The Origins: A Tale of Two Energy Units
To understand why calories remain prevalent, it's essential to look at the distinct origins of both the calorie and the joule. The calorie, derived from the Latin word calor meaning 'heat', was developed in the early 19th century, well before the SI system was established. French chemist Nicolas Clément defined the 'kilogram-calorie' around 1824 as the heat required to raise the temperature of 1 kilogram of water by 1°C. This definition tied the unit to a specific thermal property of water, making it a natural choice for early calorimetry experiments involving heat exchange.
In contrast, the joule is named after 19th-century English physicist James Prescott Joule, who established the mechanical equivalent of heat. The joule became the official SI unit for energy in 1948, defined based on mechanical work—one joule is the energy expended when a force of one newton moves an object one meter. This mechanical definition is universally applicable to all forms of energy, unlike the calorie's thermal-centric definition.
The Birth of the Nutritional Calorie
The American public's relationship with the calorie was largely shaped by W.O. Atwater. In the 1880s, his work on food composition tables and popular articles for publications like Century Magazine introduced the 'Calorie' (kilocalorie) to a broad, non-scientific audience. He leveraged the unit for public education, calculating the potential energy of food needed to sustain manual labor. At the time, the calorie was the only named energy unit available in English dictionaries, making it a practical choice for communicating nutritional concepts to the lay public. This historical use embedded the calorie deeply into American dietary consciousness long before the joule gained prominence as a universal standard.
The International Push for the Joule
As the International System of Units (SI) gained global traction, international scientific bodies, including the International Union of Nutritional Sciences, began advocating for the use of the kilojoule (kJ) for food energy. By the 1970s, the push for standardization was strong, with some countries making the transition a policy priority. The rationale was simple: adopting the SI unit would create a single, consistent measurement system across all scientific disciplines, from physics to nutrition.
However, a complete shift proved difficult. The calorie had decades of public recognition and was already the basis for existing dietary recommendations and food databases. A wholesale change to kilojoules was seen by many as disruptive and confusing for a population already comfortable with the term 'calorie'.
Factors Contributing to Calorie's Persistence
- Historical Inertia: Atwater's early adoption of the unit created a powerful legacy. Once ingrained in popular culture and policy, the term was difficult to dislodge.
- Public Familiarity: Most consumers grew up with calories on food labels and in diet books. Educating the public on a new unit, particularly one derived from mechanical work, was deemed too challenging.
- Ease of Calculation: For rough nutritional estimates, the numbers for kilocalories are often simpler. For example, the energy content of macronutrients (carbs, protein, fat) is 4, 4, and 9 kcal/g respectively, which is easier to remember than the corresponding kilojoule figures.
- Regulatory Decisions: Food labeling regulations vary globally. In the United States, the Food and Drug Administration (FDA) mandates the use of 'Calories' (referring to kilocalories), while permitting, but not requiring, kilojoules to be listed.
- Cultural Framing: The calorie became a central tool in diet culture, framing weight management as a simple equation of "calories in vs. calories out." This simplistic framework, while often misleading, cemented the calorie's role in popular thought.
Comparison: Calories (kcal) vs. Joules (kJ)
| Feature | Nutritional Calorie (kcal) | Kilojoule (kJ) |
|---|---|---|
| Definition | Amount of heat required to raise the temperature of 1 kg of water by 1°C. | Work done when a force of one newton moves an object one meter. |
| Origin | Derived from thermal studies and popularized for nutrition in the 19th century. | Based on mechanical work experiments by James Prescott Joule. |
| Standard Unit | Not an official SI unit; considered obsolete by many scientists. | The official, internationally recognized SI unit for all forms of energy. |
| Primary Use | Predominantly in nutrition and dietetics in countries like the US. | Used universally in science, including for food energy in many countries (e.g., Australia, EU). |
| Conversion Factor | 1 kcal ≈ 4.184 kJ. | 1 kJ ≈ 0.239 kcal. |
Global Labeling Standards
While the United States remains largely attached to the calorie, many other parts of the world have embraced the kilojoule. In the European Union, food labels are legally required to display energy in both kJ and kcal, offering consumers a bridge between the two systems. In countries like Australia and New Zealand, the kilojoule is the primary unit, with kilocalories often listed secondarily. This fragmented approach to labeling underscores the historical and cultural rather than purely scientific factors at play.
The Path Forward
Despite the calorie's flaws, including potential ambiguity between the small and large calorie, its deep cultural roots mean it is unlikely to disappear from nutrition discourse entirely anytime soon. However, a growing emphasis on nutritional quality beyond simple energy intake and the increasing global nature of the food industry may continue to push for greater standardization around the joule.
In essence, the reason why are calories used instead of joules is a story of history, consumer behavior, and convenience trumping scientific standardization in a single field. The calorie, though a remnant of an older system, has become a widely understood and practical tool for public health and dietary management.
For more information on the history of food energy units, see this article by PubMed Central.
Conclusion: A Unit of History
The enduring use of calories in nutrition, despite the scientific community's preference for joules, is a clear example of historical inertia and the powerful influence of public familiarity. Born from early thermal experiments and later popularized for broad consumer education, the calorie became the default unit for discussing food energy. While the joule represents a universal, modern scientific standard, the sheer effort required to re-educate the public and change decades of ingrained habits has meant that calories have remained a fixture on nutritional labels, particularly in the US. This duality—scientific correctness versus public tradition—explains why we continue to navigate two different energy systems when talking about the energy in our food.
