The Myth Of The “Slow” Metabolism

By Lance Jeffers | Fat Loss

There’s a pervasive belief that some of us have faster metabolisms while others are cursed with slower metabolisms.

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Allegedly, this is the reason that some people on the "fluffier" side of things can seemingly eat very little yet still gain weight, while other people who are leaner can almost certainly eat more without the scale moving an inch. I’m here to tell you that’s a lie.

Well, maybe not a lie, but certainly misleading.

Truth about metabolism

If only the signs were always this obvious

Right off the bat, you may be reading this and thinking that I have no clue what I’m talking about. “How can people not have faster metabolisms than others, I know plenty of skinny people that eat a ton and never seem to gain weight!”

However, the idea that smaller people can eat more than larger people may not actually be as true as you think. When you see a leaner person who can seemingly eat a lot of food, that does not mean they are some sort of genetic lottery winner who has been blessed with the ability to eat as much food as they want without ever packing any weight on.

Odds are, you’re not around them 100% of the time and so they may eat less food than you realize, they may be more active than you think, or maybe there is a missing factor that explains the mysterious difference we seem to find between individual metabolisms. We’ll come back to that in a moment.

Metabolic rates have been studied quite extensively by scientists, so we have plenty of empirical evidence that can prove whether some people truly have faster metabolisms than others. Before explaining why people don’t really have “slow” metabolisms per se, it’s important to understand what your metabolism actually is, and how many factors actually impact it.

What Exactly Is Your Metabolism?

Your metabolism is the set of life-sustaining chemical transformations within the cells of your body. It is all of the constant processes going on in your body in order to sustain your life and everything that you do.

Scientists can measure the speed of your metabolism, or it’s metabolic rate, in terms of calories. Calories are a unit of energy that you don’t need to know the exact definition of to be completely honest with you. What is important to understand is that calories measure energy, and so we measure our metabolic activity (aka the amount of energy we require) in terms of calories.

In case anything isn’t clear, think about it like this. Did you move your fingers in order to scroll to this part of the page? That required energy, which we measure in calories.

Have you inhaled and exhaled oxygen as you’re reading this article? That requires energy, which we can measure in calories.

Calories are an easy way for us to take the energy we require to do literally anything, and determine how much food/fuel is required. This is why nutrition labels have a calorie count on them. We can determine how many calories are in a food via straightforward testing, which then gives us an idea of how much energy a given food will provide.

So now that you understand that, let me highlight the importance of what I’ve said so far, in case you’re not quite putting the pieces together.

The calories from food determine how much energy our body has.

food as fuel

Food Is Fuel

Conversely, we can measure all the metabolic activity of our body in a day (sitting, running, breathing, whatever) and determine how many calories from food would be required to sustain all that activity.

So now that you know what a calorie is, and what your metabolism is, we can start to talk about what determines your metabolic rate (aka how many calories you burn in a day).

Your Metabolic Rate

Your metabolism is not just some magic “thing” like some sort of organ. Remember that your metabolism is the sum of all chemical processes in your body. So, the easiest way to measure the energy (or caloric) requirements of these various chemical processes is to classify certain factors together.

The 4 factors that determine your total metabolic rate will be:​

  • Resting Metabolic Rate (RMR)
  • the Thermic Effect of Food (TEF)
  • the Thermic Effect of Exercise (TEF)
  • Non-Exercise Associated Thermogenesis (NEAT)

    Resting Metabolic Rate

    The first factor which contributes to your overall metabolic rate is your Resting Metabolic Rate (RMR), and this is usually what people assume causes the difference between the skinny guy and the overweight guy. RMR is the amount of calories that your body burns through in a day purely for survival. It’s measured by how many calories your body uses by sitting at rest.

    As you sit in your chair, reading this absolutely fascinating article, there are thousands of interactions happening inside of your body. Your brain is using glucose for fuel, your eyes are using energy to read back and forth across each line on this page, and your heart is beating in order to keep you alive. These processes, along with numerous other ones that occur without your awareness, all contribute to the energy requirements of your RMR.

    RMR is going to primarily be determined by how much tissue is on your body. All tissues in the body are metabolically active, including body fat, and so larger individuals will actually have a higher Resting Metabolic Rate than their smaller counterparts. Lean body mass may be more metabolically active, but the difference between the metabolic rate of lean tissue and fatty tissue is not as large as people think (muscle burns approximately 9 calories more than fat per day). With this in mind, a 300-pound individual is undoubtedly going to have a higher Resting Metabolic Rate than a 160-pound individual.

