Few areas has more misconceptions or myths than pertaining to fitness or exercise. For example, it’s commonly assumed that sumo wrestlers are exceptionally strong and muscular. To the contrary, they only appear that way because their mass generates momentum when performing throwing or pushing moves. Another area of confusion or controversy is bulking.

Bulking is the process in which someone intentionally overeats, typically with the goal of adding muscle mass. This is followed by a ‘cutting’ phase to lose the extra fat, so in theory, only muscle remains. The notion of bodybuilders force-feeding themselves thousands of calories in a hurriclean struggle to add size, is enshrined in the mythos of the sport. For example, Reddit users discuss the difficulty of bulking in reference to a video of Jay Cutler’s bulking routine:

As someone interested in metabolism and dieting, the above discussion spurred me to write this post. But I have some objections or questions:

First: 75% of the US population is overweight or obese. The vast majority of dieters–around >80-90% according to the literature–fail their diets, regaining all of most of the lost weight, and then some. So it’s not as if people struggle at weight gain. To the contrary, it’s very easy, as the worldwide obesity epidemic makes clear. Why would bodybuilders uniquely struggle at weight gain, which otherwise comes naturally and effortlessly to almost everyone else?

Second: many bodybuilders and strength athletes, such as powerlifters or fighters, compete in weight classes. This favors leanness and puts a constraint on size.

Third: a common argument to explain the claimed difficulty of bulking is that ‘eating clean’ (e.g. ‘healthy food’, however that is defined) is harder than eating ‘dirty’ (e.g. ‘junk food’). This is also debatable. Healthy food can be extremely calorie dense due to high-fat content, such as nuts, meats, dairy, hummus, granolas, oils, and butters. Protein and carbs only have 4 kcal/gram compared to 9 kcal/gram for fat. By comparison a lot of junk food, such as candy, has half the calorie density as fat-laden food, due to the high-carb content.

Shown below, contrary to the popular online belief or myth that ‘fats or protein are more filling,’ studies show that in terms of self-reported satiety and fullness that the macro composition of the meal makes no difference. I experienced this myself when eating an almost all-carb diet. I simply was not able to eat that much, topping out at 2,200-2,500 kcal/day and feeling quite full for most of the day. With a more balanced, high-fat diet, I probably could have eaten closer 4,000 kcal/day without much difficulty. (Maybe I will test this later.)

The eat less diet exemplified. It doesn’t matter which macronutrients you consume or cut. Just the total energy matters. Doesn’t matter for satiety, hunger, or satisfaction either.

Comparison of Weight-Loss Diets with Different Compositions of Fat, Protein, and Carbohydrates… pic.twitter.com/o1iIjktRvy

— Emil Kirkegaard (@KirkegaardEmil) September 12, 2023

But what can explain the apparent difficulty of gaining weight in the context of bodybuilding, when ‘regular people’ seemingly gain weight effortlessly? I had observed this among others–people who genuinely struggle at weight gain or who find eating to be a chore. I see it as my job to solve problems, and I didn’t want to just dismiss such anecdotal evidence as people lying or exaggerating their claimed food intake. Upon contemplation, it occurred to me there could be some selection effects at play.

Videos on ‘how to get lean’ are very popular on YouTube, with millions of views each:

The advice is simple: just cut calories and workout, yet very achieve the desired leanness. What can explain this? Part of what motivated me to write this post was to investigate this in more detail. Why do so many fail when applying this advice?

I posit that people with the genetic potential to succeed at bodybuilding or fitness have usually fast metabolisms (when controlling for weight, height). Genetics in large part determines how many calories leads to a surplus, hence fat gain, controlling for factors such as weight and height. For some lucky people, this is set really high. I discuss this in much more detail in my 5,000-word post “Slow Metabolisms are Not Myth.” Such detail is necessary when trying to explain a relatively new concept.

