TL; DR Video at the bottom of article.
The world metabolism is simultaneously one of the most commonly used words in the nutrition & fitness community and yet one of the most misunderstood. The average Joe has at least heard the word, but very few people’s words would carry any merit when attempting to actually explain it. This article is my attempt at explaining it as simply and succinctly as possible.
Let me preface this by stating that I do not have a degree in biochemistry, physiology, or anything like that. I do not claim to have a profound understanding of metabolism enough to talk about the intricacies of cell biology at a level that would satisfy college professors. I do, however, have enough of an understanding to make it make sense for the average person. Hopefully by the end of this article you’ll feel that you could explain metabolism to your friend or family member in a way that makes sense.
Let’s first start by stating what metabolism is not. Metabolism is not some internal organ or gland that regulates your weight, determines how much food you can eat or how fat or skinny you are. I’d also like to make it clear right away that it’s a complete load of bullshit when someone advises you not to do something because you could “damage your metabolism.” There are a lot of myths out there about metabolism and they stem from a simple lack of understanding of what metabolism actually is.
The scientific definition of metabolism is: the chemical processes that occur within a living organism in order to maintain life.
By that definition, you can clearly see that metabolism is a process that occurs within a wide range of living organisms on the planet, from human beings all the way down to, for example, the germs in your mouth, and even smaller down to the cells that comprise your body. Your body is literally one giant unit composed of roughly 40 trillion cells (more or less depending on the size of your body). So for all intents and purposes, your metabolism IS your body. More specifically, your metabolism is the summation of all the chemical transactions taking place in a vast and complex network of cells which we call the human body. You ingest food, you metabolize it. Your body breaks down the carbohydrates, fats, and proteins and uses them as energy in order to carry out your bodily functions and processes. Any excess energy (a calorie surplus) is stored as fat or put towards new muscle. Any lack of energy (a deficit of calories) is compensated for by your body metabolizing the fat within your adipocytes (fat cells), and unfortunately sometimes your muscle, too.
This leads me to my next point: if you are fit and slim, you do not have a fast metabolism. If you are overweight, you do not have a slow metabolism. The words fast and slow are not even appropriate words to describe metabolism. You simply have a higher or lower BMR (basal metabolic rate), which is a number that tells us how many calories a person’s body requires in order to carry out its basic, physiological and biological functions and processes. In other words, your BMR tells us how many calories your body requires in order to function. This would be the number of calories you’d need to consume just to maintain your current weight if you were to sit on the couch all day and do absolutely nothing. Contrary to common belief, a person who is obese at 300 lbs. standing 6 feet tall would actually have a higher BMR than someone with 10% body fat at 5 foot 8. Why? Because the obese person’s body requires far more calories in order to maintain itself and stay at its current weight. A person’s BMR can vary considerably from person to person. Factors in one’s BMR are height, weight, age, and body composition, so even 2 people of the same height and weight can have different BMRs, because 2 people who are each 6 feet tall weighing 200 lbs. could have totally different ratios of muscle vs fat, and muscles require more calories than body fat.
Now let’s talk about another factor that affects your metabolism: activity level.
You’ve heard people say, “I don’t have the metabolism I used to have when I was younger”, or when talking to a teenager, “Enjoy your metabolism while you have it.” Why does it seem that younger people have better metabolisms? Couple of reasons: in general, they’re more active, and secondly their bodies are still growing, and growth requires more calories, so those excess calories may end up being put towards things other than stored fat. Activity level, however, plays a much larger role in calorie expenditure than the other reason I mentioned. Again, in general, younger people are simply more active than older people, though there are certainly exceptions to this.
For example, take Darth Maul, the badass Sith from Star Wars. Darth Maul goes around kicking people’s asses with a double-bladed lightsaber doing back flips and shit. Now take Darth Sidious, the slow, old ass man who sits in a god damn chair all day doing basically nothing because he’s the emperor. Who do you think burns more calories? Obviously Darth Maul could afford far more visits to the McDonald’s on the Death Star without having to worry about those calories going to his ass. His body expends much more energy doing all that gnarly dark side shit than Darth Sidious who, on the other hand, has to save most of his calories for the day if he wants to fit a Quarter Pounder with Cheese and a 12-piece order of chicken McNuggets (his favorite meal combo).
The point is that metabolism can be temporarily increased via exercise or other physical activity. During physical exertion, your body has to do everything harder. Your heart needs to beat faster, your muscles need to contract and expand, your lungs need to work harder to take in more air, your circulatory system needs to work harder to pump more blood & oxygen through your entire body. Everything your body does at rest, it has to do that much harder during exercise. This is why your metabolism is temporarily increased.
You might be asking, “Can you PERMANENTLY increase your metabolism?” The answer is yes. How? If you were paying attention earlier in this article, you’ll know the answer is quite simple: gain weight. The analogy I like to use is wood in a fire. If wood is food and your body is the fire and you want to make the fire bigger, what do you do? Naturally you would toss more wood into the fire. Want a smaller fire? Decrease the amount of wood it’s burning by feeding it less wood. Your body essentially works the same, in the sense that when you eat less food, you get smaller. If done right, getting smaller comes mostly in the form of fat loss (as opposed to losing muscle as well, which can happen if training is poor and your calorie deficit is too severe, but that’s a topic for another time). If you want to be able to burn more calories permanently, increase your BMR by gaining weight.
Lastly, you may also hear people talk about body types: mesomorph, endomorph, and ectomorph. I won’t get into those too much, but let’s just say that while it is true that people do have different body sizes, shapes, and different ratios of the amount of muscle vs fat, these body type labels are not an excuse to say you have a slow or fast metabolism. If you’re an endomorph, it’s because you became that way over time; your body adapted to the amount of food you were giving it. Once adulthood was reached, your body already developed its shape. So if you’re overweight, you’re already at a predisposition to gain more fat when in a calorie surplus, giving the impression that you have a slow metabolism. The simple fact is fat people gain fat more easily. The ectomorph, on the other hand, never ate enough growing up to become overweight, and so his body appears thin and skinny. Again, this doesn’t mean he has a fast metabolism. It just means he doesn’t eat enough to gain any significant weight. People will say, “I know so-and-so, and he eats anything and everything and never gains any weight.”
Are you sure about that? Do you monitor his food intake? Do you calculate his daily calorie intake vs expenditure? If you did, what you would find is that on a weekly average, his calorie intake vs expenditure comes out to be balanced. That’s why he doesn’t ever gain weight. He may gorge on foods on certain days, and eat very little on other days. That’s an energy balance.
If you’re skinny and struggling to put on weight, you either need to eat more or decrease your activity level. If you’re struggling to lose weight, you need to eat less or increase your activity level (or both).
Hopefully this article has given you at least a rudimentary understanding of metabolism, at least enough to bust the common myths people mention about the topic. Humans don’t often like to take personal responsibility for things. Body type labels, fast or slow metabolisms – these are just excuses people use to compensate for a lack of willpower or discipline. Don’t worry about losing weight out of fear of “damaging your metabolism.” Don’t worry about gaining weight because of the addition of a little fat. If you understand the if-it-fits-your-macros philosophy, then you know how exactly how to take it off or put it on.
If you are someone who struggles to lose weight, hopefully now you realize it’s not because there’s something wrong with your metabolism. It’s not slow. If you’re struggling to gain weight, hopefully now you know it’s not because you were blessed with some godly gene where your body just magically burns every calorie you put into it. Science is always there to prove your beliefs wrong. Welcome to the world of nutrition and training.