What is Metabolism?

“I guess I don’t mind so much being old, as I mind being fat and old.” –Peter Gabriel

Metabolism is one of the most misunderstood concepts in fitness.

The purpose of this article is to provide a simple but accurate explanation for one of the most important concepts you need to grasp if you want to get lean and stay lean for life. There are so many misconceptions about metabolism floating around and if you buy into them, you may very well be approaching health and fitness in a counterproductive way or at best, in a suboptimal way. If you want to optimize your body’s fat burning capabilities, you must first understand how your body uses energy. What is metabolism? The answer will help you unlock your body’s true fat burning potential.

What is metabolism? Despite what you may have heard, it’s not all genetics and age.

I am not a biochemist and I don’t pretend to be one.

So if I can understand metabolism, then you can too. It’s actually quite a complex process, but for the purpose of this post, let’s use a simple supply and demand analogy and just say that your metabolism is defined as your body’s demand for energy. A person with a high metabolism has a higher demand for energy than a person with a low metabolism. As your body uses up its energy, it needs to create more energy for basic bodily functions, activity, and to respond to fight or flight. This process of creating more energy in itself requires energy. Therefore, a person who uses more energy will need to create more energy and will use more energy in the process of creating more energy. Hopefully, you’re still with me.

The human body is extremely efficient.

It knows exactly how much energy it needs to be producing based on your age, body type, and activity level. So it knows it’s going to need to supply more energy for an active individual than it will for an inactive individual and this is exactly what it does. Think of it like this. A high-metabolism person’s energy supply and demand scenario is like a fire hose. A low-metabolism person’s energy supply and demand scenario is like a dripping faucet. There are many factors that influence your body’s energy requirements including age, calorie intake, and activity level. There are other factors such as body size and muscle to fat ratio, but I’m going to go into these three in more detail because there are some big misconceptions associated with each of them.

Age plays a very small part in your metabolism.

A lot of people I know think that age is the biggest factor driving your metabolism. This is simply not true. What’s worse, some people believe that age is the only factor driving your metabolism. People who see me eat a lot and stay lean have actually told me “Wait until you hit 30, you won’t be able to do that anymore because your metabolism will slow down.” It’s as if they believe that on the day of my 30th birthday, my body will flip a switch and abruptly shut down my metabolism. This is complete nonsense and my argument against this is that if age is the only factor, then everyone who is 30 would look the same. Everyone who is 40 would look the same. Everyone who is 50 would look the same. Clearly, this isn’t the case. There are some people in their 50s who are in much better shape than some people in their 20s. So don’t let anyone tell you that age is the only factor driving your metabolism.

If you think a fast metabolism is only for young people, think again. Take a look at this pic of Mark Sisson, taken when he was 58 years old.

If you eat more, your body will require more energy to process the food.

This makes sense, right? It requires energy to digest food and this is the logic behind the idea of eating 6 meals per day to keep your body “constantly burning fat”. Hold up, don’t get too excited. Calorie intake is another misunderstood piece of the metabolism puzzle. Yes, your body will spend a little extra energy digesting your food. However, that doesn’t give you free reign to eat whatever you want because if you eat beyond what your body’s energy needs are, you’ll still gain weight. Think about it this way, and I’m completely making these numbers up. Say you eat a 500 calorie meal and your body burns 20 calories digesting the meal. There are still 480 calories leftover. Now, if you’re active and using up these 480 calories or more either to replenish depleted energy levels from prior exercise or for future exercise then you’re in the clear. If not, then they just get stored as fat. Remember, at the end of the day it’s still a supply and demand equation.

Active individuals require more energy than sedentary individuals.

The activity level factor is really the key to your metabolism. Remember, your body will only supply as much energy as it thinks your body needs. So if you are active, it will supply more energy and doing that will require more energy. The best way to raise your metabolism is to be more active. This is tricky because depending on how fit you are, it will take some time for your body to respond. If you are overweight and out of shape, going out and working out once or twice won’t raise your metabolism overnight. At first, your body will still take its time replenishing your energy levels because it’s used to a low energy demand.