So what does this have to do with bulking? People who effortlessly hold onto fat despite not eating that much, ‘drop out’. They eventually quit, realizing they are unable to ever cut enough calories to achieve their sought lean physique. Or it’s unsustainable cutting to so few calories. However, individuals with outlier metabolisms generate tons of ‘waste heat’, and hence have to consume an absurd amount of calories to start putting on weight. But on the positive side, when cutting, the fat effortlessly melts off as the body tries to restore its lower ‘set point’. The abnormally fast metabolism acts as a sort of furnace for fat loss. The end result is a nice muscular physique, but at the cost of having to eat a lot of food, which can be a chore but is still worthwhile (and eating is fun too).

From what I have observed on Reddit and Twitter, the consensus is that a faster metabolism is understood to be good, as it makes getting lean easier despite coming with the possible downside of making it harder or slower to get bigger. The Reddit user “professor peach” describes the ‘metabolic furnace’ effect, in which the body resists weight gain. “Mikejg23” replies that this is good, because it makes getting lean easier. Having to unlimited ice cream is a good trade off for finding it harder to get bigger, when the putative goal is leanness. It’s like being as attractive as a supermodel and the downside being getting unwanted attention–it’s something you can live with.

As evidenced by how so many people are overweight or obese, most people have the opposite problem of storing fat too easily, even when not eating that much, whereas successful bodybuilders and fitness models/influencers may represent outliers whose bodies resist weight gain, requiring obscene quantities of food to get bigger compared to the typical overweight person who gains weight effortlessly and has to make a concerted effort to not get bigger.

Someone with worse genetics has a much slower metabolism and holds onto weight effortlessly despite not consuming that many calories and despite physical activity, as the below Reddit user “Loud-Marketing-1524” experienced:

The above individual–despite only consuming 1,800 kcal/day, doing 14,500 steps daily, cardio, and resistance training–is still unable to get any leaner than 17% bodyfat. His diet of 1,800 kcal/day isn’t much food and doesn’t leave much room to cut more calories.

Thus, there isn’t much more he can do, being that he’s already dieting and is active, yet the fat just refuses to come off. But someone with much better metabolism genetics would have to consume, say, 3,000 kcal/day to maintain that same weight and bodyfat percentage. If this second hypothetical individual was to cut to only 1,800 kcal/day, he would be lighter and much leaner, maybe at 10-12% bodyfat instead.

Another individual, John Goldman, at 220lbs and 6’4″ is unable to lose weight despite cutting his calories to only 1,500-1,600/day and doing lots of strenuous cardio. He tracks everything carefully using apps, so his inability to lose weight cannot be easily dismissed as miscounting or misreporting. According to calculators, for a man of his size and activity level he should be burning closer to 2,600-3,000/day. His metabolism really is this slow.

Four days straight of weight gain despite calories held at 1500-1600 and two work outs a day every day.

Why?

Here are some possibilities:

I had been resting mostly up until 9 days ago. Now I’m doing 30 min jog and 30 min cross train (row) every day. Every day no rest. This is… pic.twitter.com/bQmjc3qbeF

— John Goldman ☀️ (@JohnGoldman) November 24, 2025

The inevitable kneejerk objection is that metabolism only differs a little between otherwise homogeneous individuals, or blaming miscounting. To the contrary, as my article shows, individuals can differ by as much as 50-100% in REE (resting energy expenditure) according to studies, even when controlling for the usual factors (e.g. weight, height, physical activity and lean body mass such as muscle). Consider how much people differ in height, even within the same sex–a difference that’s obvious when you stand in line and notice some people towering above others. Now double that variation: that’s roughly how much humans differ in metabolism. It would be like routinely encountering both seven-footers and dwarfs, to a surprising extent.

At least a third of inter-individual variance of metabolism is unexplained. So you will have a 6ft male with a ‘BMI of 23’ who sustains this on 2,500 kcal/day, and an identical male who does at 3,200 kcal/day, and equal physical activity level and ‘lean body mass’. There is no apparent explanation for how the second person burns so many more calories, as the relevant variables have been controlled for, but it’s a 100% real phenomenon. The latter is at a big advantage in so far as achieving leanness.

