Comment by beagle3 | Hacker News Reader

Comment by beagle3 | Hacker News Reader

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The thermodynamics matter only as much as they are a tautology (which means: not at all), or as an upper bound.

Thermodynamics predict that you'd be better off drinking gasoline (12kcal/gr > fat's 9kcal/gr), and just as well eating wood chips (a carbohydrage) -- even though you get zero energy from either.

"But we're not using calorimeter data, we're using atwater factors"; Well, those actually DO vary from person to person, IIRC as much as 50%; and other things also matter (e.g., adding activated carbon to your food will result in less energy extracted in general; using Ally would result in less energy extracted from fat).

And that's just for the "calories in" part; metabolic rate can also vary as much as 80% along time and between people, depending on muscle volume, or other random, less understood factors - so without a proper measurement chamber you don't know the "calories out" either.

The food labels and general activity tables serve as a good general approximation that describes probably 80% of the public quite well. But invoking thermodynamics (except as an upper bound for calories in) is not justified by our current understanding of metabolism.

> But invoking thermodynamics (except as an upper bound for calories in) is not justified by our current understanding of metabolism.

The problem is when people claim that they can lose weight without running a calorie deficit. This is what's obviously wrong, and it is completely fair to invoke thermodynamics as a way to immediately and soundly refute such claims. When someone claims to have anecdotal evidence of losing weight while not running a calorie deficit, the error is not with the "calories in = calories out" equation but with their poor accounting of where the calories are going.

In my experience, people who promote their pet dieting theories (especially ones that promise dramatic weight loss without requiring exercise) using thermodynamically impossible claims are far more prevalent than people who mean lab calorimeter data when they talk about the calorie content of their food. Your comments about gasoline and wood chips are an insulting straw man, because nobody is ever referring to those numbers in a casual context. They're talking about the calorie numbers that can be found on nutrition labels, and those are meant to approximate metabolisable energy, not heat of combustion.

> The problem is when people claim that they can lose weight without running a calorie deficit.

Of course people can lose weight without running a calorie deficit. It's a commonplace experience nowadays, and absolutely trivial to demonstrate. You can test if for yourself if you are even slightly interested in checking the validity of your theories.

The death of the ridiculous thermodynamic model of human nutrition can't come soon enough.

I agree about the prevalence of pet theory, but my example wasn’t a strawman.

Your thermodynamic argument involves values that you take on faith (Atwater factors) which have been shown to have 50% variance, and values like BMR that also have 50% variance. As a result, invoking thermodynamic impossibility is not justified.

I agree with the gist of what you are saying - but not with the validity of the thermodynamic reasoning as proof.

> Your thermodynamic argument involves values that you take on faith (Atwater factors) which have been shown to have 50% variance, and values like BMR that also have 50% variance.

No, it really doesn't. Just look at the sibling comment that claims once again that it's possible to lose weight without a calorie deficit. That statement is wrong no matter how good or bad an approximation you have of what fraction of the heat of combustion is metabolisable.

If someone believes that they are losing weight while eating a diet whose nutrition labels indicate a much higher calorie count than their estimated calorie expenditures, then it's obviously wrong for them to claim that they are not running a calorie deficit. Instead, the reasonable and scientifically valid claim they could make would be that they are metabolising a much smaller portion of that diet than a normal person would, and thus they are running a calorie deficit because lots of calories pass through them unabsorbed.

Yes. Thermodynamic principles are tautological in the sense you just described and arguing against them isn’t helpful.

However, as the GP claimed the “gasoline is a strawman”, and “that’s not what people refer to when counting calories”; these two cannot live together with a thermodynamic argument in any useful sense, because it amounts to “well, if your food labels say a number higher than your exercise machine and BMR, the your numbers are wrong for reasons I cannot point to”, that’s about as helpful as a religious argument.

You can lose weight easily without caloric deficit in any sense - water weight (limited amount of course). There are several other ways in which the body loses fat - e.g. through the skin and scalp (negligible amount for most people, but not all people).

Invoking thermodynamics as an end-all be—all argument is dishonest even if it is a useful approximation.

It’s very useful when studying diets. There are meaningful differences between undigested calories, excreted calories in urine or Methane, and increased metabolism. For example increased metabolism resulted in extra body heat which needs to be removed.

Water weight is similarly important and still conserved in terms of energy. It might seem meaningless to you, but an athlete who wants to stay at absolute peak performance may want a more accurate picture of what’s going on.

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