How do metabolisms actually work?
January 30, 2020 1:05 AM Subscribe
There are a lot of commonly held ideas about metabolisms (esp with respect to age and eating habits)...I want to know what the real deal is
So in particular I'm curious about what we currently know about how eating habits relate to metabolism, but I'm also curious to have a better of what the "metablism" is. It seems like it is...the way in which we convert food into energy? But the way we think about it is a bit counterintuitive to me!
In particular about food, some common wisdom is that if you eat less, your body will go into "starvation mode" and will retain fat...this effectively means your body gets more efficient, no? Conversely, many people will say the "best" way to "have a good metabolism" is to eat lots of little meals during the day...but assuming a person is performing the same amount of work regardless, doesn't this effectively mean your body is being less efficient with respect to your caloric intake? That is all to say, generally we think of someone with a "fast metabolism" as being able to eat a lot and not gain weight, but in a sense doesn't that mean their body is being less efficient, as they take in more calories? So from a weight loss perspective that is desirable, but for the planet...isn't it better in fact if we go into starvation mode and get more from the calories we intake?
I realize there are lots of points of views. These days lots of of people are into intermittent fasting etc. It all just made me realize that I don't have a good, solid sense of what is actually going on!
So in particular I'm curious about what we currently know about how eating habits relate to metabolism, but I'm also curious to have a better of what the "metablism" is. It seems like it is...the way in which we convert food into energy? But the way we think about it is a bit counterintuitive to me!
In particular about food, some common wisdom is that if you eat less, your body will go into "starvation mode" and will retain fat...this effectively means your body gets more efficient, no? Conversely, many people will say the "best" way to "have a good metabolism" is to eat lots of little meals during the day...but assuming a person is performing the same amount of work regardless, doesn't this effectively mean your body is being less efficient with respect to your caloric intake? That is all to say, generally we think of someone with a "fast metabolism" as being able to eat a lot and not gain weight, but in a sense doesn't that mean their body is being less efficient, as they take in more calories? So from a weight loss perspective that is desirable, but for the planet...isn't it better in fact if we go into starvation mode and get more from the calories we intake?
I realize there are lots of points of views. These days lots of of people are into intermittent fasting etc. It all just made me realize that I don't have a good, solid sense of what is actually going on!
PS random anecdote to illustrate this. My friend and I were discussing how many calories one burns the other day and I showed her my latest run and she said "what, you ran 7km and burned only 600kcal, that's depressing". And I said "well, or think about what an incredibly efficient machine I am, that I can run that long on that little food!"
posted by ClarissaWAM at 2:06 AM on January 30, 2020 [4 favorites]
posted by ClarissaWAM at 2:06 AM on January 30, 2020 [4 favorites]
I was looking into fasting a little while ago, and the 'starvation mode' thing with fasting is from the Minnesota Starvation Experiment (Wikipedia) - but the thing is, you have to be really truly starving for this to kick in.
posted by freethefeet at 3:07 AM on January 30, 2020 [2 favorites]
posted by freethefeet at 3:07 AM on January 30, 2020 [2 favorites]
Yeah, “starvation mode” gets a lot of blame for why people aren’t losing weight, but it’s much more likely that —barring a significant eating disorder—someone not losing weight is simply not eating fewer calories then they’re expending.
If you were in starvation mode, it would be great for your grocery budget but terrible for your actual health.
Here’s a fact sheet about metabolism and its variance that might help: https://www.vox.com/2018/9/4/17486110/metabolism-diet-fast-weight-loss?_gl=1*1wpf27v*. Spoiler: basal metabolic rate doesn’t vary nearly as much as we think. But people’s lifestyles and behavior obviously varies a lot!
posted by chesty_a_arthur at 4:25 AM on January 30, 2020 [1 favorite]
If you were in starvation mode, it would be great for your grocery budget but terrible for your actual health.
