The TRUTH about your slow metabolism

By Aadam | September 9, 2019

The slow metabolism myth has been around forever. And while there is some truth to it, it’s not what many people think it is. In this article, I’m going to be taking a deep-dive into the research to help you understand why your lack of progress has nothing to do with your metabolism, what’s really going on, and what you actually need to be focusing on.


A few things before we begin

• Grey footnotes, like this guy → 1, are study citations and references. These are the boring footnotes reserved for the nerds who want to read the papers.

• Red footnotes titled ‘Aa’, like this guy → Aa, are extra thoughts or a further explanation of something I mentioned.

• You can download a PDF of this article for offline viewing by using the form below:


When people talk about metabolism, it can sound like this mysterious entity residing inside of us quietly dictating the fate of our body composition. 

Some people are genetically blessed with a ‘fast’ metabolism; they can eat whatever and however much they want and stay lean. Others have been cursed with a ‘slow’ metabolism; these poor souls got the short end of the stick and just looking at food has them piling on the pounds. 

You can also ‘boost’ your metabolism by performing This One Weird Exercise or eating certain foods that also happen to be weird and unbelievable, drinking special teas, or ingesting exotic supplements. And oh boy, you better be careful in case you fuck around and damage it. We definitely wouldn’t want that to happen.

But, that isn’t what the metabolism is nor how it works. And so, before we discuss slow metabolism, let’s understand what the metabolism actually is.

What we talk about when we talk about metabolism

‘Metabolism’ is a catch-all term for all the complex series of biochemical processes that convert the calories you consume through food and drink into usable energy so the body can function and keep you alive. 

Man eating food -- food being turned into energy.

While we refer to the metabolism as a single entity, it actually consists of three separate component parts.

  • Resting metabolic rate (RMR)
  • Thermic effect of food (TEF), and
  • Activity energy expenditure (AEE).  

1. Resting metabolic rate (RMR) 

Man sitting while looking at his mobile phone

As you’re sat reading this there are a bunch of chemical processes occurring inside of you. Your brain’s using calories to process this article. Your heart’s busily pumping blood all around your body. That meal you had earlier is being digested. Muscle is being synthesized and fat is being oxidised, etc. 

All of this stuff burns calories and is your resting metabolic rate.

RMR makes up the chunk of your metabolism and accounts for around ~60-70% of energy expenditure in most people. Aa

There are a number of factors that influence your RMR 

Body size (height/weight)

A larger body requires more calories to sustain. So if we took two people off the street at random, the bigger person (height/weight) will have a higher RMR. (Fat-free mass also factors in here, more on that in a second.)

Two men standing next to each other. One is large and the other is small.

Gender

On average, a woman’s resting metabolic rate is ~3% lower than a man’s of the same height and weight. 

A guy and girl standing with a description showing the difference in resting metabolic rate between the two.

This is mostly due to men having more lean body mass and less fat at any given body weight. 1

The menstrual cycle also affects RMR. Women can see a 5-10% increase in their resting metabolic rate during the luteal phase. 2 3

Fat-free mass

This includes more than just your muscles.

Your brain, heart, liver, kidneys, and residual mass (skin, intestines, bones, and lungs) all fall under the fat-free mass umbrella and contribute to your resting metabolic rate.

In fact, your organs use more energy than your muscles do. 4

A visual illustration of the human body with a description of the metabolic cost of each organ and residual mass that contributes to the resting metabolic rate

Most of the individual variance in the resting metabolic rate is due to differences in fat-free mass.

Two guys flexing their muscles next to each other -- one is a muscle man flexing and the other is a skinny man.

Fat mass

Fat mass also contributes to resting metabolic rate and is more pronounced in people with more body fat.

Adipose tissue with the energy expenditure listed next to it

Age

Your metabolic rate decreases by 1-2% per decade after the age of twenty.

This decline is closely linked with loss of fat-free mass and a reduction in activity levels, but there are other factors at play. For example, reductions in anabolic hormones such as testosterone, estrogen, and growth hormone and a decline in mitochondria activity. 5

Old guy yelling at young guy doing curls.

2. The thermic effect of food (TEF) 

This the energy expended through digesting and storing the food you eat. Carbs, fat, and protein each have varying amounts of TEF. But generally, TEF accounts for ~10% of your total calorie expenditure. 6

3. Activity energy expenditure (AEE)

Is the energy expended through activity and can be broken down into two sections: 

There are a number of other factors that can impact RMR like genetics, ethnicity, environment–namely, temperature–and even hormones. Having hypo- or hyperthyroidism, for example, can result in either a depressed or higher than normal metabolic rate.

