A brief history of calories in nutrition; a unit of energy

By Ricardo Adamo

Beyond the simplistic exterior, assessment of calories in nutrition (as a unit of energy) is fraught with error and confusion. The human body is not a heat engine…

The debate between calories and joules as units of energy in nutrition is more than a matter of terminology; it reveals deeper tensions between nutrition and its foundational roots in (bio)physics, chemistry, and biology. More than fifty years ago, both the International Research Council and the U.S. National Academy of Sciences issued a strong warning to the nutritional science community (Kleiber, 1972):

“Efforts by nutritionists to retain the calorie as the unit of energy will tend to isolate the nutritional sciences from the main body of advance in fields of physics and chemistry.”

That warning still lingers today.

Early theories of heat: Where the use of calories in nutrition began

The origins of the calorie for energy go back to the early 1800s, a time when scientific theories of heat were still evolving. The caloric theory, now obsolete, imagined heat as an invisible fluid that flowed between substances. It was in this context that French engineer Nicolas Clément introduced the term calorie to measure the thermal output of heat engines (Hargrove, 2006). The Calorie (with an uppercase “C”) was defined as the amount of heat required to raise the temperature of 1 kilogram of water by 1°C.

1 Calorie = 1000 calories = 1 kilocalorie (or kcal)

The rise of calories in nutrition

Energy as a scientific concept rose to prominence during the Industrial Revolution with the development of the steam engine. To understand the transformations of energy involved, a new discipline was born: thermodynamics, the study of how thermal energy converts into dynamic work. Somewhat belatedly, this curiosity was extended to muscle power – a source of work in animals and humans that predated the steam engine by millions of years. In 1780, Antoine Lavoisier made his famous observation, “la respiration est donc une combustion” (respiration is therefore combustion), noting that the energy released in the body through digestion and metabolism is equivalent to the energy released by burning the same food in a flame.

This concept of metabolism – how the body converts food into energy – is a natural extension of the first law of thermodynamics: energy cannot be created or destroyed, only transformed.

Physician and physicist Julius Robert von Mayer (1814–1878) was among the first to connect human physiology with these principles of energy conservation. Around the same time, British scientist James Joule (1842–1845) conducted experiments that showed how mechanical work could be converted into heat. His work led to a new understanding of energy itself and the formalisation of the first law of thermodynamics (Caneva, 2021). By 1850, Joule had calculated the mechanical equivalent of heat – approximately 4,150 joules per kilocalorie. This gave scientists a consistent way to translate between mechanical and thermal energy.

Although born in engineering, the calorie would later be adopted in biology and nutrition, where its meaning became tangled in assumptions that would prove to be limiting. Perhaps most notably, German physiologist Max Rubner adapted these principles to agriculture and animal metabolism, using bomb calorimetry to measure how much energy different feeds produced. Rubner famously proposed the isodynamic law:

“A calorie is a calorie”

Meaning that all calories, regardless of source, were treated as equal in energy value. This idea became the foundation for the Atwater system. But modern research increasingly shows that this assumption doesn’t just oversimplify how different nutrients are digested and used by the body; it’s fundamentally wrong!

From Rubner to Atwater: Calories in nutrition become code

Rubner’s work greatly influenced American chemist Wilbur Atwater, who helped develop one of the first comprehensive systems to estimate the energy content of food. Together, Rubner and Atwater measured the combustion heat of macronutrients and adjusted those values based on how efficiently the body digested them.

The results became what are known as the Atwater general factors, which are still used on nutrition labels today:

• 1 gram of carbohydrate ≈ 4 kcal
• 1 gram of protein ≈ 4 kcal
• 1 gram of fat ≈ 9 kcal

By the early 20th century, this simplified calorie-counting system had entered the public consciousness, thanks in part to Lulu Hunt Peters’ bestselling 1918 book Diet and Health (Yeo, 2021). It was one of the first to promote awareness of calories in nutrition to a general audience, solidifying the calorie’s role in policy, marketing, and popular diet culture.

Calories versus joules: A scientific debate

Over time, scientists began to question whether calories in nutrition (as a unit of energy) was still optimal, especially as other fields moved toward the joule, the standard SI (International System of Units) measure of energy.

In 1972, physician, and one of many founding fathers of nutritional science, Max Kleiber addressed this in one of his many landmark papers entitled ‘Joules vs. Calories in Nutrition’ (Kleiber, 1972). His writing offered a much more nuanced perspective:

“There is no valid reason to abandon the calorie as a unit of heat… The joule is recommended as a unit at a higher level of abstraction expressing energy per se, when different forms of energy (heat, work, electric energy) are added or subtracted.”

This was in response to a recommendation by the British National Committee of Nutritional Science, which had proposed: “The joule should be adopted as the unit for energy in nutritional work, and the calorie should fall into disuse.” Kleiber didn’t reject the calorie outright. Instead, he distinguished between specific forms of energy (such as heat, electrical, or chemical) versus abstract and unified measures like the joule, that allow scientists to calculate total energy flows across systems.

