A calorie is actually a unit of measurement, sort of like a pound measures weight or centimeter measures length. What calories measure are the units of heat energy required to raise the temperature of a single gram or a kilogram of water by 1 degree Celsius. If calorie is written with a lowercase ‘c’ as in ‘cal.’ the unit measured is 1 gram of water. If calorie is written with an uppercase ‘C’ as in ‘Cal.’ the unit measured is a kilogram of water. To make matters a little more confusing, sometimes, this Cal. is simply referred to as a large calorie or is abbreviated to kcal (short for kilocalorie). And because that’s not bad enough, know that these measurement units relate to the metric system and as such, are now considered obsolete when discussing anything other than food and nutrition. The unit of energy in the International System of Units is the joule. One calorie equates to approximately 4.2 joules, while one Calorie equates to approximately 4.2 kilojoules.

We need not be overly concerned with the joule unit of measurement because Calories are still widely discussed in terms of nutrition. As such, it is not inaccurate to simply think of a calorie as the measure of energy that comes from food. Just know that when discussing food & nutrition, we are always talking in Calories (large calories), the energy required to heat 1 kilogram of water by 1°C. In fact, this Calorie is also sometimes referred to as the food calorie, and is what shows on food nutrition labels.

Your body derives its energy (or Calories) from the foods we eat. Any excess Calories ingested are converted to and stored as triglycerides in adipose (fat) cells. We will discuss our macros and how they relate to calories as well as metabolism and our energy systems further later. For now know that approximately 3,500 calories stored in the human body, equates to a single pound of fat. Each extra pound you store is carrying extra energy.