Note: I’m mentioning this now to avoid getting annoying emails from people. This article is aimed at healthy individuals. There are certain conditions that can cause individuals to have an abnormal metabolic rate, but I’m not discussing any of those in this article. Also, holy shit this yellow background is bright as fuck.

So, to quickly sum up:

Now you’re all caught up on what the metabolism actually is, let’s get to the fun part and address if a slow metabolism is why you’re struggling to lose fat.

Uh, no, it’s not your ‘slow metabolism’

Contrary to what you may have heard, people with higher levels of body fat actually have a higher resting metabolic rate than lean people. 

So the idea that people are gaining weight despite ‘hardly eating’ is, well, verifiably bullshit. 

Your body doesn’t just decide to fuck around and create fat cells out of nothing because el-oh-el hilarious. It will only store fat when the number of calories you’re consuming exceeds the number of calories you’re burning.

So if you’re maintaining a higher body weight (and body fat) there’s only one reason: you’re eating more calories (and continuing to eat more calories) than you require which is resulting in gaining and maintaining excess body fat.

With that said, there are people with fast(er) and slow(er) metabolisms

Before you shout “SEE, I KNEW IT!” pick up your pitchforks and start some kind of slow metabolism revolt, please keep reading because it’s a bit more complicated than that.

If we took two people off the street at random, the person with the bigger body will have a higher metabolic rate than the person with the smaller body. This is known as the absolute metabolic rate. 

But there’s also relative metabolic rate. When we compare people of the same age, gender, and body composition (notably fat-free mass) you’ll find some inter-individual variance in metabolic rate; low, normal, or high. 

But this difference isn’t as much as people think it is. 

Once we account for age, gender, sex, and FFM/FM, we can predict someone’s RMR with about ~80-85% accuracy. But this still leaves an unexplained variance of ~15-20%. 7 8

To help quantify this difference, imagine we take a group of men and women and after accounting for age, sex, body composition, etc. we work out the men have an expected RMR of 1800 kcal/d and the women have an expected RMR of ~1500 kcal/d. 

A 15-20% variance based on these numbers would look like this: 

A quick note on the maths here: the total variance is above or below the expected RMR. So, for example, a 15% variance on an expected RMR of 1800 kcal/d would be a total variance of 270 kcal and a 135 kcal variance above or below the expected RMR (270/2=135). A 20% variance would be a total variance of 360 kcal and a 180 kcal variance above or below the expected RMR (360/2=180). In the examples, I’ve rounded up.

In the examples above, you can see the variance is about ~200-300 kcal above or below the expected RMR. While this isn’t ‘nothing’, it’s not as large as some people think it is. And it’s certainly not enough to justify why people aren’t losing weight despite claiming their only eating ‘500 calories per day’ or whatever. Aa

So yes, some people will have gotten the short end of the metabolic stick and will have a low relative metabolic rate. But, your resting metabolic rate isn’t a good predictor of weight gain. 

In 2016, Pimjai Anthanont and Michael D Jensen published a study in the American Journal of Clinical Nutrition that investigated exactly that: 

The researchers analysed 163 people. After adjusting for fat-free mass, fat mass, age, and sex they were split into two groups:

Researchers observed weight gain over a mean period of ~10 years and found rates of weight gain were not statistically different between groups despite there being a 500 kcal/d difference between them. Aa

This isn’t surprising when you understand resting metabolic rate is only one component of your metabolism and it alone can’t dictate whether you gain or lose fat. 

Someone with a normal or relatively high metabolic rate may not pay attention to their food intake which could lead to eating more calories than they require. A sedentary lifestyle can compound this overeating.

Conversely, someone with a relatively low metabolic rate may be diligent with their diet while engaging in regular physical activity; counteracting their ‘slow’ metabolism. 

All of this demonstrates the power of behaviour over genetics, which I’ll get to in a moment. First, I want to address the people who swear they’re only eating *insert a stupidly low number of calories here* and not losing weight.

Summary

The heavier you are, the higher your resting metabolic rate. There are variations in metabolic rate between people of the same age, sex, body size and body composition but these aren’t as large as you may think (~200-300 kcal above or below the expected RMR). Regardless, your resting metabolic rate isn’t a good predictor of weight gain.

Why you’re not losing weight despite eating *insert a stupidly low number of calories here* 

• You’re misreporting calorie intake 

To say people are bad at reporting how much they actually ate would be the understatement of the century. People suck at it. And in most instances, like, pretty much all of the instances, people under-report food intake. 

