Is a calorie really a calorie?


People who know me, are aware that I’m a critic par excellence of the fundamentalism that some health professionals put on counting calories. Counting calories in a diet, neglecting nutritional composition does not seem the best way, much less the smarter way to achieve goals. Invoking the 1st law of Thermodynamics (TD), as some health professionals strongly like to do, can be dangerous and even incipient. After all is “a calorie really a calorie”?

Will invoking the 1st law of TD, justify only counting calories, neglecting the composition of macronutrients in a diet?

In this article I will try to explain the reasons which lead me to be skeptical about the calorie count in the human body as an instrument of great precision and scientific value.

For more boring that it may seem, there is no way to explain my point of view (and of many other authors) without getting into a little of thermodynamics and bioenergetics. If these two words do trigger some kind of histamine response in you, be prepared!

What is thermodynamics?

Thermodynamics is, in simple terms, the science that deals with the concepts of heat and temperature. TD has been very successful in explaining the overall properties of matter and its relationship to the mechanics of atoms and molecules. It is almost impossible to dissociate the development of thermodynamics from the development of the atomic theory of matter.1

Already in 1827, Robert Brown reported that pollen grains suspended in a liquid, had an erratic movement from one point to another as if they were in constant movement. It was Einstein in 1905, which would use thermodynamics to explain this erratic movement, later named Brownian motion.1

Brown’s observations, together with Einstein’s explanation, would ultimately allow detecting essential information on the molecular motion as also helped consolidating the concept of the atomic constitution of matter.1

For more complicated that it may seem, TD explains: how your refrigerator works, the engine of your car, or even how your soup bowl cools down! A complicated science that explains simple things…

To conclude, we can say that TD investigates the physical and chemical transformations of matter in all its forms: solid, liquid, gas and plasma.1

Bioenergetics, metabolism and biochemistry.

These disciplines are not the result of a sudden discovery, much to the contrary, were consolidated for centuries until they become the foundation of several important areas of human knowledge, such as nutrition, medicine, thermal physics, etc..

Lavoisier, with the discovery of oxygen as an oxidant (obviously at the time he didn’t call it oxygen), establishing the undeniable relationship between oxidizer and fuel, was the main figure in establishing the important concept of oxidation.2

The word oxidation that now applies to nutrients much is due to this French chemist. With the development of the atomic theory, this concept would be extended to all reactions in which a substance loses electrons, when combined with oxygen.2

This is, in fact, the main role of this gas in Biology, to be the universal acceptor of electrons.2 This is well demonstrated, furthermore, in the respiratory chain or electron transport chain.3

Cells rely on complex mechanisms to produce energy, as well as energy obtained through chemical reactions, called metabolism.3

In relation to metabolism, I believe that the simplest definition is that this comprises two basic movements, anabolism (responsible for the biosynthesis of macromolecules from smaller precursors using energy in the form of both ATP and NADPH) and catabolism (movement guided by the molecular degradation and destruction, in which molecules of larger size are therefore reduced to smaller molecules, usually having as final product CO2and H2O).

In mammals, these reactions often require the presence of O2.3 The withdrawal of energy from food comes not from the foods themselves, but from their chemical and molecular bounds.3,4

Biochemistry is the discipline that studies, in a reductive definition, the chemical reactions in organisms.

All these disciplines are linked, meaning, they are not independent or there is any kind of limiting border. Someone who has studied chemistry is well aware that in certain areas, chemistry and physics overlap, with the same happening with all disciplines of core science.

Bioenergetics refers mainly to the application of the laws of Thermodynamics in biological systems.4

The laws of Thermodynamics in organic systems.

There are four laws of thermodynamics. The zeroth law (law of thermal equilibrium), the 1stlaw of TD (the law of conservation of energy), the 2nd law of TD (which postulates that a system tends towards maximum entropy) and the 3rd law of thermodynamics (the enigmatic law that describes absolute zero).1

Although zeroth and 3rd law are interesting to understand bioenergetics, 1st and 2nd laws are the ones we really need to pay more attention.5


According to some authors,6 isocaloric diets may not lead to weight loss, for the simple fact that “a calorie is a calorie” no matter their origin, from carbohydrates, proteins or fats. They often justify this position by invoking the 1st law of TD (conservation of energy), using this to refute any scientific work that demonstrates metabolic advantage in diets with higher protein and lower carbohydrate content.

Many scholars justify this fact by saying that such work must be wrong because it would violate the 1st law of TD.6 What makes me more astonished in this position is the fact of refuting a good scientific study,7 in the absence of methodological, formal or experimental errors, for the simple fact that “supposedly” it does not lie on a physical principle.

There are credible scientific studies that explain and validate the metabolic advantage of low carb diets in weight loss, demonstrating that there are no inconsistencies with any physical principle.8 The physical principle is probably right, however, nothing prevents its interpretation of being wrong. This is precisely what happens in this case.

