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Intermittent fasting no better than typical weight loss diets, study finds
(www.theguardian.com)
this post was submitted on 16 Feb 2026
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The one that says you cannot burn more calories than your body uses and you have to burn more calories than food you eat. It’s just tongue in cheek that the amount of energy in a (closed) system is conserved.
Of course one question is, does intermittent fasting somehow cause you to increase your base metabolic rate or cause you to digest your food less effectively per unit of food eaten, which could still satisfy thermodynamic constraints while still having an apparently larger effect. This study indicates that at a macro level, people do not have more success with this strategy vs traditional calorie restrictions, which do not support either hypothesis. They don’t disprove the hypotheses, but you don’t disprove such things, only support them. This doesn’t support them.
What you've stated is not a law of thermodynamics. The first law of thermodynamics, which is the one often misused to tell us that calories are the only thing that matter, states that within an isolated system, the total energy of a system is constant. It's well defined. The human body isn't an isolated system, and the laws of thermodynamics aren't tongue in cheek.
Our bodies don't burn calories, and you are right in saying that we do indeed eat food, not calories.
Fasting can, for example, deplete our liver's glycogen stores, and change the levels of various hormones in our body.
Sort of. Thermodynamics still definitely plays a role. You cannot have more calories than you ingest, and over time, you cannot perform more work than electrochemically possible; this is true precisely because of the laws described by thermodynamic constraints.
The laws of thermodynamics aren’t tongue in cheek. The poster saying you can’t escape the laws of thermodynamics I took to mean they’re making a tongue in cheek response; in other words, they’re sort of being witty and saying the reason this finding was observed is because of the fundamental laws governing energy consumption and use in the human body. That absolutely is rhetorically meeting the definition of tongue in cheek.
A calorie is a unit of heat energy. We cannot ingest a calorie since it has no rest mass. It is a ridiculous simplification of our biology.
We have to disagree on the wit of the poster.
Calorie is a unit of heat energy. The energetic yield of a gram of protein can be described by its ability to be burned to heat a gram of water, say. This is the definition of a calorie.
That energy is still constant, there is not some magical world where protein suddenly has more energy density in a human body than it did when it was burned in a calorimeter. That would break the laws of thermodynamics.
You can question whether there is metabolic advantage to consumption of certain types of food (that is, does the human body leverage certain foods in a metabolically more efficient fashion than others, such that consumption of the same calories but in a different composition results in differential weight gain or loss — this actually is studied in nutritional studies). But the laws of thermodynamics still apply there. The first that energy is conserved (kcal is fine for describing this as an upper bound) and the second concerning the free energy of a chemical system to perform “work”, which isn’t heating a gram of water (though it also sort of is, were warm blooded creatures) but rather describing the capacity of the substance to ultimately contribute to chemical processes in a cell, such as the generation or consumption of ATP.
And yes, every gram of food has a specific amount it can contribute to those chemical processes, and it’s tied to the total amount of energy in a gram of that material, which is conveniently calculated in a calorimeter.
Yes, this is a much better question. Our bodies do not seek energy, but the substances that are required for our biological processes.
Yes, our body temperature is regulated by our biological processes and at the same time the converse is also true. It's much more complex than doing 'work'. That would again be an oversimplification. Thinking of the body as a thermodynamic system like a bomb calorimeter is silly.
What is the error margin when measuring food? To save you the trouble, it's about 20-25%. Unless we compensate by considerably under-consuming the amount of food we eat, the sheer amount of noise alone caused by this error margin makes it meaningless.
Sure but then what is your alternative for easily assessing in a reportable way what the energy density of a food is? Bomb calorimetry doesn’t say “you will get this amount of energy from a food”, it simply says “a gram of this material has about this much energy density.” Evolution has done a remarkable job of maximizing energetic recovery from compounds, and it’s simply true that eating more energy in the form of food than is used by the body will result in the body storing said excess energy. Kcal is a convenient relative metric that does correspond to this phenomena. That is objectively true. Is it exact? No. Does it claim to be exact? Again no, but you can calculate the yield by looking at digestive and metabolic processes within constraints and the relative amount is still useful as a gross measure.
It’s fine to say that kcal aren’t what we eat. But then food isn’t really what we eat either. Food is simply compounds that our body can use to perform chemical work. You can quantify this work. And you can use a word other than work to describe chemical reactions, but the semantic point is conserved regardless.
A good proxy for this general capacity in a human body from organic digestible material is kcal. If you have an alternative to this that is easily calculated and also easily understood, I’d recommend writing it up and submitting it to a nutrition or medical journal, where it can be peer reviewed, and if it holds merit, published, to be more broadly examined, and perhaps adopted.
What I'm getting at is that measuring the 'energy' of food is meaningless. It's a waste of time at best, and when someone is mislead into thinking that they can eat 100 kcal of sugar (if we can even accurately determine that!) instead of nutrients then it can even be harmful.
In the human body? How do you propose to do this?
I'm pretty sure we generally eat food. We surely don't eat energy.
Let's start by not talking about energy at all. It's biology and chemistry.
The oxidation of sugar, which is accomplished in our bodies, can be directly measured. The end products are CO2 and H2O. If you fully combust sugar in a bomb calorimeter, the energy release is equivalent. So whether your body breaks apart something, or fire breaks apart something, they are calorimetrically equivalent, which is why people even bothered with kcals for food in the first place.
Edit: I should note just to head it off at the pass, aerobic oxidation in the body is about 35% efficient. You can measure this by looking at the metabolites formed, and the remaining 65% of the theoretical energy can directly be measured as heat, so when I say that whether our bodies do it or a calorimeter does it, I mean that the total energy is conserved because the laws of thermodynamics apply. If you eat more sugar your body will oxidize it and yield the same stuff, or possibly store it as fat. If you eat protein you can do the same measures, it’s just a slightly different process to enter the metabolic pathways. Fire is just a chemical process and reaction, after all, it’s not special. Your body also does chemical reactions.
Whether you see value in it or not, kcals observe a physical reality which has value within human cells and can be used to describe how human bodies interact with food in an energetic landscape. It does not purport to solve all of human nutrition, and nobody sane uses it for that. It is used as a gross representation of energetic equivalence, for which it serves its purpose reasonably well.
So again, if you believe your interpretation of human nutrition and biology and chemistry is novel or more useful, then submit to a journal your recommendations. Journals even accept review papers which are simply syntheses of existing papers, you don’t even have to conduct experiments. You can literally just go on pubmed and synthesize a paper and submit it for publication.
Don’t submit to a predatory journal though that uses a pay to publish format. Submit to an established, peer reviewed journal.
Conversely, just publish it on bioarxiv or something and then post it here for people to go to and review, though this is not as good as getting it peer reviewed.
If you do publish something, do post another comment here and I’ll review it.