This is why it’s really hard at first for a fat person to undergo very intense exercise. His body won’t provide enough energy to keep up. It’s almost like his stubborn body thinks he is bluffing so until he consistently proves that his demand for more energy is legit, he will get no respect. A fit person, on the other hand, gets instant respect from his body. When he begins to exercise, his body responds instantly because it has seen time and time again that he means business. More importantly, his body is supplying more energy during rest periods to replenish depleted stores and in anticipation of higher energy demand in the near future.

A person with a fast metabolism burns more calories, even when he is sitting around being a couch potato.

This is precisely why a fit person with a high metabolism burns more calories 24-7, even during rest.

Elite athletes are at the extreme upper end of this spectrum. This is why they can eat 6,000 calories per day in some cases and stay lean and ripped. Meanwhile, the sedentary cubicle worker is slowly but surely gaining weight on half of that intake.

There is one simple message I’m trying to drive home here.

What is metabolism? Metabolism is the body’s demand for energy. If you want to enjoy the benefits of a high metabolism, you need to get off your butt and be as active as possible. If you are way overweight, be smart about it and gradually work your way up. Challenge your body and your muscles without overdoing it. As you get fitter, you will notice your demand for energy increasing and that you’re burning more fat, even when you’re sitting around. Eat the right amount to meet your energy needs, not too much. If you are already lean and fit, keep doing what got you there because the body adapts quickly and can easily go the other way if you start to slack off.


About admin

I'm an introvert, number nerd, food junkie, personal finance enthusiast, and fitness blogger. I specialize in fat loss and stress-free fitness for busy people.


  1. Great you have posted this as metabolism is a funny subject most people don’t realise that science has adequately proven the reasons why metabolism functions as it does, but they prefer to listen to people in the gym.
    Age is a contributing factor but not the only one.
    I would say Mark is an exception to the rule as most 58y/o don’t look like him although he makes it a real possibility. And when people who are overweight say they a slow metabolism or even a skinny guy says he has a fast one .. I say its more related to their food intake than any thyroid issue.
    Raymond-ZenMyFitness´s last blog post ..How To Use Advanced Weight Lifting Techniques to Break Through Plateaus

  2. @ Raymond,

    I think people get a little intimidated by the science, so they turn to word of mouth instead. There are definitely people with thyroid issues but they are a small minority. In most cases, activity level is the biggest factor.

  3. My whole life people have told me how lucky I am to have been born with a fast metabolism. Genetically, I am on the skinny side and I do believe genetics can play a huge roll in body shape. But that doesn’t mean fat people can blame their parents for their weight. You do a great job at identifying the metabolism as not being the root of the cause, like so many people believe. It is much, much more than that.
    Nate Armstrong´s last blog post ..Exercising Indoors

  4. @ Nate,

    Genes and body type do influence your metabolism to some degree, but people have much more control over it than they realize. Young kids are always on their feet, running around, crawling under things, climbing onto things. It’s no wonder their metabolisms are so high! Imagine if adults were to do the same.

    It’s easy to blame a slowed metabolism on age or genetics, but if people take a closer look, they’ll see that the movement of their bodies has decreased dramatically over time and has had a much greater impact on metabolism than aging or genes.

  5. I’m really glad you touched on the subject of metabolism. It really is a misunderstood concept and often people think that it is something that comes with age and that is difficult to control.

    I remember when I was in my early 20’s and looking thin people would tell me, wait til you’re 25. Well 25 came and went and then I heard, “wait til you’re 30.” I’m now 35 and I was told my metabolism would come to a grinding halt by now. Well, the truth is I have been an active gym person and work out since my early 20’s.

    So looking at my case I can vouch that it is a matter of staying active if you want to keep a higher than average metabolism.

