Fast or slow metabolisms are not a “lie”. It’s time to put this myth to rest.

I saw this Reddit post going massively viral, ““Fast metabolism” is a lie, stop making yourself feel bad.”

It got 1829 votes and 288 comments, indicative of considerable discussion and controvery. As I wrote in the posts “Individual differences of metabolism are real and matter” and “Slow metabolisms are not a myth“, to deny that fast or slow metabolisms exist or are a myth, is tantamount to denying that smart or dull people exist, that tall or short people exist, or any other trait in which humans vary, which is all of them.

Two otherwise identical people matched for height, weight, sex, and activity level can vary by as many as 1-2 thousand calories/day of RMR (resting metabolic rate). That is a lot, or about as much intraindividual variance as something like IQ. Like, IQ, there is no obvious explanation for this. No one can explain why humans vary so much in terms of metabolism, but it’s real. Like IQ, individual differences of metabolism are downplayed or denied. And like IQ, there is not much that can be done to raise metabolism (without also getting fatter in the process, such as by eating more, which I assume is not what people have in mind when the putative objective is weight loss).

A 2024 study from China found “mean RMR for men and women was 1825.2 ± 248.8 and 1345.1 ± 178.7 kcal/day, respectively.” Dividing the mean by the variance gives a ‘coefficient of variance’ of 7.3, which is comparable to IQ. Shown below, this represents enormous intraindividual variance of RMR (resting metabolic rate) when either adjusted for FFM (fat free mass) and relative to BMI (body mass index). Those at the top decile have as much as 2x as fast of a metabolism compared to those at the bottom decile when controlling for sex, BMI, and FFM. This is the exact opposite of the claim “fast metabolisms are a lie”. As the meme says, “de-boonked!”

Again, it’s not known why otherwise physically identical people vary so much in metabolism. It could be due to differences of NEAT (non-activity exercise thermogenesis), which describes non-purposeful movement such as fidgeting, or the internal organs. Muscle only explains a small bit of it, as skeletal muscle burns few calories relative to the metabolically-hungry organs such as the heart, liver, and brain. What about physical activity? The study accounted for this by instructing the participants to abstain from vigorous activity leading up to the study:

Participants were instructed to visit the Sir Run Run Hospital (Jiangsu, China) following a minimum 10 h fasting period, during which they refrained from consuming any stimulant substances (e.g., caffeine, tobacco) for at least 4 h. Additionally, participants were required to abstain from engaging in vigorous physical activity for 24 h prior to their visit23. All measurements were performed between 8:00 and 10:30 in the morning.

It’s funny how this topic keeps coming up and is so contentious. This suggests there is some underlying truth to metabolism actually mattering, in contrast to the insistence by some that metabolism does not matter. We all know those people–friends, family, or acquaintances-who blithely eat a lot and do not gain weight, whereas others track food meticulously and are still fat–and it cannot just be explained by the failure to adequately count calories.

This is a topic that evidently a lot of people care about, and even seemingly innocuous comments can generate enormous controversy and discussion, similar to IQ. For example , the tweet “losing weight is super simple: you just eat less” garnered a staggering 120,000 ‘likes’ at the time of the publication of this post. To put this in perspective, this is half of Trump’s 4th of July tweet.

This is more ‘likes’ and comments than even news related to Epstein, Iran, tariffs and other high-stakes stuff such as that. People really care a lot about this. So I will keep writing about it (and I care too).

Metabolism is similar to IQ in the sense that it’s always being debated and denied, and yet we can similarly relate our personal experiences of knowing people who seem to just “pick up” concepts naturally, whereas other struggle at learning even simple concepts like addition. Things which are manifestly false are seldom debated vigorously (although religion is a possible exception). Things which are obviously false does not need constant reassurance of being false. No one debates the existence of leprechauns, for example. Disbelief in Santa or the tooth fairy does not need periodic recapitulation.

