How the Calorie Entered the Kitchen
The calorie is such an ordinary word now that it is easy to forget it was borrowed. It began in nineteenth-century physics and chemistry as a unit of heat, defined by how much energy it takes to warm a fixed quantity of water by one degree. Nothing about that definition has anything to do with dinner.
What carried it into the kitchen was a change in what scientists were measuring. Once laboratories began burning dried food samples in sealed chambers and reading off the heat released, food itself could be described in the same units as coal or gas. Food became, on paper, a fuel.