The 1st law, despite being a universal law, is clearly a theoretical law.5 It states that energy can change form, however the total is always maintained.1,3

The 2nd law, in turn, is a law of the dissipation. It is defined by a designated value entropy (S) usually associated with increased disorder or high probability.1,3

Entropy can be seen as energy in a system that is not able to produce work (W).3 All processes, whether they are strictly chemical or biological, tend to the maximum of S or disorder. In open thermodynamic systems (in which there are exchanges of matter and energy), it is literally impossible to quantify the changes in entropy (∆S), because these systems are rarely in balance.3

In human body it is the 2nd law which moves chemical reactions.3,5 As already mentioned, the 1st law is theoretical in nature, does not say whether the reaction will occur, even less about the kind of energy generated.5

According to the 2nd law, we conclude that in any irreversible process S will increase.3Entropy (S) is identified with irreversibility and the balance is not expected under these conditions.It is the 2nd law that will tell us whether a reaction is favorable or otherwise. In these circumstances ∆S>0 (entropy tends to increase).1,3

Due to the difficulty in quantifying entropy, a new concept was introduced in order to assess the available energy to perform work (W), Gibbs free energy (G).3

So this was written: ΔG = ∆H – T∆S

When ΔG> 0, the reaction is endergonic (not spontaneous), meaning that absorbs energy from the environment.

When ΔG <0, the reaction is exergonic (it is spontaneous), meaning that releases energy into the environment.

When ΔG = 0 the reaction is at equilibrium (which rarely happens in the human body, only at a transient state).

In order not to make this article too technical and hard to the general public, I will refrain myself from further developing pure thermodynamics. I found, however, important to explain these concepts in a very simplistic way, to make the reader understand why a calorie may not be a calorie, at all.

Metabolic advantage in low carb diets.

The work of Greene et alseems to prove beyond any doubt, that there are benefits in weight loss with low carb diets.

In isocaloric diets, how does the manipulation of macronutrients explain the weight loss? What happened to the lost energy? It cannot simply disappear! Remember the 1st law?! Remember that energy is conserved? There is nothing more dangerous than core science in the hands of pseudo-experts! Usually we point out the moon while they look at the finger. In this case, someone pointed out the 1st law, and they totally forgot the 2nd

By the way, the 2nd law appeared before the1st law.9 Regardless of important contributions being made ​​by Lavoisier in the 1st law, it might be fair to give some merit to that which I believe (along with Leonardo da Vinci) was one of the brightest minds of all times. I am referring to the inevitable Isaac Newton, who started the conservative approach from linear momentum.9

As already mentioned, several authors have shown plausible mechanisms for the metabolic advantage in low carb diets.8,10 According to them, the biochemical pathways involved in gluconeogenesis, together with increased protein turnover, may explain the missing energy. I couldn’t agree more! The human body is not a pump calorimeter.

In human body, substrate is interconvertible, and enzymatic synthesis also “costs” energy. Thus, the allosteric potential of a substance can lead to different energy outcomes. The ironic part is that the dietary intervention, which ignores macronutrients valuing only the calorific value of the diet, violates the 2nd law of TD.5

The thermogenic potential of different macronutrients, by itself, should give clues about the “mysterious” disappearance of energy in these “supposed” isocaloric interventions.

The thermogenic potential of a nutrient is, in summary, the amount of heat generated when being processed by the body. According to some authors,11 this thermogenic potential is approximately 2-3% for fats, 6-8% for carbohydrates and 25-30% for proteins.

Diets low in carbohydrates have, inevitably, higher protein content, you see now where this is going to end? The greatest mistake of the concept, “a calorie is a calorie” is precisely this idea.

The issue here, is that gluconeogenesis from amino acids in proteins (relax, I will not speak of the glucose-alanine cycle, and even less of the deamination of alanine to pyruvate), costs energy. It is well established in the scientific literature that the use of one mole of alanine requires about six molecules of ATP.3,12

If we look at Figure 1, we can easily understand that ΔG3 is not 0. Unlike the values ​​of the calorimeter, which provide approximately 4 kcal for carbohydrates and protein, our body makes other calculations. The conversion of protein into carbohydrates “costs” energy, so a calorie is not a calorie in the strict sense of a calorimeter.5



Figure 2 shows the theoretical metabolic advantage of reducing carbohydrate in the context of isocaloric diets. Notice that at 21% carbohydrates, you spend up to more 100kcal/day, when comparing with a diet with 60% carbohydrates. At 8% carbohydrates your losses are now around 140kcal/day. This theoretical graph was built considering the thermogenic potential, already mentioned in the scientific literature.11   


If in a pump calorimeter, proteins and carbohydrates are equivalent, in the human body this is not the case.

