Those calories go in and they have a specific chemical property that creates an additional reaction. They cannot be separated, those 200 calories are not the same as 200 calories worth of kale or steak or even pasta. We can't go by the calories in/calories out model for long term health. Sure, you could consume 10 shots of pure grain alcohol for your 2000 calories per day, but the effect it would have on your body would be far different than 2000 calories of real food. Alcohol is an example from a basic a&p class, when you delve into more intricate biological study, you'll see much more. It shouldn't be hard to extrapolate that different macronutrients affect the human body differently in different ratios.
Sent from my DROID4 using Marks Daily Apple Forum mobile app