“That's loaded with calories!", "Are you counting your calories?” You must’ve heard these sentences repeatedly. So, when people talk about the calories in food, what do they exactly mean?
A calorie is nothing but a unit of measurement. A calorie is a unit of energy. When you hear something contains 100 calories, it's a way of describing how much energy your body could get from eating or drinking it.
We usually tend to associate calories with food, but they apply to anything containing energy. For example, a gallon (about 4 liters) of gasoline contains about 31,000,000 calories. Scientifically it is defined as “the energy needed to increase the temperature of 1 gram of water by 1 °C.” One calorie is equal to 4.184 joules, a common unit of energy used in the physical sciences.
In most fields its use is outdated, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy. Food energy is the amount of energy obtained from food that is available through cellular respiration.
Like other forms of energy, food energy is expressed in calories or joules. The calorie is a very small measure of energy so the food calorie (kilocalorie, kcal), 1kcal = 1000 calories, is more often used and is what food packaging usually refers to when showing calorific value. 1 kcal is equal to 4.184 kilojoules (kJ). The kilojoule is the unit officially recommended by the World Health Organization and other international organizations. In some countries only the kilojoule is used on food packaging, while in others the calorie is the most common unit.