Here’s a fact sheet about metabolism and its variance that might help: https://www.vox.com/2018/9/4/17486110/metabolism-diet-fast-weight-loss?_gl=1*1wpf27v*. Spoiler: basal metabolic rate doesn’t vary nearly as much as we think. But people’s lifestyles and behavior obviously varies a lot!
posted by chesty_a_arthur at 4:25 AM on January 30, 2020 [1 favorite]
Part of the reason you don't have a good sense of what is going on is that the science is contradictory, often driven by capitalism, and terribly, terribly susceptible to the common problem where 18-30 year old white American men are used as stand-ins for the whole species. The thing I've learned, after 30ish years of being really interested in this question from a layman's perspective, is that there is a ton of variation in different people, we don't fully understand how things like hormones interact with metabolism, people process food very differently from each other on many different axes, and the entire diet industry is blowing smoke up your ass.
(And yeah, it's "better" from a pure efficiency standpoint to need less food. It's dreadful from a capitalist standpoint, because then you are consuming less.)
posted by restless_nomad at 5:01 AM on January 30, 2020 [29 favorites]
(And yeah, it's "better" from a pure efficiency standpoint to need less food. It's dreadful from a capitalist standpoint, because then you are consuming less.)
posted by restless_nomad at 5:01 AM on January 30, 2020 [29 favorites]
At a high level the whole area is a classic example of "The less a topic is understood, the more is said about it."
If you’re interested in digging a little deeper, I highly recommend The Second Brain, which among other things gives a history of research into the gut and talks about what we actually know vs. what people extrapolate and turn into the diet o’ the week. It can get a bit technical but overall it is surprisingly accessible for a book written by an academic.
"The whole area" by the way largely consists of the enteric nervous system. That’s a good google term if you want to poke around.
posted by Tell Me No Lies at 5:44 AM on January 30, 2020 [7 favorites]
If you’re interested in digging a little deeper, I highly recommend The Second Brain, which among other things gives a history of research into the gut and talks about what we actually know vs. what people extrapolate and turn into the diet o’ the week. It can get a bit technical but overall it is surprisingly accessible for a book written by an academic.
"The whole area" by the way largely consists of the enteric nervous system. That’s a good google term if you want to poke around.
posted by Tell Me No Lies at 5:44 AM on January 30, 2020 [7 favorites]
Here's a fascinating article. It melds a nice scientific overview with the journalist's experience of spending time in a metabolic chamber.
posted by Mr.Know-it-some at 7:06 AM on January 30, 2020 [5 favorites]
posted by Mr.Know-it-some at 7:06 AM on January 30, 2020 [5 favorites]
Reread restless_nomad's comment.
Regarding some things that are generally true; muscle tends to demand more calories, so having more muscle will raise both your body's base metabolic rate (how much energy it takes for you to exist in your current state), as well as can help you be able to use more energy. Within the muscle cells, via aerobic exercise, you can also promote an increase in mitochrondia, which are what makes fuel for the muscles. So doing endurance aerobic work will also further raise your base metabolic rate when you're not doing anything because even when there isn't a strong call for energy, your mitochondria don't hibernate doing and consuming nothing.
Both muscle, and mitochrondria are expensive for the body to maintain, so if you're not using them, you will lose them (over a process of weeks/months). Some way to look at efficiency from an evolutionary perspective would be how quickly you can expend energy in X amount of time. Being able to spend twice as much energy (more mitochondria, more muscles), may enable one to escape that tiger (or at least outrun your peer who gets eaten by the tiger). But you still need to find enough energy to not die when food is less available.
Further with age, starting in one's 40's, and accelerating when one's in their 60's, your body will really start to shed muscle mass. Even if you're using it. So even if you were strictly watching your weight/diet combo and weighed X at age 30, and X at age 50, your body composition has changed such that you likely have (I had done calculations for a specific example in an ask answer a few months back if you want to see specific; this is a remembered guess) anywhere from 10-15% less muscle mass. So you might have had 45 lbs of muscle, but now have 38 lbs of muscle. And if you're the same weight, that means an extra 7 lbs of fat. Your metabolism will have lowered because fat takes less energy to maintain.
Exercising and strength work can help reduce the muscle loss, but there's only so much we can fight against. There are advanced agei lifetime muscle/body builders, and while they look heads and shoulders above their same aged peers, and even above many average 20-30 year olds, they also look like twigs compared to their prime.
