What is Metabolism?

This word “metabolism” is thrown around a lot these days. Many slim people claim they have a fast metabolism. You may think you gain weight more easily if you have a slow metabolism.

But what exactly does metabolism mean, and can we alter it?

Well technically “metabolism” is the word to describe all of the biochemical reactions in your body.  It’s how you take in nutrients and oxygen and use them to fuel everything you do.

Your body has an incredible ability to grow, heal, and generally stay alive.  And without this amazing biochemistry ‘you’ would not be possible.

Metabolism includes how your body:

  • Influences activities you can control (e.g. walking, dancing, moving ).
  • Keeps activities you can’t control going (e.g. heart beat, wound healing, digestion and so on).
  • Allows storage of excess energy for later (it does this by storing glycogen in the liver and muscles or fat in fat cells… more about this in another post!)

So when you put all of these processes together into your metabolism you can imagine that these processes can work too quickly, too slowly, or just right.

Which brings us to the “metabolic rate”.

Metabolic rate

This is how fast your metabolism works and is measured in calories (Apologies. You know I normally hate talking about calories!).

The calories you eat can be used up by the body in 3 different ways:

  • Providing the energy for Physical Work (i.e. exercise and other activity).
  • Providing Heat (i.e. from all those biochemical reactions).
  • Storage (i.e. extra leftover “unburned” calories stored as fat).

As you can imagine the more calories you burn as work or to create heat the easier it is to lose weight and keep it off because there will be fewer “leftover” calories to store for later.

There are a couple of different ways to measure metabolic rate.  One is the “resting metabolic rate” (RMR) which is how much energy your body uses when you’re not being physically active. So this is the amount of calories that you would burn even if you stayed in bed all day.

The other is the “total daily energy expenditure” (TDEE) which measures both the resting metabolic rate as well as the energy used for “work” (e.g. exercise) throughout a 24-hour period.

What affects your metabolic rate?

In a nutshell: a lot!

The first thing you may think of is your thyroid.  This gland at the front of your throat releases hormones to tell your body to “speed up” your metabolism.  Of course, the more thyroid hormone there is the faster things will work and the more calories you’ll burn.

But that’s not the only thing that affects your metabolic rate.

Size does matter! How big you are counts too.

Larger people have higher metabolic rates; but what matters is your body composition.


As you can imagine muscles that actively move and do work need more energy than fat does.  So the more lean muscle mass you have the more energy your body will burn and the higher your metabolic rate will be.  Even when you’re not working out, those muscle cells will be using up more calories than the fat storage cells.

This is exactly why weight training is often recommended as a part of a weight loss programme.  Because you want muscles to be burning those calories for you.

The thing is, when people lose weight their metabolic rate often slows down which you don’t want to happen.  So you definitely want to offset that with exercising and keeping up the muscle mass.

Aerobic exercise also temporarily increases your metabolic rate.  Your muscles are burning fuel to move so they’re doing “work”.


The type of food you eat also affects your metabolic rate!

Your body actually burns calories to absorb, digest, and metabolise your food.  This is called the “thermic effect of food” (TEF).

You can use it to your advantage when you understand how your body metabolises foods differently.

Fats, for example increase your TEF by 0-3%; carbs increase it by 5-10%, and protein increases it by 15-30%.  By trading some of your fat or carbs for lean protein you can slightly increase your metabolic rate.

Another bonus of protein is that your muscles need it to grow.  By working them out and feeding them what they need they will help you to lose weight and keep it off.  But this doesn’t mean you should go all out on the protein and ignore the carbohydrates and essential fats. Your body still needs them. Just make sure you are having good sources of protein with every meal and snack.


And don’t forget the mind-body connection.  There is plenty of research that shows the influence that things like stress and sleep have on the metabolic rate. We discuss this more in the Wellness section.

This is just the tip of the iceberg when it comes to metabolism and how so many different things can work to increase (or decrease) your metabolic rate.