How Did Humans Come Up with Calories?

January 14, 2026 •

4 min read

The concept of measuring energy in food dates back to the 18th century, with initial experiments linking respiration to combustion. The journey to understand how did humans come up with calories involves a surprising cast of scientists and engineers, and was not a straightforward process, evolving significantly over centuries of scientific inquiry.

Quick Summary

The measurement of calories began with early calorimetry experiments in the late 1700s, evolving from a unit of heat for steam engines in the 19th century into a standard for food energy. Key figures like Nicolas Clément, Max Rubner, and Wilbur Olin Atwater developed the methods and systems that led to modern nutritional science.

Key Points

French Roots in Calorimetry: French chemist Antoine Lavoisier established the concept that animal respiration was a form of combustion, using an ice calorimeter to prove that living organisms produce measurable heat.
The First Formal Definition: The word "calorie" was formally defined in the 1820s by Nicolas Clément, initially as a unit of heat energy related to the efficiency of steam engines.
German Physiology Pioneers: German scientists, including Max Rubner, were instrumental in applying calorimetry to nutrition, determining the energy values of proteins, fats, and carbohydrates in the late 19th century.
Wilbur Olin Atwater's Impact: American chemist Wilbur Olin Atwater popularized the calorie for nutrition in the U.S. in the 1890s, using a respiration calorimeter to calculate human energy expenditure and developing the Atwater Factors (4-4-9) still used today.
The Shift to Modified Atwater System: The modern method for determining food calories uses the Atwater Factors in conjunction with chemical analysis of digestible macronutrients, providing a more accurate reflection of the energy the human body can absorb.

From Caloric to Calorimetry: The Early Pioneers

The idea that food provided measurable energy originated in the late 18th century with French chemist Antoine Lavoisier. He, along with Pierre-Simon Laplace, used an apparatus called an ice calorimeter to measure the heat produced by a guinea pig's respiration, proving it was a form of slow combustion. They correctly linked the energy of life to the physics of heat. While Lavoisier's "caloric theory"—which proposed heat was a weightless fluid—was later debunked, his foundational work in calorimetry paved the way for future discoveries.

The First Definition of the Calorie

The word "calorie" itself emerged from the world of steam engines, not dieting. Around 1824, French physicist Nicolas Clément defined the calorie as a unit of heat. His lectures, though unpublished, defined a "large calorie" as the heat needed to raise the temperature of one kilogram of water by one degree Celsius. The term was later adopted and formalized in physics and engineering contexts throughout the 19th century.

The 19th Century and the German Influence

By the mid-19th century, the focus shifted toward applying the principles of energy and metabolism to nutrition. The German school of physiology, centered in Munich, became a hub for this research.

Justus von Liebig (1840s): This pioneer developed a method for creating a concentrated beef extract to feed the poor, emphasizing cheap, energy-rich food and inspiring his students to measure the energy from food.
Max Rubner (1880s): A student of Carl von Voit, Rubner meticulously measured the heat of combustion for different foods, establishing that proteins, fats, and carbohydrates had different heat values. This foundational work directly led to the energy values we still use today.

The Atwater System: Bringing Calories to the Masses

The transition of the calorie from a scientific curiosity to a popular dietary metric is largely thanks to American chemist Wilbur Olin Atwater. After studying in Germany, Atwater began pioneering nutrition research in the United States in the 1890s.

Atwater and his team utilized sophisticated respiration calorimeters, large sealed chambers where subjects lived for days at a time. These devices measured the intake of oxygen, output of carbon dioxide, and the total heat produced by a person performing various activities, from rest to vigorous cycling. These experiments proved that the law of conservation of energy applied to humans and led to the creation of the famous "Atwater Factors".

The Atwater Factors

The Atwater Factors are the widely used averages for the caloric content of macronutrients:

Protein: 4 calories per gram
Carbohydrates: 4 calories per gram
Fat: 9 calories per gram

These factors, first published for a general audience in articles in Century magazine in 1887, made calorie counting accessible and became the basis for the nutritional information found on food labels. Atwater also conducted comprehensive food composition studies, which provided the first reliable nutritional databases.

Modern Measurement and Ongoing Refinements

Today, the energy content of food is still determined using principles derived from the Atwater system, though with a key difference. The original method, called direct calorimetry, involved burning a food sample in a bomb calorimeter and measuring the heat produced. However, this overestimates the energy our bodies actually absorb, as we cannot digest every component completely. The modern, or "Modified Atwater system," uses chemical analysis to determine the amount of digestible macronutrients and applies the established Atwater Factors, excluding indigestible components like fiber.

Comparison: Atwater vs. Modern Measurement


Feature	Direct Calorimetry (Original)	Modified Atwater System (Modern)
Method	Burns a dried food sample in a bomb calorimeter and measures heat directly.	Chemically analyzes food for macronutrients (protein, fat, carbs) and applies Atwater Factors.
Accuracy	Overestimates actual caloric availability as it counts all combustible material, including indigestible fiber.	More accurately estimates digestible energy by focusing on usable macronutrients and excluding fiber.
Application	Initially used in research to establish the energy values of specific foods.	The standard method for calculating nutritional information on food labels today.
Result	Provides a total potential energy value.	Provides a more realistic and usable net energy value for humans.

The Takeaway: From Lab to Label

The journey of the calorie shows how a unit of measurement created for steam engines was adapted and refined by scientists to understand the human body. From Lavoisier's early observations linking respiration to combustion to Atwater's systematic approach to food composition, each step built upon the last. While modern science continues to explore the complexities of metabolism beyond simple calorie counting, the Atwater system remains a foundational and practical tool for nutrition. The simple number on a food label represents a deep and complex history of scientific innovation and human curiosity.

Conclusion: The Evolving Science of Energy

Ultimately, humans came up with calories through centuries of scientific inquiry driven by curiosity and necessity. The calorie began as a purely physical measurement of heat, evolved to quantify the energy in food, and was ultimately popularized to help people understand nutrition and manage their health. The story is a testament to the scientific process—beginning with a simple observation, expanding through rigorous experimentation, and culminating in a practical system that continues to shape our relationship with food today. The transition from direct measurement to the modified system reflects a deeper understanding of human biology and metabolism, ensuring that the calorie remains a relevant and useful tool in a constantly evolving field.

Frequently Asked Questions

While Antoine Lavoisier first linked respiration to combustion in the 1780s, the first recorded definition of the word "calorie" as a unit of heat energy was by French physicist Nicolas Clément around 1824.

A calorie (with a lowercase 'c') is the energy needed to heat one gram of water by one degree Celsius. A Calorie (with a capital 'C'), often used on food labels, is actually a kilocalorie, or 1,000 small calories.

Early scientists used a method called direct calorimetry, which involved burning a dried food sample inside a sealed chamber called a bomb calorimeter. The heat released during combustion was used to calculate the energy content.

The Atwater system was developed by American chemist Wilbur Olin Atwater in the late 19th century. He established the calorie values for macronutrients that form the basis of modern nutritional labeling.

The need for a unit of food energy arose from a desire to scientifically understand nutrition and metabolism. Quantifying the energy in food allowed for the creation of nutritional tables and informed public health efforts related to diet and disease.

The calorie counts on food labels are based on the Modified Atwater System, which provides a reliable estimate of digestible energy. However, individual variations in metabolism and gut bacteria mean the exact energy absorbed can differ from person to person.

The joule is the standard international (SI) unit of energy used by scientists. The calorie persists primarily in common speech and public nutrition education, particularly in the United States, but many countries use kilojoules on food labels.