Over the years, several authors have drawn attention to this fact:

When one form of energy can spontaneously convert to another form, the “value” of that energy is degraded; the transfer of energy from place to place also “costs”. This is why perpetual motion machines do not work.


Christopher B. Scott, PhD in Essentials of Sports Nutrition and Supplements

Calories are a measure of the amount of heat produced when you burn food in a crude instrument called a bomb calorimeter. Your body is not a bomb calorimeter. It doesn’t burn anything. It Works by controlled nuclear power.


Michael Colgan, PhD in Sports Nutrition Guide

In the biological “machine”, the nature of the fuel, as well as the processes involved are absolutely crucial in the process of obtaining energy.


Reasons which lead me not to use calories as a prime reference:

1-The caloric values only refer to the calorimeter, and are not valid for the human body. The measures in calorific value are attained in a pump calorimeter at constant volume, where work (W) is 0. The volume in human body is not constant and W is not 0. It would make more sense to speak of enthalpy, using a calorimeter at constant pressure, and kJ instead of kcal. If there is volume variation, there is work, which seems much more appropriate to the human body.

2-Standard deviation is high, especially when it comes to measuring the calorific value of fats. Depending on the degree of saturation, the standard deviation reaches disturbing values, suggesting poor precision in the method.

3-You can only measure the energy expenditure in humans accurately from direct calorimetry. This method is mainly experimental, not being applicable to the “average customer”.

4-As already explained, calorimeters do not synthesize enzymes or have metabolism. A calorimeter makes direct combustion, nothing that resembles the human body.

5-The biochemical pathways and substrates are interconvertible also the biochemical reactions are not unidirectional. You can go from glucose to pyruvate in the same way that you can go from pyruvate to glucose (just an example). The reaction path depends on the substrate and also on several allosteric factors.

6-It is known that certain genes influence the synthesis of some enzymes (lactase and amylase are clear examples). By influencing the enzyme substrate, you are inevitably influencing metabolism. For this reason, the differences between individuals may be more or less significant.

In my opinion the dogma of calories was implemented in two ways:

1-It was necessary a guideline for nutritionists and health professionals. The simplification rule always seems desirable to those who lack critical sense. When you’re sitting in the dark, a light beam can really look like a lighthouse.

2-People, who really understand the concepts involved, spent too much time sending “messages” in scientific papers. In the end, they created a climate of conflict rather than an atmosphere of reflection.

In war, truth is the first casualty.

Aeschylus Greek tragic dramatist (525 BC – 456 BC)

With this article, I do not want you to stop counting calories, or embark on a hyperprotein diet. I just want you to realize, that the method itself has many limitations and contradictions.

Do not make a dogma out of the caloric value. Accept its limitations and make a useful and positive management of it, putting aside the fanaticism, of course.


Filipe Teixeira

Technical Manager, Body Temple Ltd.

The Tudor Bompa Institute, Portugal.

Nutrition & Performance Department of TBI.

The opinions contained herein reflect only the opinion of the author and not necessarily of Body Temple Ltd/Tudor Bompa Institute. Always consult your doctor or health professional before embarking on any supplement, diet plan or treatment.



1. Serway RA. 2 Physics for Scientists and Engineers with Modern Physics. New York: Updated Printing; 1996.

2. Teixeira P et al . Nutrição, Exercício e Saúde Lousã: Lidel; 2008.

3. Devlin TM. Textbook of Biochemistry with Clinical Correlations 7th edition. New Jersey: John Wiley & Sons; 2011

4. Wolinksy I, Driskell J. Sports Nutrition: Energy metabolism and exercise. Boca Raton: CRC Press, 2008.

5. Feinman RD, Fine EJ (2004). “A calorie is a calorie” violates the second law of thermodynamics. Nutrition Journal, 3:9

6. Bray GA (2003). Low-Carbohydrate Diets and Realities of Weight Loss. JAMA, 289: 1853-1855

7. Greene P et al (2003) . Pilot feeding 12-week weight-loss comparison: Low-fat vs. low-carbohydrate (ketogenic) diets. Obesity Research, II: A23

8. Feinman RD, Fine EJ (2003). Thermodynamics and Metabolic Advantage of Weight Loss Diets. Metabolic Syndrome and Related Disorders, I :209-219

9. Antonio J et al. Essentials of Sports Nutrition and Supplements. New Jersey: Humana Press; 2008.

10. Willet WC (2004). Reduced-carbohydrate diets: no roll in weight management? Ann Intern Med, 40:836-837 I

11. E. Jequier Pathways to obesity (2002). Int J Obes Relat Metab Disord, 26 Suppl 2: S12-7

12. Voet D, Voet JG. Fundamentals of Biochemistry 3rd Edition. New York: John Wiley & Sons; 2004.


4 responses to “Is a calorie really a calorie?

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