A half-eaten jam doughnut

Killing off the calorie

Find out more about the history of this term

Food packaging lists how many ‘calories’ are inside, but scientists consider this term obsolete, opting instead for the kilojoule (kJ) when discussing energy. How did the term fall from favour? Jennifer Trent Staves chomps through the history of the calorie.

Your chocolate bar’s wrapping says it contains 278 calories. What does that actually mean?

In scientific terms, the calorie (kcal) is the amount of energy necessary to raise the temperature of 1 kg of water by 1 degree. The term was first coined as a unit of heat in 1825 in France, but it wasn’t until 1896 that Wilbur Olin Atwater locked a student into a chamber and measured thermal energy fluctuation as the student ate various foods and performed various tasks. He also burned the food, measuring the ash and heat produced. Three years later, Atwater began publishing tables assigning specific energy values to certain foods and activities – one experiment even set out to find “how far a man could ride a bicycle on one egg”.

The calorie concept quickly caught on with the general public, it but also became a political tool. Governments now understood exactly how many calories a person needed to do a specific job, for example.

The unit was used during World War I to identify the food that was both the easiest to transport and the most calorie dense – the cheapest foodstuff to keep the soldiers ‘fighting fit’. Bread, therefore, became the carbohydrate of choice. Calories were instrumental in rationing back on the Home Front, too. And later, according to Nick Cullather in ‘The Foreign Policy of the Calorie’, the calorie enabled world hunger to be quantified.

However, these caloric calculations are one-dimensional. When vitamins were ‘discovered’ in the 1920s, it spelt the end of the calorie being the sole way of assessing a food’s value. The concept of the ‘empty calorie’ – found in high-energy foods that lack additional nutritional content, such as many types of fast food – was born. Today, nutrient-dense foods are considered a healthier choice than those containing empty calories.

Despite these findings, ‘calorie counting’ is still a popular form of weight management. The methodology behind this process is ‘calories in minus calories out’. A pound of flesh contains approximately 3,500 calories, so a person needs to consume 500 calories a day less than he or she uses to lose one pound in a week. This method of dieting is still popular, alongside others that focus on limiting particular nutrients, such as low-fat or low-carbohydrate diets.

Calorie confusion

The term ‘calorie’ is in the everyday vernacular – but the terminology can get a little sticky when investigated more closely.

What we know as a ‘calorie’ on most food packaging is actually a kilocalorie (kcal). According to Dr James Hargrove at the University of Georgia in the USA, calories are used to describe food mainly because they were the only named energy unit in English dictionaries in 1897, the time of Wilbur Olin Atwater’s experiments.

However, you can convert kilocalories to the SI unit for energy, the joule (J). One kilocalorie is equal to 4.184 kilojoules (kJ), or 4,184 joules. The joule is the preferred unit after scientists’ adoption of the SI unit system in the 1950s.

Scientists have suggested the calorie be discarded as a term because it cannot be derived from base units. Since 1970, it has been advised that kilocalorie should not be used in scientific publications. According to Hargrove: “The only valid use of the calorie is in common speech and public nutrition education. To avoid ongoing confusion, scientists should complete the transition to the joule and cease using kcal in any context.”


Further reading

About this resource

This resource was first published in ‘Food and Diet’ in June 2011 and reviewed and updated in August 2016.

Physiology, History, Biotechnology and engineering
Food and Diet
Education levels:
16–19, Continuing professional development