Simply put, metabolism is the body’s way of converting food into energy, and then using that energy to sustain and build the body. It takes a complex and interrelated series of chemical and physical processes to accomplish that. A simple way to understand your body’s metabolism? It works a lot like the engine of a car.

To get up and go, your body needs food, much like a car engine needs fuel. Your body’s engine is your metabolism. When you rev the engine—say, when exercising—you burn more fuel. And when your engine is a finely tuned metabolism, your body burns its fuel most effectively, and with less waste. That means less fat and greater fitness for you.

Fueling your system is a calories in/calories out proposition. The “calories in” come from food fuels: fats, carbohydrates, proteins. The body uses each of those fuel sources for different purposes, and excess calories are stored in a way that is unique to each fuel source. (Take fat, for instance: The body has seemingly unlimited capacity to store excess fats, and it does so in a decidedly bulky way.

The “calories out” are burned off through physical activity such as exercise, the digestion of food, and resting metabolism. The amount of calories burned via physical activity is largely up to you: your activity levels and your workout routine. The energy it takes to use the food you eat requires a nominal 10% of the total calories out. Your resting metabolism—meaning the amount of energy required to do all the things a body does to stay up and running, from the breathing you hardly even notice to the thinking you’re doing at this very moment—commands most of the calories out, at 60% to 70%. Learning how to maximize that number gets to the heart of New Leaf’s metabolic assessment.

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