I posit biological factors such as metabolism is the underlying factor for weight loss. Yes, although anyone can lose weight with dieting, biology can explain why some people are much more successful at dieting compared to others. Studies consistently show considerable variability in terms of weight loss when controlling for factors such as starting weight and activity level. Some people really are ‘metabolically blessed’ in the sense of either resisting weight gain, or losing weight faster, or keeping the weight off longer. Some individuals are deemed ‘diet resistant’, in that their bodies stubbornly hold on to the weight, instead of easily shedding the pounds on the opposite extreme of being ‘diet sensitive’.

Consider the above tweet about “eating less”–as it turns out, having a faster metabolism means you can eat more without getting as fat. Same for willpower–a faster metabolism means less willpower is required, because you can eat more. There is less margin of error when your body burns more calories. To say this does not matter seems like a willful denial of reality.

So where is the evidence? From a 2002 study, “Decreased Mitochondrial Proton Leak and Reduced Expression of Uncoupling Protein 3 in Skeletal Muscle of Obese Diet-Resistant Women”:

The highest and lowest quintiles of weight loss were defined as diet responsive and diet resistant, respectively. After body weight had been stable for at least 10 weeks, 12 of 70 subjects from each group consented to muscle biopsy and blood sampling for determinations of proton leak, UCP mRNA expression, and genetic studies. Despite similar baseline weight and age, weight loss was 43% greater, mitochondrial proton leak-dependent (state 4) respiration was 51% higher (P = 0.0062), and expression of UCP3 mRNA abundance was 25% greater (P < 0.001) in diet-responsive than in diet-resistant subjects.

And in 2021, “Metabolic adaptation characterizes short-term resistance to weight loss induced by a low-calorie diet in overweight/obese individuals”. The first paragraph acknowledges how much people vary despite adhering to the same protocol:

Although low-calorie diets (LCDs) (800–1400 kcal/d) can reduce weight by 8% over a 6-mo period (4, 5) there is notable response heterogeneity among cohorts. This weight loss response heterogeneity exists despite adherence and compliance to a weight loss diet (6). Characterizing the metabolic components of controlled weight loss success will be crucial in predicting the efficacy of weight loss interventions in obese populations and developing future personalized treatment plans.

This again suggests actual biological factors for individual response to dieting. “Increased mitochondrial proton leakage” is a fancy way of saying “more waste heat energy,” meaning the body is less efficient in people who lose weight faster. Such leakage means impaired ATP production, similar to the process by which DNP (2,4-dinitrophenol) works, minus the the overheating and occasional lethality.

The second study concludes:

Metabolic adaptation refers to reduced RMR compared with what is anticipated based on body composition (8, 9). ODR participants had a greater metabolic adaptation than ODS participants. In other words, the ODR group had greater reductions in actual RMR compared with what was expected based on FFM, age, and race. This difference existed despite both groups having comparable RMR throughout the LCD. Metabolic adaptation after long-term weight loss has been established (29, 30, 31, 32, 33, 34). In the present study we have shown that metabolic adaptation exists over a short duration (28 d) in agreement with previous research (35) and more notably characterizes individuals who have a blunted response to weight loss.

Metabolic adaption is increased in subjects who are resistant to dieting (ODR) compared to participants sensitive to dieting (ODS). Even if the mechanism for this is not entirely known, it’s undeniable there is considerable variability that is of a biological origin, being that the environment has been controlled for.

Differences of metabolism have also been observed in animal studies, such as livestock. This is called “residual feed intake“, which measures how much feed an animal consumes compared to how much it’s predicted to consume for its weight or for a given yield (e.g. eggs, per pound of meat, etc.). This has been studied extensively since the concept was first conceived in 1963 by Robert M. Koch, with some studies having hundreds of citations. The implications are obvious: more metabolically efficient animals need less feed, meaning higher profit margins, so it makes sense to investigate this, especially at scale in the context of factory farming. Like variation of human metabolism, there is no smoking gun or obvious risk factors. It’s not like metabolically efficient or inefficient animals are in any way different in terms of activity level or health. Yet when it comes to differences of human metabolism, we’re led to believe such differences are illusory or only tiny. This logic makes no sense: why would humans be an exception when metabolic variation is observed everywhere else?