A likely source of discrepancies for how much someone can eat vs someone else, is going to be in the digestive system. We'll have different ratios, strains etc of bacteria. Different thicknesses in intestinal walls. Different strengths/regulation for peristalsis. So not only may some people digest/absorb something faster, but they also might unlock more total energy. Similar to the muscular mitochondria systems, this is likely regulated by use it or lose it. I.E. if I'm regularly shoving 3500 calories a day down my gullet for my base needs and exercise needs combined, my digestive system has probably undergone some adaptations that are different from someone who regularly eats 2000-2400.
posted by nobeagle at 7:55 AM on January 30, 2020 [4 favorites]
Regarding some things that are generally true; muscle tends to demand more calories, so having more muscle will raise both your body's base metabolic rate (how much energy it takes for you to exist in your current state), as well as can help you be able to use more energy. Within the muscle cells, via aerobic exercise, you can also promote an increase in mitochrondia, which are what makes fuel for the muscles. So doing endurance aerobic work will also further raise your base metabolic rate when you're not doing anything because even when there isn't a strong call for energy, your mitochondria don't hibernate doing and consuming nothing.
Both muscle, and mitochrondria are expensive for the body to maintain, so if you're not using them, you will lose them (over a process of weeks/months). Some way to look at efficiency from an evolutionary perspective would be how quickly you can expend energy in X amount of time. Being able to spend twice as much energy (more mitochondria, more muscles), may enable one to escape that tiger (or at least outrun your peer who gets eaten by the tiger). But you still need to find enough energy to not die when food is less available.
Further with age, starting in one's 40's, and accelerating when one's in their 60's, your body will really start to shed muscle mass. Even if you're using it. So even if you were strictly watching your weight/diet combo and weighed X at age 30, and X at age 50, your body composition has changed such that you likely have (I had done calculations for a specific example in an ask answer a few months back if you want to see specific; this is a remembered guess) anywhere from 10-15% less muscle mass. So you might have had 45 lbs of muscle, but now have 38 lbs of muscle. And if you're the same weight, that means an extra 7 lbs of fat. Your metabolism will have lowered because fat takes less energy to maintain.
Exercising and strength work can help reduce the muscle loss, but there's only so much we can fight against. There are advanced agei lifetime muscle/body builders, and while they look heads and shoulders above their same aged peers, and even above many average 20-30 year olds, they also look like twigs compared to their prime.
A likely source of discrepancies for how much someone can eat vs someone else, is going to be in the digestive system. We'll have different ratios, strains etc of bacteria. Different thicknesses in intestinal walls. Different strengths/regulation for peristalsis. So not only may some people digest/absorb something faster, but they also might unlock more total energy. Similar to the muscular mitochondria systems, this is likely regulated by use it or lose it. I.E. if I'm regularly shoving 3500 calories a day down my gullet for my base needs and exercise needs combined, my digestive system has probably undergone some adaptations that are different from someone who regularly eats 2000-2400.
posted by nobeagle at 7:55 AM on January 30, 2020 [4 favorites]
Ah, my previous answer - a 160 lb male who weighs the same at age 35 and 55 has approximately 10 lbs less muscle and correspondingly 10 lbs more fat.
posted by nobeagle at 8:22 AM on January 30, 2020 [1 favorite]
posted by nobeagle at 8:22 AM on January 30, 2020 [1 favorite]
Muscle does raise your metabolic demands, but it's not going to be a world changer. A pound of muscle burns about 6 calories a day (or maybe 10-15). Going with 6, gaining 10 pounds of muscle - which would be a huge achievement - would be offset by eating 1/4 of a candy bar a day.
And nobeagle's point is important, but a bit pessimistic. Yes, the average person loses muscle mass, and yes, your maximum potential muscle mass declines, but it's also true that the average person can maintain or even increase fitness and muscle mass. No direct evidence, but I'd bet that if you took a typical 40-year old man who'd never done weight training and had him do a couple of 30-40 minute weight training sessions every week, he'd be noticably stronger and larger at age 50 than at age 40.
posted by Mr.Know-it-some at 8:52 AM on January 30, 2020 [2 favorites]
And nobeagle's point is important, but a bit pessimistic. Yes, the average person loses muscle mass, and yes, your maximum potential muscle mass declines, but it's also true that the average person can maintain or even increase fitness and muscle mass. No direct evidence, but I'd bet that if you took a typical 40-year old man who'd never done weight training and had him do a couple of 30-40 minute weight training sessions every week, he'd be noticably stronger and larger at age 50 than at age 40.
posted by Mr.Know-it-some at 8:52 AM on January 30, 2020 [2 favorites]
almost none of the popular Internet/magazine-level understanding of metabolism is correct.