This effect is more pronounced in overweight individuals than lean individuals, and women tend to under-report more frequently than men. What? Don’t look at me like that. I don’t make the science I just report it. 

One study quantified the under-reporting between normal-weight and individuals with obesity and found normal-weight individuals under-reported by ~150 kcal/d while individuals with obesity under-reported by almost 600 kcal/d.

And in a now-famous 1992 study, researchers investigated energy intake in self-proclaimed ‘diet-resistant’ individuals who failed to lose weight despite claiming to eat fewer than ~1200 kcal per day using doubly-labelled water. 

The subjects were tested before the study to ensure there were no metabolic abnormalities. None of the subjects’ RMR was more than 10.4% below the predicted values. 

The mean self-reported intake was ~1028 calories but the groups actual mean intake was ~2081 calories per day. The participants underreported their energy intake by ~47% 9 

The image below shows reported and unreported intake in all ten subjects. Every single one of these ‘diet-resistant’ individuals underreported food intake. 

Seriously, there’s an overwhelming body of literature that has shown just how much people suck at reporting their food intake time after time after time after time after time after time. Do you want me to carry on? Because I can do this all day. Here have one more.

Misreporting of food intake isn’t exclusive to the general public, either. 

Here’s a review of a study by James Krieger of Weightology that found even dietitians misreport calorie intake.

The results showed that the dietitians underreported their food intake by an average of 223 calories per day, while the non-dietitians underreported their intake by an average of 429 calories per day. 

Thus, while being a dietitian improves the accuracy of self-report of food intake, it does not eliminate the phenomena of underreporting.

Yes, even people whose job involves dealing with these things on a daily basis aren’t immune to misreporting.

And the internet is filled with videos and articles of people claiming they have a slow metabolism when in fact they were misreporting food intake.

Like this British actress who was adamant she had a slow metabolism. Turns out she was under-reporting by almost 2,000 kcal. Aa

The point: everyone lies about their food intake.  

You lie. Your friend who swears she’s only eating 500 calories per day AND SHE CAN’T LOSE WEIGHT! lies. 

Of course, it’s not always intentional. Sometimes you may be eating more without realising. But if you’re struggling to lose weight, the first thing you need to do is get honest about how much you’re actually eating because whether you realise it or not, you’re eating too much. 

So, I dunno, stop playin’ yo ‘self maybe?

• You think you’re burning more calories than you actually are

Just like people under-report how much they eat, they over-report how physically active they are. 10

The 2008 Health Survey for England asked adults to recall how much physical activity they had performed over the previous four weeks. A subsample of the adults wore an accelerometer for the week following the survey.

The self-reported data showed that ~40% of men and ~30% of women met the minimum required recommendations for physical activity, but the data from the accelerometer told another story: only 6% of men and 4% of women actually met the recommended levels of physical activity. 

Overestimating energy expenditure wouldn’t be a big deal if that’s all it was–people ‘thinking’ they burned more calories than they actually did (and incessantly gloating about it on Instagram).

But, people overestimate energy expenditure and compensate for the calories burned by eating more after exercise. 

Fenzl et al. demonstrated this in an interesting 2014 study.

They took 96 participants and split them into two groups: one group was told they were doing ‘fat-burning’ exercise and the other group was told they were doing ‘endurance exercise’. 

Once the researchers analysed all the data, they found participants burned an average ~96 calories but ate ~37%-96% more calories. 

But that wasn’t the only interesting finding. 

Subjects in the fat burning group who perceived the exercise bout as stressful and unenjoyable consumed more calories than subjects who perceived the exercise as enjoyable. 

This could explain one reason why people who are unfit (often those who are also overweight) tend to compensate by eating more after exercise.

If you’re basing the number of calories you burned on how tough the workout felt, it’s easy to justify eating more. Aa

The number of calories you eat after a workout can also be grossly exacerbated if you’re ‘eating back’ the calories your smartwatch is purporting you’ve burned. Spoiler: you didn’t burn 1000 calories doing yoga, fuckface. 

There’s also a phenomenon known as ‘compensatory behaviours’. People unconsciously move less after an intense exercise session. They take the elevator instead of the stairs, sit and lie down more, etc. 

• You have low levels of NEAT

At the start of the article I mentioned NEAT (Non-Exercise Activity Thermogenesis). The component of your metabolism that accounts for all movement that isn’t intentional exercise. 

