Why Calories Count

I have run into a lot of nonsense recently claiming that calories don't matter in weight management for various reasons. As an engineer I disagree.

A calorie is by definition a unit of energy. A food calorie is actually a kilocalorie and equivalent to the heat energy needed to raise the temperature of 1 kilogram of water by 1 °C. 

Every food item contains energy locked away in chemical bonds. That energy is measured in calories. Once the food is ingested it is metabolized by the body to release the stored energy. Cells then capture and use this energy to fuel normal bodily functions. 

The reason you can't consume wood and coal, as one individual claimed should be possible if calories mattered, is the body can't metabolize wood and coal. 

The first law of thermodynamics states that when energy passes into or out of a system the system's internal energy changes in accordance with the law of conservation of energy. 

In other words if you ingest 2000 calories then your body just gained 2000 calories of energy. The body either uses the energy, expels it as waste or stores it. 

About 10% is expelled as waste and the rest is either used or stored as fat or muscle. It will usually only be muscle if your diet is high in protein and you're engaging in resistance training. For most people excess calories are stored as fat. 

If you consume fewer calories than the body needs then it will literally begin to consume itself to make up the difference. Typically it will first consume stored fat and then muscle. 

That's how it works. Everything else simply changes the rate at which a body consumes energy or affects the body's ability to metabolize and release the energy in the food consumed.