A Calorie Isn’t a Calorie(?)

Ever since Nicholas Clement defined the calorie as a unit of heat back in the 19th century, we’ve used it as a measure for the energy available to our bodies in the food we eat. The so-called “kilogram calorie” (or kilocalorie), which you see on nutritional labels, equates to the energy needed to raise the temperature of one kilogram of water by one degree Celsius.

These capital-C Calories serve as an approximation for the amount of energy we take in from our food, as well as the amount of energy we expend over the course of our day, through a combination of essential life-processes and any additional physical or mental activity we add on top of the baseline.

Later on in the 19th century, chemist Wilbur Atwater used oxidation reactions to test the energy content of various nutrients, inclusive of corrections for rates of digestion and the production of urea. Atwater’s values, roughly 4 calories per gram for protein and carbs and 9 kcals/gram for fats, remain in use today.

Lately, however, there’s been a trend towards rejecting this model. Not only are calories thought to be insufficient — or outright irrelevant — in explaining the continuing rise in obesity, but the deeper reason is that “a calorie isn’t a calorie”.
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