    Key Point

    Larger individuals will almost always have a higher Resting Metabolic Rate because they have more tissue on their body, and all tissue requires energy/calories.

    Now the science is quite clear on how much variability exists between 2 individuals’ RMRs—it’s not much. In fact, when you’re looking at the RMR of 2 people of the same sex and bodyweight, RMR varies by about 200 calories per day at the absolute most [1]. That difference in metabolic rate shrinks to around 5% when you compare 2 different people of the same sex, bodyweight, and bodyfat.

    In case you’re not familiar with the math, that’s not much.

    The 200 calorie per day difference between individuals of the same sex and bodyweight is approximately 1 medium sized donut, 1 flour tortilla, or 1 large apple. If that makes 200 calories seem like a really small difference in RMR between individuals, that’s because it is a small difference.

    So if individuals of the same bodyweight only vary their RMR by maybe 200 calories, then why do some leaner individuals seem to be able to eat anything they want without gaining fat while some heavier individuals (who have a higher RMR by default) seem to gain weight while eating even less?

    We need to look at the other factors in determining Metabolic Rate to begin seeing why this might be happening, if it is actually happening in the first place.

    The Thermic Effect of Food

    The Thermic Effect of Food is the next factor which influences your overall metabolic rate. It refers to the increase in metabolism due to breaking down and utilizing the nutrients found in food. For example, breaking down protein has to occur in the liver, which requires energy. Carbohydrates and dietary fat also undergo some processing before we can utilize them. This means that even the act of eating requires calories. The more you eat, the higher your TEF will be, however this is offset by the fact that the more you eat, the more calories you consume (if you eat 1000 calories, TEF will only account for roughly 100 calories).

    Like RMR, the Thermic Effect of Food (TEF) varies very little between individuals who are eating the same diet. If two people are both eating the same 2000 calories a day, their TEF will be almost identical. The only real variation in TEF occurs when one individual has a higher protein intake, because protein increases TEF more than the other nutrients. Even then, the difference in TEF is insignificant.

    You can safely assume that TEF is largely irrelevant in terms of differences in metabolic rates between people, so long as the individuals are eating the same diet. We need to dig deeper to discover why some individuals appear to have slow metabolisms.

    The Thermic Effect of Exercise (TEE)

    TEE represents the number of calories burned from exercise, and at least with this factor we can begin to see some variability in overall metabolism. We’ve finally discovered the main reason some people are skinny with fast metabolisms, and others are overweight with slow metabolism’s.

    Except it’s probably not the primary culprit.

    TEE is obviously determined by the amount of exercise you perform. Whether it’s 30 minutes of jogging, an hour in the weight room, or even 45 minutes of Zumba (hey, I won’t judge). All dedicated exercise is going to burn calories, with some methods potentially burning more than others.

    I guess you have to start somewhere

    The main determinant of how many calories you burn via TEE is the intensity and the duration of your exercise. 30 minutes of walking burns some calories, but 30 minutes of jogging at a brisk pace will burn more calories (due to a difference in the intensity of the exercise). And from there, 60 minutes of jogging will burn more calories than 30 minutes of jogging if the pace is the same.

    So, if you’re a sedentary person, and your friend is lifting weights 4 days a week and jogging 2 days a week, there is going to be a big difference in your metabolic rates. His TEE will be very elevated, while your TEE will be very low, even if you were both the same body weight, body fat, and height.

    This is where we start to see some differences in metabolic rates. People who exercise are going to burn more calories (and therefore have a higher metabolic rate) than people who don’t. Hopefully that is not a shocking revelation to you.

    An interesting side note about TEE is that heavier people and people who are out of shape actually burn more calories during exercise. While this might seem surprising at first, it makes sense when you think about it.

    A 250-pound man is going to have to generate a lot more force to move his body along the street than a 150-pound man. There is simply more weight that needs to be moved, and so the heavier man’s muscles will need to work harder and use more energy to move the body.

    Additionally, an out-of-shape person will burn more calories than an in-shape person while doing the same exercise, because one of the adaptations to exercise is improved efficiency. Due to the fact that we become more efficient at movements as we learn them, we also require less energy (calories) to perform them.

