4 ms·

> Humans aren't ideal furnaces and food/drink energy content isn't just its temperature output when burnt in a furnace. Yes we are. We get all our energy from

by kmm 10y ago

> Humans aren't ideal furnaces and food/drink energy content isn't just its temperature output when burnt in a furnace. Yes we are. We get all our energy from oxidizing reduced carbon. The chemical equation is exactly the same and thus the energy produced must be as well. Burning a gram of sugar will yield 4 calories of heat, whether in a furnace or in a mitochondrion. And because the waste heat is useful in keeping us warm, it's a 100% efficient process. The sugar, fats or protein you ingest cannot simply stay in your body, and it's not leaving you either (feces has no nutritional value and unless you have diabetes your urine won't be sweet either). So if your body is already at the right temperature, it is going to be stored as fat. There is simply no other way for it to go! There is no significant interperson difference in metabolisms. Any difference would raise your body temperature, which is what banned dieting aids used to do (https://en.wikipedia.org/wiki/2,4-Dinitrophenol https://en.wikipedia.org/wiki/2,4-Dinitrophenol) Your comparison to phlogiston is frankly insulting. As a physicist, I'm not too fond of the practices in nutrition either, but this is just basic biochemistry that has been known for decades.

WorldMaker 10y ago
«Your comparison to phlogiston is frankly insulting.» That's why I like the comparison: it's designed to be insulting to anyone that has studied science. It's also a pretty good summary, I feel, of the state of nutrition "science". Key foundational figures of nutrition "science", especially in America, were fellows like Mr. Kellogg that very much were phlogiston-based snake oil salesmen, and we still use a lot of the same methods to view nutrition, just slightly updated. So in your analysis we have a black box system where the body is a machine (which you claim to be highly efficient) whose inputs are only food and outputs are merely heat, feces, urine, and stored fat. Looking at even just the Wikipedia page on Biochemistry [1], and assuming I have no further training on this subject, this analysis seems to me to be such a gross oversimplification of a lot of very complex processes. Can you honestly tell me, as a person of physics, that this doesn't look like the biochemical equivalent of the "spherical cow fallacy"? Assume the body is a furnace / assume the cow is spherical. If we want to cherry pick simplifying assumptions, allow me to bring up the toroidal simplification of the human body. The toroidal simplification suggests that the human body can be simplified to a donut: the gastrointestinal tract being the "hole" in the donut. This assumption is equally "silly" to the "spherical cow", but I think is also a useful simplifying assumption to point out some immediate holes in bare CICO "furnace" assumption. From the toroidal point of view, the GI microbiome is "outside" the body, it's complex signals and hormone productions and biochemical processes are definitely inputs into the body. That brings up the question of how much input the microbiome on the epidermis/skin may input/output, and the obvious other factors that your simplified model above fails to account for outputs such as sweat and exhalation... Yes, calories can be a useful approximation model, but I have a hard time believing its the best possible approximation model science can build for us. [1] https://en.wikipedia.org/wiki/Biochemistry https://en.wikipedia.org/wiki/Biochemistry
- kmm 10y ago
  When considering just the energy balance, calories aren't an approximation. You see, energy is what we call a state function, it depends only on the state of the system and not how we got there. The catabolism of lipids, sugars and proteins into carbon dioxide, water and urea is an extremely complicated process, which I'm not sure we've even completely mapped yet. Last time I counted, the oxidation of glucose in our body takes about 33 steps. However, because we know the difference in energy (actually enthalpy but let's not split hairs) between the input and the output, we know the exact total amount of energy released in all those steps. And it has to be exactly the same as the energy released when simply burning the food in a furnace, because even though the process is wholly different, the begin and end products are the same. This is barely nutrition, this is just basic thermodynamics. Again, because energy is a state function, no assumptions need to be made about the efficiency of any of the intermediate processes or about how the muscles/mitochondria/liver/kidneys/... work, energy conservation keeps us from having to mess with that. The reason this analysis is so useful is because we're an endothermic species. The core of all of our bodies are at pretty much the same temperature. And as anyone who's had to pay a heating bill knows, heat isn't cheap. In fact, the bulk of our food energy is expended on keeping us warm. This is why there are no such things as slow or fast metabolism. If your metabolism were too slow to burn your resting energy expenditure in 24 hours, you'd die of undercooling. If your metabolism were significantly faster than normal, you'd die of overheating (or at least you'd feel significantly warm to the touch, be constantly sweating/flushed). In fact, a person's resting metabolic rate can be accurately predicted from their body composition [0], and doesn't differ between age or gender, as would be expected. This puts a pretty high lower limit on one's energy expenditure, absolutely nobody can maintain their weight on, say, 600 calories per day. You simply can't keep a 70 kg sack of mostly water at the right temperature with that energy budget, not with how badly isolated we are. Even if it does seem oversimplified, it's no spherical cow. The energy balance is exact, because of basic physics and biochemistry: we have only one way of generating energy (oxidizing reduced carbon), only one way of storing energy for the long term (long chains of reduced carbon), no energy is lost (because heat generation is a 100% efficient process) and our digestion is very efficient (not feces, urine nor sweat contain significant amounts of carbohydrates, lipids, protein or any other high-energy compound). It's also useful because we have an unflexible lower limit to the amount of energy we expend in a day because of the way our bodies work. The wiggle-room on the previous assumptions only serves to make calories-in smaller (e.g. malabsorption) or calories-out larger (e.g. exercise), and thus would only make it easier to lose weight. I won't deny the microbiome is important. There is a probably a correlation between gut biome and obesity. But that can only because it influences things like satiety or mood, because none of those bacteria seem to change the eventual energy equation, nor do they change how much heat you need to produce to keep warm or energy expended when lifting a weight. They're relevant because willpower is a finite resource, but they don't change the fact that any diet will in some sense need to consist of eating less calories than you burn in a day 0: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3139779/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3139779/