    Sam- Look Like An Athlete´s last blog post ..Barbell Complex Workout: Build Muscle And Burn Fat In Less Time

  6. Great review of metabolism. I’m just working on an article about G-Flux, the notion that your body burns more fat by eating more and exercising more rather than doing either in isolation. Exercise increases metabolism, eating increases metabolism, therefore there are some synergies to combining the two rather exercising a lot and eating a little. I’m still looking into it but it makes some sense for the long term. In the short term, I think you can work your body pretty hard without significantly altering your metabolism too badly.
    Dave – Not Your Average Fitness Tips´s last blog post ..Best Fitness Tips

  7. @ Sam,

    I bet 20 years from now, people will still be trying to tell you that your metabolism is bound to slow down. Or maybe by then, they will actually buy into it… this is just another huge benefit of living a healthy, fit lifestyle.

    @ Dave,

    This subject is really fascinating to me. I think for people who are really overweight, exercising a lot and eating a little still works well because their bodies don’t burn and replenish energy as efficiently during intense exercise. So in theory, they could still gain weight by exercising and eating a lot.

    Once people become fitter and eventually super fit, the large calorie deficit is counterproductive because they are using energy very efficiently so if their activity level is very high while their intake is too low, they’re not supplying enough energy to maximize their workouts and this would encourage the body to store more fat for energy instead of burn it.

    Looking forward to reading your post!

  8. Saw your website on Rusty’s training site, very good post, people get metabolism confused too much, I hope you check me out at http://www.somebodylied.com, my website has only been active just over a week, but I have been working hard at getting articles up before University begins.

  9. @ Michael,

    Thanks for visiting and reading the post! I checked out your site and it looks great. Keep up the good work!

  10. Fair play taking on what often seems a complicated subject. I use exercise as a bonus when it comes to fat loss and concentrate on eating right for my goals as the main aim. After all there is only so much exercise we have time to do.

    Michael- The Underwear Body´s last blog post ..What Body Signals Are You Sending?

  11. I think people get too hung up on the science of weight loss/fat burning. I agree with your comment that people get confused and tend to rely on word of mouth. People should just start with the basic of eating less calories than their body requires (less calories in and more calories out = weight loss). Once they understand and have put this concept into practice, then it is time to examine the more complex side nutrition and how to manipulate your metabolism.

  12. @ Michael,

    I do the same. For most of us, diet is the most important factor for staying lean. I still believe that you cannot outexercise a terrible diet, although with a high enough activity level and metabolism, I think you can outexercise a somewhat poor diet. More on this in my next post!

    @ Niko,

    Right on. People who struggle with obesity need to first and foremost create a large calorie deficit to get back on track. For these people, this is the quickest and most efficient way to lose weight. It was only after I dropped down to my target weight when I was able to start manipulating my metabolism and increasing calories once again.

  13. That’s good info! I didn’t really get the idea of eating several times a day and I did learn that eating healthy and 6 times a day also requires portion control… ooops. I realized and rallied! I also see the lesson of your body not believing you are really going to use it. I was working out 2 a day every day for two weeks and eating well and only lost one lb. A little discouraging, but I could feel myself getting stronger so, I shrugged it off and just kept going…the 3rd week… 5 lbs. Now, I guess it is actually starting to wake up from the sugary salty inactive coma I put it in for the last 10 years… Thanks for the simple break down AND the 58 yr old pic. I’m 40 and I can check off that excuse.

  14. @ Brenda,

    Glad to hear you are making progress. Your body will eventually adjust to your higher metabolism and that’s when eating more will be okay. You really need to get a feel for your body, though, to be able to know when this is happening. These days, I feel like I’m always hungry and it’s real hunger (like when my stomach growls), not just made up hunger. This is when I know it’s reasonable to up my calories a little. Elite athletes need to eat 6 times a day just to maintain thier energy levels but for most people who are inactive, this will actually be counterproductive unless they practice very stict portion control, as you alluded to.

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