One question that has always intrigued me is: why do calories work? A calorie is defined as the amount of energy required to raise the temperature of 1 gram of water by 1 °C (ignoring factors like starting temperature and atmospheric pressure for simplicity). To my understanding, the calorie content of food is determined by burning it in a calorimeter—an insulated, oxygen-filled chamber surrounded by water. The heat released from combustion raises the water’s temperature, and this change is used to determine the food’s caloric value.
However, this process seems completely disconnected from what actually happens in the human body. Digestion and metabolism involve a complex series of biochemical reactions, the details of which could easily fill an entire textbook.
So, how is it that counting calories works so reliably, to the point that essentially every diet ultimately boils down to this concept?
As background information, I studied physics and completed my PhD thesis in theoretical nuclear physics in 2012, though I have since left the field.
If I’m understanding the question correctly, you’re asking why do Calories predict weight change so well?
Short answer, I suspect that because energy balance is what governs body mass, any consistent measurement of energy would likely work. In this case, Calories or joules.
Long answer, W.O. Atwater introduced the Calorie to US audiences in an 1887 article in Century magazine. Note that he defined the Calorie as potential energy needed to support a given amount of physical work against gravity, which he calculated as foot-tons.
In explaining the Calorie, he indicated that the heat unit was equivalent to about 1.53 foot-tons mechanical energy. The measurements of energy from protein, carbohydrates, and fat are called “Atwater factors”.
About 50 years later, The Bureau International des Poids et Mesures (BIPM) proposed that a calorie should equal 1/860 watt-hours or 3600/860 joules.
A number of subsequent committees and professional recommendations have tried to banish the Calorie in favor of the joule, but Calorie persists nonetheless worldwide
As a measurement of energy given off as heat, Calorie seems to work okay…even in the body.
Hi Jordan,
Thank you for taking the time to provide such a detailed response.
My point wasn’t about which system of units to use, as that’s ultimately just a matter of convention. What surprises me is that the energy released from burning different types of food to heat water turns out to be such a reliable predictor of the weight a person gains from eating that food, despite the vastly different chemical processes involved. Yet, somehow, it clearly works.