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What is a Calorie?

A calorie is a unit of energy, like the meter is a unit of length. Calories are the mostly adopted energy unit in nutrition, with most parts of the world including India adopting it. There are two calories in use today:

• small calorie or gram calorie, defined as the quantity of heat required to raise the temperature of 1 gram of water by 1°C¹. It is written as calorie with a lowercase c, abbreviated as cal.
• large calorie or food calorie of kilocalorie is 1000 times the small calorie — hence the name kilocalorie. It is written as Calorie with an uppercase c, and abbreviated as kcal or Cal. To put this into perspective, it's similar to a kilometre being 1000 times the meter.

The reason for two calories is two groups of scientists defining calories differently for their specific works. Here is the timeline of definitions²³:

• In 1819, Nicolas Clément defined calorie as the quantity of heat needed to raise the temperature of 1kg of water by 1°C.
• But in 1852, two scientists called Pierre Favre and Johann Silbermann defined calorie as the amount of energy needed to raise the temperature of 1g of water by 1°C.
• To distinguish between them, sometime in 1870s, another scientist called Marcellin Berthelot referred the first definition as Calories with an uppercase c, thus forming the convention of small and large calories.
• In 1894, Joseph Raymond proposed calling large calories as kilocalories since they are 1000 times the small calories (1 kilogram equals 1000 grams).

However, Raymond's proposition wasn't widely adopted perhaps because calories and Calories were indeed distinguishable in writings. Whatever the reason is, orally small and large calories were not distinguishable since letter cases aren't pronounced. Overtime, a convention was accepted in food and nutrition that calories orally refer to kilocalories.

Takeaway

So, the takeaway is that, in the context of nutrition and food science:

• when you hear the word calorie, it is referring to the large calorie or food calorie or kilocalorie.
• when you read "cal", it is referring to small calories
• when you read "kcal" or "Cal", it is referring to large calories.

Not all countries use calorie as the unit of energy (some use joules). Although not all of these countries implement the above convention as a rule in their nutrition domain, most countries including India do.

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