How do we know how many calories are in a food?

A Calorie on a food label is a unit of heat. The small calorie is the heat needed to warm one millilitre of water by one degree Celsius, and the food version is a kilocalorie, a thousand of those, which equals 4,184 joules. So the label's number is really answering a question: how much heat could this food give off? The first answer came from setting food on fire. A bomb calorimeter measures a food's gross energy by adding up the heats of combustion of its protein, fat and carbohydrate.

A body is not a furnace, though, because some of what you eat is never absorbed. The man who sorted this out was Wilbur Olin Atwater, an American chemist often called the father of modern nutrition research. He measured how much of each nutrient is digested and how much energy leaves in faeces and urine, and subtracted it. What is left is the energy your body can actually use. From his work it became common practice to use round numbers: 4 kcal per gram for carbohydrate and protein, and 9 kcal per gram for fat. Alcohol comes in at 7.

Atwater did not stop at the lab bench. With two colleagues he built the first calorimeter big enough to hold a person for days, and from 1896 they ran close to 500 experiments in it, measuring the heat a person produced while doing set physical activities. One result got him into trouble: his findings showed that people get heat from alcohol just as they do from carbohydrate. Atwater was prominent in the temperance movement, and he was upset when the liquor trade used information from his experiments to promote alcohol.

The honest caveat is that the number on the label is a tidy convention, not a measurement of you. Real foods vary, and even careful weighing in a lab is rarely accurate to better than about 5 percent.