Why was it decided to differentiate kcals from calories with a capital 'C'? It seems kind of odd to me.
1 Cal = 1 kcal = 1000 cal
What were the reasons behind that decision and who decided to that?
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The rocky history of the calorie is explained fairly well in
Does the history of food energy units suggest a solution to "Calorie confusion"? J. L. Hargrove, Nutr. J. 6, 44 (2007).
In short, it appears that
both the "small" and the "large" calories (the energy used to raise the temperature of 1 g and 1 kg of water by 1°C, respectively) were in use, and
there was no standard spelling for either,
for the long stretch of time between their initial uses in the first half of the 19th century until the systematization of unit systems in the mid-twentieth century. In particular, large stretches of nutritional science and the public were using the name Calorie (I believe for the large calorie) for several decades during the twentieth century, in parallel with a limited use of the small calorie by scientists and engineers.
By the time nutritionists sat down to sort things out in the 1960s, it was pretty much too late. They resolved to
The first three are easy, as they make the system confusing but at least it is unambiguous. The fourth point is a lot harder, because it requires the public to make widespread use of this weird kcal thing. (Moreover, it was already behind the times, and they should have switched to kJ while they were at it.) Because of these difficulties, the fourth point was not really completed.
In general, the use of the large calorie as a heat unit is pretty strongly discouraged, but this does depend on where you are. In the UK, for example, nutrition facts labels must indicate energy content using kcal and kJ (source). The USA regulations, on the other hand, are pretty ambiguous, and there probably are a lot of brands out there that use the large calorie in their labels to avoid big, intimidating numbers.
The small calorie or gram calorie (symbol: cal) is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie, nutritional calorie, Calorie (capital C) or food calorie (symbol: Cal) is approximately the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius. The large calorie is thus equal to 1000 small calories or one kilocalorie (symbol: kcal)
Simplicity for diets, not to carry so many significant figures which make no difference in dietary programs.