Like IQ, people intuitively know metabolism matters. On ‘fitness/health Twitter’, when the discussion of metabolism comes up, there are similar attempts at downplaying its importance or the exitance of fast or slow metabolisms. When someone claims to have a fast metabolism or whose reported daily calorie consumption is indicative of a fast metabolism, the response by others is that the person is lying or miscounting. It invokes the same envy response as someone claiming to have a high IQ. This suggests people, correctly, at some intuitive level know metabolism matters for attractiveness and being lean, and that people with faster metabolisms are indeed privileged in this regard.

Millions of people worldwide consume fitness/health content, such as popular videos on “how to get ripped” or “how to get abs,” but few achieve the sought leanness. Metabolism can in large part can explain why.

As I discuss this in detail in the post “Metabolism , fitness influencers, and looking good into adulthood“, many of these guys in their 30s or 40s who are persistently lean, although it can be explained by drug use, excessive cardio, or extreme dieting–metabolism is likely a key factor too, that all too often is overlooked .

‘Fast metabolism’ is just another trait, like athleticism, attractiveness (such as facial bone structure), tallness, or high IQ, which confers status and other social benefits. The ability to ‘generate lots of waste heat’ or a heightened response to caloric restriction (ODS), is as genetic as the ability to have a ‘sub-4.5 second 40-meter dash’ or a ’40-inch vertical leap’. But whereas athleticism matters early in life, having a fast metabolism is useful for the rest of one’s life in terms of not getting fat and being attractive, long after high school or college. Because metabolism cannot be meaningfully raised, it makes sense to downplay it, compared to things which can be changed like physical activity level or diet. You cannot sell a ‘coaching plan’ to fix something that is unfixable.

Sure, ‘too much NEAT’ or ‘too much waste-heat’ will not earn one a Heisman trophy, but not being ridiculed on social media for having a fat profile pic is nice too. You cannot see waste heat energy like replaying an NFL highlight reel, but it’s every bit as genetic.

In practical terms, to give an example of how metabolism matters, the guy who is burning as many as 4,000 calories/day at rest (RMR) has 2x bigger metabolic furnace and hence much more room to cut calories compared to an otherwise identical guy who burns only 2,000 calories/day at rest to maintain his weight. The first guy will generally have an easier time losing weight and lose more weight given that his body is burning more calories. Starting at only 2,000 calories/day leaves much less room to cut without nutritional deficiencies and fatigue from eating so little. These metabolic outliers at the far-right tail of the distribution are at a major advantage for weight loss, just as people with high IQs are at an advantage when it comes to learning, say calculus.

Although the above example is contrived, this is not an exaggeration and is consistent with the aforementioned studies and abundant anecdotal evidence of people who burn fewer calories in terms of RMR than predicted by calculators (which are based off of regression equations) for their weight and activity level, or who otherwise stall despite counting their calories carefully. For example, a user on HackerNews “HorizonXP” describes stalling out despite still being >200lbs and only cutting to 1,800-2,000 calories/day, which is low relative to his height and weight (about comparable to a woman, not an active 5’10” male).

Another user “BizarroLand” relays a similar experience of burning fewer calories than predicted by calculators. This explained why he stalled at 250lbs, because he was too hungry to cut anymore, and yet the weight was not coming off due to having such a slow metabolism. When he went off his diet, he rapidly regained 20lbs despite not eating that much more.

Yes, being more active can raise metabolism, but this also varies greatly individually. Moreover, studies and anecdotal evidence also show a form of metabolic adaptation in response to exercise, called ‘adaptive thermogenesis’, so this means burning fewer calories at rest to compensate for the increased exertion. The Apple watch app may say you burned 400 calories from a vigorous run, but due to adaptation effects, it is closer to 100-200, or even zero for some especially unlucky folks. And then there is also the increased hunger reported by many after exercise. Again, many people can attest to this:

Same on Twitter. There is tons of of anecdotal evidence of people failing to lose weight despite cutting their calories aggressively and doing tons of cardio, or who have to consume absurdly few calories to not regain:

And:

In the replies, these people are often accused of lying or failing to adequately track everything. Yet burning so few calories, especially due to metabolic adaptation and having a low baseline metabolism to start with, is perfectly consistent with the literature of individual metabolic variation, which like everything else in life, follows a normal distribution. Yes, they could very well be lying or miscounting, but genetics are real. Some people must get the short end of the stick.