by kmm 10y ago

WorldMaker 10y ago

«Your comparison to phlogiston is frankly insulting.» That's why I like the comparison: it's designed to be insulting to anyone that has studied science. It's also a pretty good summary, I feel, of the state of nutrition "science". Key foundational figures of nutrition "science", especially in America, were fellows like Mr. Kellogg that very much were phlogiston-based snake oil salesmen, and we still use a lot of the same methods to view nutrition, just slightly updated. So in your analysis we have a black box system where the body is a machine (which you claim to be highly efficient) whose inputs are only food and outputs are merely heat, feces, urine, and stored fat. Looking at even just the Wikipedia page on Biochemistry [1], and assuming I have no further training on this subject, this analysis seems to me to be such a gross oversimplification of a lot of very complex processes. Can you honestly tell me, as a person of physics, that this doesn't look like the biochemical equivalent of the "spherical cow fallacy"? Assume the body is a furnace / assume the cow is spherical. If we want to cherry pick simplifying assumptions, allow me to bring up the toroidal simplification of the human body. The toroidal simplification suggests that the human body can be simplified to a donut: the gastrointestinal tract being the "hole" in the donut. This assumption is equally "silly" to the "spherical cow", but I think is also a useful simplifying assumption to point out some immediate holes in bare CICO "furnace" assumption. From the toroidal point of view, the GI microbiome is "outside" the body, it's complex signals and hormone productions and biochemical processes are definitely inputs into the body. That brings up the question of how much input the microbiome on the epidermis/skin may input/output, and the obvious other factors that your simplified model above fails to account for outputs such as sweat and exhalation... Yes, calories can be a useful approximation model, but I have a hard time believing its the best possible approximation model science can build for us. [1] https://en.wikipedia.org/wiki/Biochemistry https://en.wikipedia.org/wiki/Biochemistry

kmm 10y ago
When considering just the energy balance, calories aren't an approximation. You see, energy is what we call a state function, it depends only on the state of the system and not how we got there. The catabolism of lipids, sugars and proteins into carbon dioxide, water and urea is an extremely complicated process, which I'm not sure we've even completely mapped yet. Last time I counted, the oxidation of glucose in our body takes about 33 steps. However, because we know the difference in energy (actually enthalpy but let's not split hairs) between the input and the output, we know the exact total amount of energy released in all those steps. And it has to be exactly the same as the energy released when simply burning the food in a furnace, because even though the process is wholly different, the begin and end products are the same. This is barely nutrition, this is just basic thermodynamics. Again, because energy is a state function, no assumptions need to be made about the efficiency of any of the intermediate processes or about how the muscles/mitochondria/liver/kidneys/... work, energy conservation keeps us from having to mess with that. The reason this analysis is so useful is because we're an endothermic species. The core of all of our bodies are at pretty much the same temperature. And as anyone who's had to pay a heating bill knows, heat isn't cheap. In fact, the bulk of our food energy is expended on keeping us warm. This is why there are no such things as slow or fast metabolism. If your metabolism were too slow to burn your resting energy expenditure in 24 hours, you'd die of undercooling. If your metabolism were significantly faster than normal, you'd die of overheating (or at least you'd feel significantly warm to the touch, be constantly sweating/flushed). In fact, a person's resting metabolic rate can be accurately predicted from their body composition [0], and doesn't differ between age or gender, as would be expected. This puts a pretty high lower limit on one's energy expenditure, absolutely nobody can maintain their weight on, say, 600 calories per day. You simply can't keep a 70 kg sack of mostly water at the right temperature with that energy budget, not with how badly isolated we are. Even if it does seem oversimplified, it's no spherical cow. The energy balance is exact, because of basic physics and biochemistry: we have only one way of generating energy (oxidizing reduced carbon), only one way of storing energy for the long term (long chains of reduced carbon), no energy is lost (because heat generation is a 100% efficient process) and our digestion is very efficient (not feces, urine nor sweat contain significant amounts of carbohydrates, lipids, protein or any other high-energy compound). It's also useful because we have an unflexible lower limit to the amount of energy we expend in a day because of the way our bodies work. The wiggle-room on the previous assumptions only serves to make calories-in smaller (e.g. malabsorption) or calories-out larger (e.g. exercise), and thus would only make it easier to lose weight. I won't deny the microbiome is important. There is a probably a correlation between gut biome and obesity. But that can only because it influences things like satiety or mood, because none of those bacteria seem to change the eventual energy equation, nor do they change how much heat you need to produce to keep warm or energy expended when lifting a weight. They're relevant because willpower is a finite resource, but they don't change the fact that any diet will in some sense need to consist of eating less calories than you burn in a day 0: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3139779/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3139779/