The cost of oversimplification

The persistence of the calorie in nutrition and public health has less to do with scientific accuracy and more to do with simplicity and convenience. Calories are easy to display on food labels, track in health apps, and communicate in everyday language. Although this ease comes at an extreme cost to accuracy of food energy assessment, it is one that has been overlooked, fundamentally misbalancing global health and food policies.

Oversimplification of calories as a unit of energy in metabolism

The popular formula “calories in = calories out” is appealing, but biologically misleading. The human body is not a heat engine. It’s a complex, adaptive system where energy is:

• Absorbed differently depending on the food source
• Processed differently based on the microbiome and hormonal influences
• Expended through multiple overlapping pathways

Calories in nutrition, as they are currently used, oversimplify this human dynamic and fail to account for energy losses, inefficiencies, and inter-individual variability.

The human body does not want to lose heat. The body has many goals, and it is not to gain or shed heat, but to maintain homeostasis – a stable internal temperature measured in degree Celsius. Heat loss or retention is simply the mechanism the body uses to adapt to its environment. If calories are used to capture total metabolism, what they are really measuring is inefficiency: energy from food that is not directed into useful biochemical work, but dissipated as heat. From the perspective of thermodynamics, this heat loss is inevitable in an open biological system. But from the perspective of nutrition, treating heat loss as the central unit of measurement reduces human metabolism to a study of wasted energy, not productive energy.

This is why the calories as a unit of energy in nutrition is conceptually limited. It was designed to quantify heat, yet heat is only one of many ways that food energy is transformed. The body also channels energy into chemical bonds, electrical gradients, mechanical work and more. Collapsing these diverse outputs into a single ‘heat unit’ isolates nutrition from the broader physical sciences, where the joule already captures all forms of energy.

Moreover, all nutrients are not equivalent: their metabolic fate depends on species, physiology, and individual lifestyle. Energy efficiency is shaped by underlying physical processes, such as the cardiovascular system, which is moulded by lifestyle more so than genetics. ATP yield from glucose varies with how many protons the electron transport chain pumps across membranes, and also with which electron carriers are used: for example, NAD⁺ in the liver yields more ATP than FAD⁺ in the brain.

In short, by measuring metabolic inefficiency as opposed to productive energy, calories reduce metabolism to a measure of wasted energy. Treating calories as the master unit of nutrition oversimplifies the adaptive, nuanced, and highly individualised ways that our bodies manage energy.

What are we even measuring?

This brings us to a set of unresolved questions that exists in nutritional science today:

• What is actually being measured in the units of nutrition?
• Are we tracking heat, work, or chemical energy?
• What are the correct units for these measurements?

Addressing these questions can effectively give nutritional science a foundational framework. That will be necessary if nutrition is to remain a science, and not just a system of conventions – notably, many scientists agree that nutritional science should align closely with the fundamental principles of biophysics.

Reconnecting nutrition with the physical sciences

While the calorie has historical significance, and possibly intra-disciplinary significance in nutrition, as a measure of energy it is the joule that belongs to the language of modern science.

The joule integrates seamlessly into the SI unit system; the standard units of measurement defined by the International System of Units (NIST, 2025).

• The unit of joules is derived from force (newtons), which comes from mass (kg), distance (m), and time (s).

In contrast, the calories as a unit of energy is disconnected.

The joule allows energy to be traced, quantified, and compared across disciplines: from biochemistry to astrophysics. The calorie does not. This conceptual isolation of the calorie is not just a measurement issue: it represents a philosophical separation of nutrition from the sciences that could help it evolve. As Kleiber (1972) put it:

“The major source of confusion among those who advocate to get rid of the calorie as a unit of heat seems the lack of understanding of the difference in rank of generalization and abstraction between a unit for a form of energy (such as heat, chemical energy, work, electric energy) and a unit for the abstract energy per se which represents all forms of energy by calculating all results to a common denominator”

His insight is key: we must differentiate between units used for measuring specific energy forms, and those used as universal representations of energy. Without this clarity, nutritional science simply won’t evolve, it will stay a calorie-centric model, isolated from the scientific body. Let’s take Figure 1 as an example. Where is the distinction between exogenous and endogenous food energy indicated, as described by Elia and Cummings (2007)? How is nutrient efficiency being measured, what is it being compared against, over what temporal and spatial scales, and using which methodology? These questions are critical because traditional ‘calories in, calories out’ models often oversimplify energy balance and fail to account for thermodynamic realities, substrate-dependent efficiencies, and tissue-specific energy transduction (Feinman and Fine, 2004; Hall and Guo, 2017; Howell and Kones, 2017; Ludwig and Ebbeling, 2018; Yeo, 2021).

Figure 1 – Forms of Food Energy and units

Energy in the human body has never been and will never be a simple ‘calorie count’. Food ‘energy’ is metabolised into multiple forms of matter, energy and information, not combusted into a single event of heat being released. By tracing these flows in joules rather than calories, nutrition can be reconnected to the same universal language of energy that already unites physics, chemistry, and biology. The cultural weight of the calorie in nutrition as a unit of energy won’t vanish overnight; it’s embedded in textbooks, apps, labels, and institutional policies. But scientific understanding is evolving, and nutritional science must evolve with it.

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