Sam pulls into the gas station and up to the pump. He selects the fuel grade and begins to fill up. When his tank is full he gets in the car, turns the key to start the engine which combines the gas with air, a spark ignites it and the combustion releases the energy that allows the car to go. (At least I think this is how it happens…it's close enough to make my point). Sam is fueled up and on his merry way to engage in the rest of his day.
It's not all that different from how food works as fuel in your body. When our tank (stomach) runs low on fuel (food) we need to fill back up. The macronutrients are the combustibles – the carbohydrates, proteins and fats that yield energy from their calories. That's what a calorie is – the unit of energy for your body.
To keep your engine going to must fill up at regular increments. Running on fumes frequently will cause your body to lower its metabolism because it is inconvenient and inefficient for your body to use backup sources of fuel.
Next time you are approaching empty stop and fill up.