    Taken together, we can see that if anything, an out-of-shape and overweight people would burn more calories during exercise than a slimmer and fitter individual. The primary difference in TEE levels does not appear to be how many calories each person burns when doing the same workout, but instead the difference appears to be caused by how likely it is that someone will work out in the first place.

    Basically, some people will have higher levels of metabolic activity from TEE than others simply because they enjoy working out more, and thus workout more frequently (and burn more calories) than people who don’t like to exercise as much.

    This leads us to something obvious but important; the first thing to ask yourself when comparing your own metabolism to somebody that you think has a faster metabolism than you should be “Does this person have a faster metabolism than me, or are they simply more active than I am?”

    It’s very common for people to assume that other individuals have a faster metabolism, when it is simply that other individuals are exercising more regularly. I do not say this to be mean, but I have noticed a strong correlation between people believing they have a slower metabolism and those same people not working out as much as they probably should if they want to lose weight.

    But wait.

    Even without accounting for exercise it still seems that some people do have faster metabolisms than others. You probably know a slim individual who barely works out, while eating ridiculous amounts of food.

    Since Resting Metabolic Rate varies very little between individuals of the same body size, the Thermic Effect of Food has almost no variance between individuals, and TEE is irrelevant since we’re using someone that doesn’t work out as a comparison, then how is it that they seem to be able to eat so much more?

    The final factor that determines how many calories we burn in a day seems to reveal the answer: NEAT.

    Non-Exercise Associated Thermogenesis

    NEAT, or Non-Exercise Associated Thermogenesis, entails all the non-exercise related movements we make in a day. This can be biting your nails, jiggling your foot, fidgeting in your seat, parking further away in a parking lot, using the stairs, and anything else that may not be intentional exercise (so not running, lifting weights, or playing basketball).

    NEAT appears to hold the key to all of our differences in metabolism.

    While Resting Metabolic Rate varies very little, NEAT can vary by as much as 2000 calories per day. Yup, someone who weights the exact same amount as another person may be able to live off 2000 calories per day more [2]. Unlike RMR, which varies by maybe 200 calories at the most between individuals, NEAT can vary by several meals worth of calories.

    In perhaps one of the more interesting studies to compare overeating between individuals, a group of scientists overfed people by 1000 calories per day over their original metabolic rate [3]. What happened next demonstrates the power of NEAT.

    On one hand, one individual in the study increased their energy burned through NEAT by 692 calories per day. So, while they were originally being overfed by 1000 calories over baseline, they started fidgeting and moving around so much more that they increased the amount of calories burned through NEAT by 750 per day. So what was originally an overfeeding of 1000 calories, wound up being an overfeeding of only 300 calories. This individual is the perfect example of someone who can probably get away with eating much more than other people, without needing to exercise more.

    In contrast, one of the individuals in the same study reduced their NEAT levels by 98 calories when they were overfed. So they were overfed 1000 calories per day more than they needed, and burned less calories than they did before the study. Unlike the fast metabolism guy from the above example, this individual is probably an example of someone who can overeat by seemingly small amounts and yet still gain a lot of weight over time.

    So what does all of this mean?

    Basically, NEAT explains almost all the variability in metabolic rate when comparing people of the same size who are performing the same exercise program. This idea that some people are born with a dramatically faster Resting Metabolic Rate simply isn’t true. However, what does appear to be true is that some people are genetically programmed to burn far more calories than others because they move so much more, even if they don’t necessarily exercise more.

    I’m a pretty good example of this. Even when I had to stop exercising during physical therapy, I was still burning a significant number of calories due to my activity levels at work and just being a fidgety guy in general. I managed to maintain my weight on a caloric intake that would make many people heavier than me gain weight. I was lucky enough to win the fast metabolism lottery, but many people don’t.

    I think it’s legitimate to wonder why it is that NEAT seems to account for such high levels of metabolic activity. After all, isn’t purposeful exercise supposed to be the main thing that burns calories?

    For starters, most people that are not professional athletes aren’t exercising more than 90 minutes a day, and even that is on the higher side. In case you didn’t know, jogging burns approximately 400-600 calories per hour. Yes, the treadmill at the gym has been lying to you.

    However, just going from standing to walking at 2 miles per hour (which is slow enough to piss of anyone stuck behind you) more than triples your calories burned per hour.. That's a big difference, and if we’re comparing 6-hour work shifts with one individual sitting at a desk, while the other person has to stand and move the entire time, then the individual who has to move during work will burn over 1000 calories more during a work shift..

