We think about calories as just a number on a food container, and it’s easy to forget that a calorie is actually a unit of energy. It’s the amount of energy required to raise the temperature of one kilogram of water by one degree. So how did they determine how many calories were in food in 1910? The article describes a pretty outrageous experiment:

In making these experiments a man is shut in an apartment that is sealed hermetically and supplied with air, food and drink…

While the man within is reading or writing or attending to the numerous small duties connected with the care of himself, for he must weigh himself, stripped and dressed, twice a day, must note his bodily temperature and keep track of many other conditions connected with the experiment, those on the outside are busy with a long programme of work that must be done regularly every hour of the many days that some of the experiments are under headway. For instance, the thermometer in most of the experiments is read every two minutes, and the reading set down carefully.

The air is removed, measured, and recycled. The subject’s heat emission is measured during work, rest, sleep, reading, etc. All of this is used to measure… something.

At this point the article loses me, because I’m not quite sure how you go from that information to the amount of calories in food. I have an easier time understanding how tests like this could be used to measure calories burned by various activities, and indeed tests like these are used today for that very purpose. But how you go backwards to the food he ate is beyond me.

Slate.com has a pretty good writeup about how we measure calories in food today.

SCIENCE MEASURES THE ENERGY STORED IN VARIOUS FOODS: Interesting Results Given from Recent Government Experiments with the Calorimeter (PDF)

From June 26, 1910