"Starvation mode" is only real when it refers to people who are actually starving, like in concentration camps. Their metabolism slows down because they are being starved of nutrients (very different from a healthy person choosing to eat fewer cookies, etc.) and as a result their bodies' metabolic functions, including movement, do slow down. Out of weakness. This is not actually a state one can induce in oneself, short of anorexia. Hunger will make you eat enough to not hurt yourself, if you have the choice.
In terms of who's getting it right, it's hard to say, because metabolism turns out to be wildly complicated and variable, with genetics being a prime variable. You can get a sense of some of the questions, and the extent to which the calorie reduction=weight loss model is wrong because it doesn't take into account endocrine and other effects, from Gary Taubes' work. But that's not to say he's the authority on what's correct. He's just a good reporter on what's gone wrong in the science so far.
posted by fingersandtoes at 12:40 PM on January 30, 2020 [1 favorite]
"Starvation mode" is only real when it refers to people who are actually starving, like in concentration camps. Their metabolism slows down because they are being starved of nutrients (very different from a healthy person choosing to eat fewer cookies, etc.) and as a result their bodies' metabolic functions, including movement, do slow down. Out of weakness. This is not actually a state one can induce in oneself, short of anorexia. Hunger will make you eat enough to not hurt yourself, if you have the choice.
In terms of who's getting it right, it's hard to say, because metabolism turns out to be wildly complicated and variable, with genetics being a prime variable. You can get a sense of some of the questions, and the extent to which the calorie reduction=weight loss model is wrong because it doesn't take into account endocrine and other effects, from Gary Taubes' work. But that's not to say he's the authority on what's correct. He's just a good reporter on what's gone wrong in the science so far.
posted by fingersandtoes at 12:40 PM on January 30, 2020 [1 favorite]
"Starvation mode" is only real when it refers to people who are actually starving, like in concentration camps.
Just so I'm clear here - are you claiming that adaptive thermogenesis is only observed in people who are starving to death? Because that's simply not true.
Yeah, “starvation mode” gets a lot of blame for why people aren’t losing weight, but it’s much more likely that —barring a significant eating disorder—someone not losing weight is simply not eating fewer calories then they’re expending.
Look, this is just straight up ignorant horseshit, and you should stop.
Just so I'm clear here - are you claiming that adaptive thermogenesis is only observed in people who are starving to death? Because that's simply not true.
Yeah, “starvation mode” gets a lot of blame for why people aren’t losing weight, but it’s much more likely that —barring a significant eating disorder—someone not losing weight is simply not eating fewer calories then they’re expending.
Look, this is just straight up ignorant horseshit, and you should stop.
The over 80% recidivism rate to pre-weight loss levels of body fatness after otherwise successful weight loss is due to the coordinate actions of metabolic, behavioral, neuroendocrine, and autonomic responses designed to maintain body energy stores (fat) at a CNS- defined “ideal”...The inability of most otherwise successfully weight-reduced individuals to sustain weight loss reflects the actions of potent and redundant metabolic, neuroendocrine, and autonomic systems...posted by some little punk in a rocket at 5:09 AM on January 31, 2020 [3 favorites]
...Maintenance of a 10% or greater reduction in body weight in lean or obese individuals is accompanied by an approximate 20%-25% decline in 24-hour energy expenditure. This decrease in weight maintenance calories is 10–15% below what is predicted solely on the basis of alterations in fat and lean mass. Thus, a formerly obese individual will require ~300–400 fewer calories per day to maintain the same body weight and physical activity level as a never-obese individual of the same body weight and composition.
This thread is closed to new comments.
When your body goes into "starvation" mode tho, what it does is take its energy from muscle. Muscle consumes energy, so you (the body) wants to reduce muscle mass, making you more "efficient" in surviving a period of starvation, sure. But you're not a more efficient organism, generally. You become weak (not just physically, other functions will also be just about sustained so you don't die).
Someone will come along and say all this in a much more scientific way, I'm sure. But I've typed it now!
posted by ClarissaWAM at 2:02 AM on January 30, 2020 [1 favorite]