    This is part of why NEAT can be such a large determinant of body weight In one of the studies I cited earlier, the researchers concluded that one of the largest determinants to NEAT is going to be the occupation that we have. There is a reason that construction workers generally aren’t obese, while people who pick up an office job often pack on the pounds without changing their diet.

    What’s interesting is that the 2000 calorie difference in NEAT is largely attributed to different jobs, however even when we look at people with the same occupation we can see differences in NEAT of several hundred calories. This is where the genetic component to NEAT really makes itself evident; someone who is the exact same body weight, body fat, height, and gender as you, who is performing the exact same exercise program as you while also working the same job as you can still wind up burning hundreds of calories per day more than you simply because they tend to move around/stand/fidget more than you do.

    So, do people actually have slower metabolisms than others?

    Yes and no.

    Let's recap the differences in metabolic factors that compromise our overall metabolic rate.

    200
    RMR Variability
    100
    TEF Variability
    500
    TEE Variability
    2000
    NEAT Variability

    Resting Metabolic Rate will vary by approximately 200 calories between two people of the same height, weight, and gender, but that difference becomes dramatically smaller when we also make sure the individuals being compared are the same body fat percentage. The Thermic Effect of Food shows almost no variability when people are eating the same amount of calories, but can vary slightly if one individual is eating a high protein diet (which increases TEF) while another individual is eating a lower protein diet. The Thermic Effect of Exercise may account for an average of 500 calories per day depending on the duration and intensity of the exercise, but this difference virtually disappears if we compare individuals on the same exercise program, Lastly, NEAT can represent huge differences in calories burned, and even when comparing individuals of the same bodyweight, height, gender, and body fat percentage, who are doing the same exercise program and who work the same job, NEAT can still vary by several hundred calories.

    With this in mind, we can finally answer whether some people have faster metabolisms than others:

    No they do not, because people of the same body-weight only vary their resting metabolic rate by about 200 calories at the most. That skinny person who can eat anything without gaining weight probably wouldn’t be able to do that if they were stuck in a bed all day, unable to move.

    At the same time, some people demonstrate clear differences in how many calories they burn through NEAT, and NEAT does appear to have a somewhat genetic component. So, when some people overeat, they also start to move around much more during the day and burn off the extra calories. However, when other people overeat they may not increase their activity levels at all, leading all the calories they overate to be stored as body fat.

    This seems to be the main reason that some people can clearly get away with more food even without purposefully exercising more. This advantage goes away however if the person with higher NEAT levels is unable to move around as much. Simply switching from an active job to a desk job can result in dramatic drops in NEAT, and a corresponding increase in weight unless the diet is adjusted.

    Now what?

    The primary point in writing all of this is to dispel the myth that some people have magic metabolisms, while other’s do not. The reality is that some people move more than others, and that may be partially determined by genetics, but is not 100% genetic. Regardless of whether you were born to burn a lot of calories via NEAT or not, you can still increase your overall metabolic rate via regular exercise.

    It's important to remember that there are two mindsets we can have in life: a Fixed Mindset or a Growth Mindset.

    When we choose to have a Fixed Mindset, we fully believe that we are victims to circumstances beyond our control, so we may as well not even try to make the most of the hand we've been dealt.

    When we choose to have a Growth Mindset, we acknowledge that we are largely in control of our own outcomes in life. Rather than complain about a "slow metabolism" or the fact that some other people may be genetically prone to avoiding fat gain, they choose to exercise more and alter their food habits. After all, the research is quite clear that while some people are less likely to become fat, virtually everyone is capable of losing any fat they put on via diet and exercise.​

    Author's Note: Throughout this article, I chose not to mention certain disorders such as Hypothyroidism or PCOS. I readily acknowledge that there are medical conditions that can legitimately slow down an individual's metabolic rate far below what it should be, however those conditions are quite rare. Additionally, you would most likely be fully aware of such a condition if you had one; they tend to show up on the blood work performed during an annual physical and the side effects are hard to miss even if you haven't been officially diagnosed yet. For individuals with such medical disorders, pharmaceutical drugs are available that can help to normalize their metabolism and make it easier to maintain a healthy body weight.

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    About the Author

    Coffee Lover, Puppy Petter, and Nutrition Geek. When I'm not dealing with technology issues or watching Game of Thrones, I'm busy learning about nutrition and exercise science.

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