What Is Metabolism and How Does It Really Work?
17 min read

What Is Metabolism and How Does It Really Work?

What Is Metabolism and How Does It Really Work?

“You are worth about 3 dollars worth in chemicals.” ― Carl Sagan

what is metabolism mri scan obsese woman

It’s very trendy nowadays for people in the fitness community to talk about metabolism.

Lose fat by boosting your metabolism!

Don’t diet, just improve your metabolism!

The 8 foods that supercharge your metabolism!

And so on and so forth.

We’ve all seen the magazine covers.

When it’s not being used as a magic pill to promote some new weight loss shake, it’s recycled into the biggest fat loss excuse out there.

Believe it or not, I don’t eat that much.  I’ve tried everything. I just can’t lose weight. I have a slow metabolism. It’s my genetics, and it stops me from losing weight.

I’m not saying that you can’t speed up your metabolism or that there are never individual variances, but don’t you think it would be wise, before forming an opinion on the matter, to at least have some idea how the thing works?

That’s what this post is all about.

The word ‘metabolism’ gets used in the fitness industry in just the same way that the word ‘quantum’ gets used in certain niches of the self-development industry.

“Interdependence arises and subsides in quantum energy”— Deepak Chopra

Both are scientific words that describe a complicated series of scientific processes, which few understand properly.

Both get used in vague contexts that seem enlightening but aren’t upon closer inspection because the people using them, don’t understand them themselves.

“The great American physicist Richard Feynman once said ‘If you think you understand quantum theory, you don’t understand quantum theory.’ Isn’t Deepak Chopra just exploiting quantum jargon as plausible sounding hocus pocus?” — Richard Dawkins 

The same gurus who can’t seem to stop telling us about ‘the super foods that speed up our metabolisms’, always seem run out of words when you question them about cellular respiration, glycolysis, the electron transport chain and the role of adenosine triphosphate in the Krebs cycle.

Pretty strange considering all of these processes are fundamental to the way in which we manufacture energy from calories, store fat and gain muscle.

Textbook stuff every nutritionist ought to know.

Don’t get me wrong, the biochemistry of human metabolism is very confusing and complicated if you’re studying for a Ph.D.:

complicated biochemistry equation


But for us health and fitness enthusiasts who just want to improve our physique – this won’t help us.

What will, is understanding the big picture of what is metabolism and how it works, so we can make better informed nutrition decisions and use our critical thinking the next time a guru bombards us with metabolic gobbledegook.

*The idea for this article came from the popularity of my comprehensive nutrition primer, The Truth about Calories, Macronutrients and Weight Loss. If all you’re after is solid practical nutrition advice I’d recommend that article first, it will have everything you need. This article goes a little deeper down the nutritional rabbit hole.

We’ll start with an overview of the metabolic process, before zooming into each subsection separately. If you notice anything being repeated, it’s been done on purpose to help cement in the lessons.

The keywords that you should pay particular attention to in the following overview are emboldened.

 This article goes very in-depth with metabolism, so if your goal is just to lose weight, check out my fat loss resource page instead.

A Brief Overview of Metabolism

Scientific definition: Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.

Put simply, a metabolism is the total of all the chemical reactions in the human body. The two most important of these reactions are those that build things up (anabolism) and those that break things down (catabolism).

On top of that, our metabolism gives us energy to move, adapt, repair and do all the things humans do. The running costs of powering our metabolism isn’t free either. 

We need energy to make energy. Even power plants have bills to pay.

The energy currency of the body is a compound called ATP.

ATP = Biological Energy

Deriving ATP from food is one of the chief purposes of catabolism. In the absence of food (calorie deficit), our stored fuel sources (skeletal muscle, body fat, liver glycogen) can also be catabolized and turned into ATP.

You could say that a person who is losing weight is partly ‘feeding’ off themselves.

Anabolism is all about building things back up. Repairing and generating cells, gaining weight, and storing the building blocks of ATP are all dependent on the anabolic reaction.

Speaking of building blocks…

The Four Essential Nutrients

In order for our metabolisms to carry out the processes mentioned above we need to have eaten. But it’s not really food our metabolisms are after.

Our metabolism doesn’t speak the language of food. It only understands nutrients.

The requirements of life boil down to our bodies ability to use the following four essential nutrients (biochemists call them macromolecules):

  1. Proteins
  2. Fats
  3. Carbs
  4. Nucleic Acids (the stuff of DNA)

You may recognise some of these as the macronutrients from your food labels. But, while all four of these essential nutrients are present in food, they’re not necessarily in the configuration that the body needs them in to be able to make use of them.

Carbohydrate Note: Dietary carbs are not essential. We can survive without eating them. However, glucose – the broken down form of carbohydrate – is essential to fuel our brains. When we are without dietary carbs, our body can make their own out of pyruvate, lactate, glycerol, glucogenic amino acids, and fatty acids in a process called gluconeogenesis.

