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Comment by belltaco | Hacker News Reader

> In 2016, Pontzer and colleagues published a study putting forward the constrained energy model: Energy expenditure does increase with more activity, but only to a point. Once physical activity gets really high, the body will adjust other components of the metabolism to keep your daily energy expenditure within a narrow range

How do people who constantly repeat the mantra of thermodynamics in such threads explain this? How is the body burning the same calories with increased physical activity? Not to mention that people can extract more or less calories from the exact same food and quantity consumed.

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A couple of non-exclusive theories from a non-physiologist:

Your body needs to burn less to maintain / manage the fat you are or rather were carrying around.

Your body may be engaged in various processes that it turns down. It's possible, for example, that it may be engaged in various anti-cancer activities that aren't necessary as your body fat decreases (higher body fat is correlated with some cancers, though I don't believe a a direct causal link has been established...but this comment is all speculation anyway). Or the excess calories could signal your body to do other "discretionary" processes that it dispenses with in time of calorie deprivation. It's even possible that such discretionary processes could be statistically bad for you in our current environment, which might be a reason why calorie deprivation may extend lifespan, at least in modern humans.

Your gut could expend less effort to extract certain nutrients or vitamins from your food in a low calorie regime ("bah, opportunistically grabbing that extra Vitamin E is not worth the effort right now").

Just some off the top ideation, not meant to imply that these ideas are necessarily what happens but are ideas why simple thermodynamics is inadequate on its own to explain the phenomenon. The body is more than a simple calorimeter.

That could be explained if you start with an inefficient body. If you suddenly start running 5K races, you'll likely won't finish the first one, and it will hurt a lot the following days. As your body becomes used to the new activity, you'll start finishing your races and getting less drained afterwards.

You're improving your fuel economy.

> How is the body burning the same calories with increased physical activity?

As someone who runs marathons, the answer to “how does the body burn the same calories with increased activity” is: It doesn’t. You will feel a marathon metabolically no matter what. And if you don’t eat like an absolute monster that day, you’re literally going to feel insatiable hunger for multiple days until you get those calories back.

Feeling hungry when your stomach is so full that you can’t take another bite is quite an experience I’ll tell you that.

And yes, your grocery bill does get higher when training for a marathon. It's very noticeable.

Most people who quote thermodynamics don't understand it. They are just saying calories in = calories turned to fat + calories burned. Which is true, but not helpful. The body has many choices, including burning it and then using sweat to get rid of the extra heat. Since we have minimal control over if the body will do this there isn't anything more thermodynamics can input into weight.

The only plausible explanation is that it's at a cost of other functions that are vital long-term but can be turned down temporarily? Such as reduced creation of new blood cells etc.

Couldn't your body burn more or fewer calories passively and release the energy as heat? Or you could make more or fewer micro movements throughout the day, in a way that adds up to whatever your body thinks energy expenditure should be?

Lowering body temperature would be precisely one of these things you can do short term to maintain the same calories expenditure but would be unhealthy if prolonged.

yes, both of these things happen! I don't have the studies at hand, I remember I read this as part of Slime Mold Time Mold series on obesity

This is it! Well based on what I have learned across books and podcasts, namely Dr. Andy Galpin's discussion with Andrew Huberman, Ultra Processed People and Racing Weight. The shutdown of these systems is believed to give them a break and is one of the reasons that regular exercisers have better health (just think why is exercise even healthy? It wouldn't seem so. Spike cortisol, burn a ton of energy, stress joints, jostle around the organs. Seems bad to me but of course its not).

Or you can find out for yourself. Touch your stomach during a run on a hot day. Its cold. Digestion is slowed and blood pulled away from the gut.

N of 1. But I do not eat much more on the days I run 15 miles on the trail vs days off. I do not need to due to these adaptations.

That the body is a complex buffered system of variable effeciency is not in dispute - nor is it intractable with the thermodynamic position.

It is always realistic to achieve a thermodynamic deficit, that there is some buffer or hysterisis to the curve (i.e non linearity) is not a big deal. The key insight is that the variability in the effeciency of your metabolism is not the bounding factor. A standard deviation of a few hundred calories in the natural variability of metabolism will meaningfully increase the difficulty of weight loss, but this simply means modulating the duration of your activity taking into account the diminishing returns of increased exercise and natural variability in metabolism. Now if this is psychologically or practically easy is an entirely different question.

Because it’s not. Thermodynamics is a law, not a political or mental thing!

My maximum effort in a day was an 8 hour off road bike ride. I burned 3500kCal just from the riding, as measured by a power meter. The body can’t even absorb enough carbs to replace that in the same time.

There is some difference in efficiency between people for muscular exercise - between 20% and 25% of energy burned actually gets converted to work.

And no people cannot extract more or less. They can absorb more or less in total for carbs - between 80 to 120 g/hr balanced between glucose and fructose is the limit for most people.

Have you ever seen a diagram of the Kreb's cycle?

This is just to say: cellular metabolism is stupidly complicated. There is so much we either do not understand yet, or have difficulty piecing together in a consistent way.

the most obvious anecdata is that for some female endurance athletes, menstrual cycles delay or stop altogether. that's one of many things the body can turn off to reduce energy expenditure.

The straightforward hypothesis would be "by reducing other body functions". For the duration of a Barry's Bootcamp class, my watch (probably over-)estimates my calorific expenditure at 1000 kcal. I know that right after that, I could probably not do too much Mathematics. Frequently, I encounter visual snow. But most of the time, I can do Mathematics.

And then there's healing, and saliva generation, and digestion, etc. I'm sure there's a lot of room for energy savings.

I’m like that too sometimes after a particularly difficult workout. I can’t make conversation or make sense of things I would normally have no trouble with (programming related).

I definitely notice brain fog when exercising heavily or on caloric restriction. It's like my ADHD goes into hyperdrive. Probably great for hunting down food on the savannah though.

In summary - the body adjusts. The increased hunger stimulated by the vigorous exercise will usually outweigh any calories burned off

Seems in line with other phenomena such as resting heart rate decreasing after sustained cardio training regimens. Your body just starts expending less energy during resting periods. This doesn't entail loss of function because the body is capable of operating in a wide variety of energy expenditure regimes

The body burns fewer calories later, like on immune system and digestion and other things

> constantly repeat the mantra of thermodynamics in such threads explain this?

As someone that took three semesters of thermo. I don't understand thermo in a deep way and they have a Randall Munroe School of Science understanding.

The answer is: leptin. I just wrote a post about it, which you may find intriguing: https://news.ycombinator.com/item?id=38942521

It says it right there in your quote. Your body neglects other stuff.

The only thing that makes sense to me is that the body gets more efficient.

That what I imagined as I was reading, the act of breathing and pushing blood around the body just becomes easier when you are fit. (requires less energy)

Your nervous system also becomes more efficient at zapping the right muscles at the right time. The effect of this can be pretty dramatic too. Even a slightly out of sync orchestra just sounds wrong.

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