I’ve noticed on social media that some people who maintain impressive leanness seem to consume significantly more calories than others of similar size who are far less lean, indicative of better metabolism genetics. For example, Nick Norwitz below says his daily intake is “> 2X the Harris Benedict calculated BMR,” which is a popular formula for predicting metabolism based on someone’s weight and height. His genetics are clearly much better than the aforenoted Reddit user. Being that he graduated from Harvard and possesses other signifiers of high IQ, this also agrees with my hypothesis that smarter people have faster metabolisms, which also cannot be explained by environmental factors such as physical activity. (See how everything comes together on this blog.)

Then why am I so small 🧐😜? My daily intake is > 2X my Harris Benedict calculated BMR and I only do resistance training ~1hr per day. Currently minimal walking. What gives?

Avg from last 3 tracked days: – 3498 kCal – 80% fat – 18% protein – 2% carbs

Maybe we should have a… pic.twitter.com/wdnqvu4fCb

— Nick Norwitz MD PhD (@nicknorwitz) February 8, 2025

Sure, the elephant in the room here is drug usage, such as anabolic steroids, but even still, when controlling for steroid usage, there is still considerable inter-individual REE variance. The same selection effects apply, as steroid users with poor or average metabolism genetics fail to get lean and quit. Moreover, steroids does not necessarily promote leanness. This is another common myth. For example, men and women have identical obesity rates and equally struggle at weight loss despite men having much more testosterone. Bodybuilders have to diet by still cutting calories.

As for the added muscle mass due to steroids, this only boosts metabolism a little bit. People greatly overestimate the ease of adding muscle and how much calories muscle burns. Yes, muscle is 2x as metabolically active compared to fat, but much less than the hungry internal organs, especially the liver and the brain. An extra whopping 30 lbs of muscle, which would represent quite an impressive physical transformation, only adds up to an extra 300 kcal/day, or about the same as three Reece’s peanut butter cups.

For those who don’t aspire to be as large as a bodybuilder, but just want to be lean, the same metabolism argument applies, minus the bulking. Such selection mechanisms may also apply to sumo wrestling, but in the reverse: aspiring wrestlers whose metabolisms are too fast are unable to ever put on enough weight to be competitive and consequently drop out of training.

As someone who lost weight and was always looking for insight or advice, I had read and watched a lot of fitness and dieting content, and tried to formulate a theory as to why some people are much more successful than others when following the same generic advice, e.g. “cut calories and do cardio”. I suspect those who are able to get lean easily, like on social media, are just the metabolic outliers. The guys genetically have fast metabolisms and growing up could eat a lot without getting too fat, and now find it much easier as adults to get lean without starving too much. The calculators and apps work best for those special people.

So if you wanted to measure potential for leanness, the pertinent phenotype is ‘excessive production of waste-heat energy’–basically the human equivalent of an old, inefficient car. Which is weird because it’s not obvious and even maladaptive, because efficiency is generally understood to be ‘good’, yet for this contrived scenario in which the putative goal is leanness, it is advantageous to be inefficient. You want your body to be burning fat, instead of storing it for the famine that never comes. Likewise, for overall athleticism, the relevant phenotype is being either fast or slow-twitch muscle dominant. Or for academic potential, that would be IQ.

If your metabolism sucks (or, specifically, at the left-side of the normal distribution), then weight loss will be exceptionally hard. Even when cutting calories absurdly low, the weight does not come off , even with cardio and other exercise as otherwise predicted by the online TDEE/RMR calculators and apps, as the above individual on Reddit experienced. And there just isn’t much margin to cut in the first place if your metabolism is naturally slow to begin with (‘slow’ in this context means relative to your height and weight). People get mad when you mention this, but faster metabolism means a steeper weight loss trajectory and more room to cut calories.