While the other components of your metabolism are pretty much set and you can’t do much to change them, NEAT is the one component you can change. And after fat-free mass, most of the variability we see in energy expenditure between individuals is due to differing levels of NEAT which can vary by up to 2000 calories per day. 11

This is why long-term studies have found high levels of NEAT to be protective against weight gain and people with higher levels of NEAT weigh less.

For example, this study by Shook et al. found “individuals with low levels of physical activity experienced the largest gains in fat mass of all individuals, resulting in a 1.8-3.8 higher risk of gaining clinically significant amounts of FM over a 12-mo period.” 

Levine et al. (2005) observed obese participants were seated longer and stood and moved less than their lean counterparts despite both groups identifying as ‘couch potatoes’.  

The researchers commented: “If the obese volunteers adopted the NEAT-enhanced behavior of their lean counterparts, they could expend an additional 350 kcal per day. Over a year, this alone could result in a weight loss of ~15 kg, if energy intake remained unchanged.”

And Johannsen et al. (2008) found no difference in resting metabolic rate between lean and obese women, but an almost ~400 calorie difference in levels of energy expenditure between the two groups. 

Other studies have also found obese individuals spend more time engaging in sedentary activities like laying down, sitting, watching television, etc. 12 13 14

So it’s not hard to see how low levels of NEAT can affect your progress. 

The average person will only be engaging in a few hours of intentional exercise per week.

And unless you’re a professional athlete, exercise won’t make much of a difference to your weekly energy expenditure.

Here’s a cute pie chart to help illustrate this. 

3-4 days of intentional exercise is only ~2% of the week. A very small sliver of time in the scope of the entire week.

Even 5-7 days of exercise is still only ~4% of the week spent in physical activity.

In contrast, if you increase your levels of daily NEAT you could be burning hundreds of calories every day. Over a week, this could result in thousands of additional calories expended. 

Of course, most people reading this will have a job that’s pretty sedentary. But that doesn’t mean you can’t increase your NEAT. Try to move more throughout the day like interspersing sitting with standing, taking frequent breaks to walk and move, taking the stairs instead of the elevator, etc. During an eight-hour workday, all of these small, seemingly inconsequential movements, could lead to hundreds of additional calories burned.

In both individuals, the energy expended from the resting metabolic rate, thermic effect of food, and physical activity is the same. The difference in their energy expenditure comes wholly from NEAT.  

And yes, this may just be hypothetical but it’s not too different from what we see in the real world. As I already explained, the variation in resting metabolic rate is only a few hundred calories either way; certainly not enough to justify a ‘slow’ metabolism. But the huge variations in NEAT is why some people gain fat easier than others. 

Research into NEAT and fat gain has consistently shown people who increase NEAT in response to overfeeding gain less fat than people who don’t increase NEAT as much (or at all). 15

Taking all of the above together, it’s not hard to see why people ‘think’ they have a slow metabolism. But it’s not their metabolism that’s the problem.

Rather, they:

  • Underestimate how many calories they’re eating, 
  • Overestimate how many calories they’re burning through exercise, 
  • And have extremely low levels of NEAT. 

Or, put another way: they’re not in a calorie deficit despite believing they’re in a deficit. 

Summary

Your metabolism isn’t slow. You’re simply eating more than you think you are, overestimating calories burned through exercise, and have low levels of NEAT.

Genetics play a role, but…

Let’s take a quick break from our regular scheduled programming to very quickly discuss ‘genetics’.

Yes, genetics do play a role which can make it harder for some people to lose fat than others. One example relevant to this topic is the spendthrift/thrifty metabolism. 

  • Some people have a ‘spendthrift’ metabolism: they move more when overfed and see a lower reduction in metabolic rate with caloric restriction. These people gain the least fat when overfed and will lose fat faster when restricting calories. 
  • Others have a ‘thrifty’ metabolism: they don’t move as much when they’re overfed and see the biggest drop in metabolic rate with caloric restriction. These people gain the most fat when overfed and will see slower rates of fat loss when restricting calories because they move around less. 16
thrifty-spendthrift-slow-metabolism
In the image above you can see an example of the spendthrift/thrifty metabolism in response to a calorie surplus and calorie deficit.

But, my response is always going to be the same: so what?

