Metabolism: What Does It Actually Mean?

NUTRITION

Jul 16

Written By Ben Waters

You've probably heard somebody say:

“I've got a slow metabolism.”

Or:

“He can eat whatever he wants. He must have a fast metabolism.”

We tend to talk about metabolism almost exclusively in relation to bodyweight.

Usually when somebody is trying to lose it.

But metabolism isn't simply how quickly you burn calories.

It's much bigger than that.

What Is Metabolism?

Metabolism is the collective term for the enormous number of chemical reactions constantly taking place within your body to keep you alive and functioning.

These processes allow your body to do things like:

Produce and use energy.

Build and break down tissues and molecules.

Maintain your cells.

Support growth and repair.

Process nutrients.

Keep your organs functioning.

All of this is happening continuously without you consciously thinking about it.

So when people talk about having a “fast” or “slow” metabolism, what they usually mean is something more specific:

How much energy does my body use?

What Is Metabolic Rate?

Your body uses energy constantly.

Even if you spent the entire day lying completely still, you'd still require energy to:

Breathe.

Circulate blood.

Maintain body temperature.

Run your brain.

Keep your organs functioning.

Maintain and repair tissues.

The energy required to keep you alive at rest makes up a large proportion of your daily energy expenditure.

But that's not the whole picture.

You also use energy through:

Physical activity and exercise.

Everyday movement.

Digesting and processing food.

Put everything together and we get your total daily energy expenditure.

That's the number we're generally interested in when discussing calories and bodyweight.

Does Having a Bigger Body Mean a Slower Metabolism?

Here's where things get interesting.

People sometimes assume that someone carrying more bodyweight must have a slower metabolism.

That's not necessarily true.

Larger bodies generally require more energy to maintain and move than smaller bodies.

Body composition matters too.

Muscle is more metabolically active than fat tissue, although adding muscle doesn't suddenly transform you into a calorie-burning furnace.

Age, sex, genetics, body size, body composition and activity can all influence energy expenditure.

So the person who describes themselves as having a “slow metabolism” may actually burn more calories per day than a considerably smaller person.

Bodyweight alone doesn't tell us whether someone's metabolism is unusually fast or slow.

What About Genetics?

People do differ.

We're not identical machines.

Genetics can contribute to differences in metabolic rate, body composition, appetite and other factors related to bodyweight.

But the enormous differences people sometimes imagine between “fast metabolism people” and “slow metabolism people” are often more complicated than resting metabolism alone.

For example, people can differ considerably in how much they move throughout the day.

Some naturally walk more.

Stand more.

Fidget more.

Take the stairs.

Generally spend less time sitting still.

Those small differences can accumulate.

And then there's food intake, appetite, environment, sleep, training and lifestyle.

Bodyweight is influenced by considerably more than one metabolic number.

Calories Aren't the Enemy

A calorie is a unit of energy.

That's it.

Calories aren't a substance hiding inside your food trying to make you fat.

Your body needs energy.

You need it to train.

Walk.

Think.

Recover.

Work.

Play.

And stay alive.

If you're trying to lose body fat, energy intake becomes particularly important because fat loss requires an energy deficit over time.

But the objective isn't necessarily:

“Eat as few calories as possible.”

It's to create an appropriate deficit that you can actually sustain while still supporting your health, training and everyday life.

Can Dieting Slow Your Metabolism?

This is another area where things get exaggerated.

When you lose weight, your energy requirements generally decrease.

There's simply less body mass to maintain and move around.

Your body can also adapt to an energy deficit. Energy expenditure may decrease somewhat beyond what we'd expect from the change in body size alone, a phenomenon generally referred to as adaptive thermogenesis.

You may also unconsciously move less when you're eating considerably less food.

But this doesn't mean dieting has “broken your metabolism.”

And it certainly doesn't mean fat loss has become physically impossible.

It means energy expenditure is dynamic rather than fixed.

Don't Crash Diet

This is where I agree with the philosophy behind the original version of this article.

If you're trying to lose fat:

You don't get bonus points for eating the least food possible.

A more aggressive deficit might produce faster weight loss initially, but it can also make training, recovery, hunger management and adherence considerably more difficult.

If a moderate calorie intake is producing appropriate progress, there's no automatic reason to slash another 500 calories simply because you want everything to happen faster.

Use the smallest intervention that produces an appropriate result.

Then adjust when necessary.

Does “Food Quality” Increase Your Metabolism?

Not in the way this is sometimes presented.

Eating more nutritious foods doesn't “nourish your cells” into running a faster metabolism.

But food quality still matters enormously.

Your diet needs to provide adequate:

Protein.

Essential fats.

Vitamins and minerals.

Fibre.

Energy.

And eating patterns that contain plenty of nutritious foods can make it easier to support health, performance, recovery and body composition.

As we covered in Food Quality, quantity and quality both matter.

They're answering different questions.

Does Eating More Protein Boost Metabolism?

There is a small but interesting piece of the puzzle here.

Your body uses energy to digest, absorb and process the food you eat. This is known as the thermic effect of food.

Protein generally requires more energy to process than carbohydrate or fat.

But I wouldn't turn that into:

“Eat protein to hack your metabolism.”

Protein is useful for much better reasons—particularly supporting muscle growth and maintenance and helping with satiety.

The thermic effect is simply part of the bigger energy-expenditure picture.

What About Strength Training?

Strength training is fantastic.

You probably don't need much convincing of that from me.

Building and maintaining muscle can contribute to your energy requirements, but claims that adding a little muscle will massively increase your resting metabolism are exaggerated.

That's not why you should lift.

Lift because being strong is useful.

Because muscle matters.

Because training can improve physical capacity.

Because you enjoy it.

And because maintaining muscle becomes particularly valuable when you're losing weight and as you get older.

You don't need to pretend every benefit of lifting comes from burning more calories.

Can You “Boost” Your Metabolism?

This phrase sells an enormous number of supplements.

Usually the promised solution is:

A particular food.

A tea.

A supplement.

A spice.

A detox.

A secret morning routine.

The boring answer is that there isn't a useful metabolic switch that transforms your body into a dramatically faster calorie-burning machine.

Instead, concentrate on things that actually matter:

Be physically active.

Strength train.

Eat enough protein.

Build a nutritious diet.

Sleep appropriately.

Avoid unnecessarily aggressive diets.

Use a sustainable calorie deficit if fat loss is the goal.

Those things aren't as exciting as a metabolism hack.

They're considerably more useful.

Metabolism and Fat Loss

If fat loss is your goal, I like this general principle:

Don't automatically eat less than you need to.

We need an energy deficit.

But if you're steadily losing fat while eating 2,000 calories, there isn't necessarily a benefit to immediately dropping to 1,500 simply to make the process faster.

We want enough food to support:

Your training.

Your recovery.

Your nutrition.

Your lifestyle.

And your ability to stick to the plan.

Fat loss isn't a competition to discover how little food you can survive on.

The Bottom Line

Your metabolism isn't simply “fast” or “slow.”

It's the enormous collection of chemical processes that keeps your body functioning.

When we're discussing bodyweight, we're usually talking more specifically about energy expenditure.

And that's influenced by things including:

Body size.

Body composition.

Activity.

Food intake.

Age.

Sex.

Genetics.

And adaptations to changes in bodyweight and energy intake.

You don't need to hack your metabolism.

You don't need to cleanse it.

And you probably haven't broken it.

Eat appropriately for your goal.

Move.

Lift.