7.1 What Are Calories?

A calorie (technically a kilocalorie, kcal) is a unit of energy. One kilocalorie is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius. When we say a food “contains 200 calories,” we mean that the chemical bonds in that food, when broken down by your metabolism, release approximately 200 kcal of usable energy.

Your body needs this energy for everything it does: pumping blood, breathing, thinking, walking, and—most relevant to this book—repairing and building muscle tissue after training. Every physiological process in your body runs on the energy extracted from the food you eat.

The energy content of food comes from three macronutrients, each with a different caloric density (known as the Atwater factors):

Macronutrient	Energy per gram
Protein	4 kcal/g
Carbohydrate	4 kcal/g
Fat	9 kcal/g
Alcohol	7 kcal/g

These values are averages. The actual energy yield depends on the specific food and how your body processes it (the thermic effect of food, which we’ll cover next). But these numbers are close enough for all practical purposes.

Alcohol is listed here because it does contain energy, but it is not a macronutrient your body needs. It is metabolised as a toxin, and its calories come with significant downsides that we will discuss in Section 7.4.5.