Because the truth is, genes rarely, by themselves, have the power to dictate your future. And despite your genetic predisposition, your behaviours are going to have a bigger impact on your goals. Aa

A number of studies, for example, have found individuals with the FTO genotype (a gene most consistently linked with obesity due to its effects on reduced satiety after a meal and stronger appetite-related responses in brain areas when looking at pictures of food) lose weight as expected with lifestyle changes like controlling food intake and exercise. 17 18 19 20 

Speaking of exercise, physical activity–including intentional exercise and NEAT–can minimise or completely eliminate weight gain among individuals carrying the FTO gene. 21 22 23

Echoing the studies above, Galgani and Ravussin reviewed the data from long-term studies of weight gain in the Pima Indians of Arizona–a population with one of the highest reported prevalences of obesity in the world–and found a low RMR only contributed to about 30% of the weight gain in this population. Leaving 70% of the weight gain to behavioural and environmental factors. 24

All of these studies demonstrate your behaviours matter far more than your genetics and your genetics are not your destiny. 

Even though the resting metabolic rate makes up a large portion of your metabolism, it’s still only one component. And it’s one component you can’t control or change.

However, there are things you can control that, despite the genetic hand you were dealt, will ensure you make as much progress as is possible for you. 

So, you have two real-world options: 

  1. Choose to blame your genetics and use that as an excuse to not try and change, OR
  2. Put on your big boy/girl/whatever-gender-you-wish-to-identify-as pants and focus on the things you can control. 

Summary

Genetics do play a role and this can make it harder for some people to lose fat. HOWEVER, genetics only play a very tiny part and your behaviours are going to have more of an influence on whether you make progress or not.

And fortunately, there are things you can control.

1. Track your calories

Before you blame your lack of fat loss on your metabolism, you need to get radically honest about how much you’re eating. 

Download a calorie tracking app and track every morsel of food that you put in your mouth every day for one week. Then read this article and compare how much you’re eating with how much you should actually be eating. 

Once you recover from the shock caused by the truth slapping you in the face, continue eating that amount and watch the pounds magically drop off. 

This is you before and after reading this article. Image taken from my Instagram page

2. Move more

I don’t think it’s controversial to say we’re becoming increasingly sedentary as technological advances make our lives easier.

Which is obviously awesome because washing clothes with my hands? I think I’ll pass, thank you very much. But this increasing sedentariness is leading to fewer calories expended through daily activity. Aa

While structured exercise is awesome and everyone should exercise, it doesn’t increase energy expenditure as much as people think. However, increasing your non-exercise activity alongside your weekly exercise can make a substantial difference. 

Increasing non-exercise activity can also, to an extent, attenuate the reduction in NEAT in those of you with a ‘thrifty’ metabolism. By quantifying your activity levels at the start of the diet (e.g. 10k steps per day) you can consciously keep one part of the ‘energy out’ equation consistent. As the diet progresses and you notice a reduction in your daily activity (in this case, your step count), you can work to offset this. 

And let’s not forget increasing your daily activity can allow you to diet on slightly higher calories and improve diet adherence. 

Some ideas for increasing NEAT:

  • Use the activity tracker on your smartphone and aim to hit a certain number of steps per day. If you own a Fitbit or smartwatch you can use that. But for the love of God, please don’t eat back those calories. A minimum of 7k steps is very realistic for most people, so start there.
  • Get up from your desk and move for a few minutes every few hours. Protip: drink a ton of water so nature forces you to get up.
  • If you own a standing desk, switch between sitting and standing throughout the day.
  • Plan activities that involve being active like hiking or playing a sport.
  • If feasible, take public transport instead of driving to work a few days per week. Not only will you help the environment, but you’ll also increase energy expenditure. 

Summary

If you’re struggling to lose fat, the first thing you need to do is get super honest about your food intake to ensure you’re not eating more than you need. Increasing daily activity (NEAT) will also play a huge role in fat loss by increasing the number of calories you burn every day.

***

In 2016, I flippantly posted “You don’t have a slow metabolism, just a fast appetite” to Facebook.

It was in response to the growing number of emails I was receiving from people claiming they were unable to lose weight despite ‘hardly eating’. 

While it was a bit facetious, the core message still stands.

In most instances, people’s metabolic rate is where we’d expect it to be based on their height, weight, age, sex, FFM, etc. but there’s a huge discrepancy in their energy intake. 

Yes, there are variations in metabolism and some people may have a slightly slower metabolic rate than others.

But regardless of genetics, anyone wanting to change their body and improve their health will have to do the same things as everyone else. 

A ‘fast’ metabolism doesn’t excuse people from watching their food intake or improving the quality of their diet; it doesn’t excuse them from engaging in regular physical activity; it doesn’t inoculate them from weight gain if they fail to do the above; and it doesn’t mean they won’t experience fat loss stalls or screw up from time to time. 

The behaviours you need to adopt to make progress don’t discriminate. So the more you focus on the things under your control, the more progress you’re going to make. 

And if you ask me, that’s an infinitely better way to approach your goals.