People greatly underestimate just how much human diversity there is. When people online claim “It’s impossible to live on only 1,200 calories/day” or “That seems way too low…you’re obviously miscounting,” I’m like “You fail to understand how much humans differ.” The belief that the body cannot adapt to so few calories shows a lack of imagination of the spectrum of human adaptability and diversity. Humans are remarkably adaptable: there are stories of humans surviving in some of the most harrowing circumstances of extreme food scarcity.

This is not to say they are all obese or that dieting totally failed them. They lost weight, which is consistent with “CICO”, but now they stall at well-above their desired weight and are not lean. Or they can only remain lean by recreating the Minnesota Starvation study or the Dutch famine, which sounds really unpleasant and unsustainable.

Does this mean these people have bad genetics? Not really. It’s more like they have the genetics that served humanity well for hundreds of thousands of years, until only very recently. There is a sort of myth, promulgated by popular health influencers, that Americans in the 1950s were all skinny, and then fast-food came along during the ’70s and ruined everything. Americans have been gaining weight and trying (and usually unsuccessfully) too lose weight for as long as it has been tracked. Pudgy middle-aged people were pretty common during the ’40s and ’50s. It’s just that before the visually-driven modern era of social media, few cared or noticed. For much of human history, a propensity to being overweight would have probably been beneficial to survive famine. Now social media, especially post-Covid, has created a weird sort of artificial environment that favors those lucky individuals who generate too much waste heat, are picky eaters, or have heightened fullness signals, when for the rest of the history of humanity, this was a net-negative for survival.

Of course, there are other variables aside from metabolism, such as ‘hunger signaling’. Some people simply do not feel a ‘fullness sensation’ until they have eaten too much. The interplay between ‘the mind and the gut’ is somehow deregulated, so that the brain does not get the message that the stomach is full or that sufficient calories have been consumed. Or the stomach empties its contents too fast, leading to hunger too soon. But metabolism tends to set a hard line as to how much you can lose. You cannot lose more than you burn. Cutting below some threshold is not only unpleasant, but also potentially unsafe due to muscle and bone loss and various nutritional deficiencies.

But won’t GLP-1 drugs, like Ozempic, level the playing field? Sorta. Metabolism matters here, too. GLP-1 drugs suppress appetite, so like above, the guy or gal with the faster metabolism is going to still lose more weight. Similar to other weight loss interventions, there is extreme intraindividual variability in terms of how much weight is lost on GLP-1 drugs, with some individuals losing a lot (so-called hyper-responders) and others losing a little or none (non-responders). Yes, there are obviously other factors at play too, as these drugs seem to affect the brain and behavior, too. But I posit people with slow metabolisms relative to starting weight, yet still obese, find it hard to lose enough weight, even with the help of these drugs.

For example, consider an obese person (BMI>40) who eats 6,000 calories/day due to a binging disorder brought upon by a sudden setback in life, compared to someone who eats ‘only’ 3,000 calories/day to maintain the same weight and has always struggled with obesity his or her whole life despite not eating ‘that much food’ (such variance is again consistent with the aforenoted studies). The first person with the faster metabolism will be at a big advantage, owing to simply burning more calories at rest, so the differential/deficit is greater under calorie restriction.

When people tell their weight-loss stories using GLP-1 drugs, there is no mention of metabolism or starting calories. Is this obesity in the context of a binging disorder or of short onset, or a lifelong struggle at obesity even when not eating that much or even when dieting?
Those on social media who get shredded or otherwise lean on GLP-1 drugs likely have fast metabolisms to begin with, which when combined with the calorie restriction induced by these drugs, have very impressive results. Metabolism is still again the deciding factor, only that these drugs make it easier to diet.