How are calories in food calculated?

Asked by: Gerard Henry, Birmingham

Calories are a measure of the energy content of food, and as such play a key role in the science of nutrition. During the late 19th Century, scientists began the laborious task of measuring the calorie content of food by burning it in a sealed container and measuring the heat released. While studying the results, a rule of thumb emerged: weight for weight, fat contains nine calories per gram, around twice that in protein or carbohydrates. This led to the so-called Atwater system for calculating the calories in food without lab tests: work out the proportions of fat, protein and carbs it contains, and multiply by the relevant ‘Atwater factor’ giving the calories contained in each component. But while it’s quick and cheap, there’s growing concern the Atwater system misses subtleties of how the body uses calories.

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