To turn the nutrients we eat into the macromolecules we need, we must first digest them, break them down, and reconfigure their structure through catabolism.

Our metabolism kicks off this process by breaking these essential nutrients into their component parts – the smaller structures that make the bigger ones.

The result of this first digestive step is as follows:

 Proteins > Amino Acids

Fats > Fatty Acids

Carbs > Glucose

Nucleic Acids > Nucleotides

From there, now that these molecules have been broken down into their component parts, the body has three options with what to do with them:

  1. Break them down further and convert into ATP. (Catabolism)
  2. Repair, renew, rebuilt, adapt, heal, etc. (Structural Anabolism)
  3. Store for later use. (Storage Anabolism)

That’s the metabolism overview over.

You should now have a basic foundational knowledge of what is metabolism.

Let’s go a little bit deeper.

Metabolism In Action — ATP: The Currency of Biological Energy

Scientific definition: Adenosine triphosphate is a nucleoside triphosphate used in cells as a coenzyme. It is often called the “molecular unit of currency” of intracellular energy transfer. ATP transports chemical energy within cells for metabolism.

ATP is one of the most important molecules in the human body.

Many refer to ATP as the currency of biological energy because the creation of ATP is impossible without spending a few in the process. It’s sort of like a biochemical stock market.

ATP in action allows us to do pretty much everything, but the three most important uses of ATP are:

  1. Biosynthesis (synthesising macromolecules)
  2. Muscle Contraction (movement)
  3. Ion Movement (sending nerve signals)

How does ATP work?

The unabbreviated name for ATP is adenosine triphosphate.

It sounds more complicated than it is. The ‘tri’ in triphosphate means that there are three molecules of phosphate. If you take one molecule of adenosine and add three molecules of phosphate you get a compound that looks like something like this:


That’s ATP.

ATP is called the currency of biological energy because it’s not actual ‘energy’, rather it’s stored energy. To make it become active energy, we must first hydrolyze it.

Hydro = Water

Lysis = Break Open

Hydrolyze = Break Open With Water

When ATP ishydrolyzed, one phosphate molecule pops off — turning ATP into ADP, otherwise known as adenosine diphosphate (two phosphates).

Here’s what that looks like:

A+P+P+P + H2O  =  A+P+P  

It’s the actual popping off of this phosphate molecule that causes the spark of energy.

Think of it like a flint scraping against a rock. When they separate, sparks fly.

Much like hydrolyzed ATP.

sparking a rock animation

After the ATP becomes ADP, some ADP cycles back around to regain a phosphate molecule and the process repeats itself.

This cycle between ATP and ADP is part of a bigger process called cellular respiration. It’s often useful when trying to understand a system to start at the outcome before breaking down the process that gets you there.

So now that we’ve looked at ATP let’s find out how we manufacture it.

Cellular Respiration

Scientific Definition: Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate, and then release waste products.

If ATP is the most important biochemical in the human body, then cellular respiration, by default, is the most important biochemical process. It’s essentially how we derive energy from the food.

Or as we now understand, manufacture ATP.

If cellular respiration goes to plan, in theory, we can manufacture an astounding 38 ATP’s for just one molecule of glucose, in reality it’s a bit less.

Cellular respiration is not the same thing as digestion.

Digestion can be a preliminary stage of cellular respiration that breaks down our macronutrients into their component parts. The difference between cellular respiration and digestion is that…

Cellular respiration is going on right now in your nose.

And in your biceps and your big toe. It’s going on everywhere there are cells, every second of your life.

It’s a very complicated process with lots going on, but biochemists agree that there are three main stages:

  1. Glycolysis
  2. The Krebs Cycle
  3. Electron Transport Phosphorylation

The attainment of ATP from food is a catabolic reaction. And what we do or don’t do with that ATP is typically left over to anabolism.

Biochemistry textbooks always teach cellular respiration through the catabolic process first because biomolecules can not go through an anabolic reaction unless they have first been catabolized.


Scientific definition:  Catabolism is the set of metabolic pathways that breaks down molecules into smaller units to release energy.

man knocking down wall like a catabolic reaction

This house is going through a catabolic reaction.

The attainment of ATP is more or less the whole point of catabolism.

A good way to think about catabolism other than the breaking down and building up analogy is in terms of weight gain and weight loss.

If you are losing weight, you are by default in a net catabolic state. You are breaking down and utilising more energy than you are expending.

When you gain weight, you are in a net anabolic state. You are building up your body, be it fat or muscle gain. Think anabolic steroids.

Catabolism can also occur in our stored energy reserves such as body fat, skeletal muscle and muscle glycogen. When we are in a calorie deficit, for example, the food we eat will not supply sufficient energy to the body, so we catabolise stored fat (and muscle in a bad diet) and convert that into ATP.

Stage 1: Glycolysis

Scientific Definition: Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− H+. The free energy released in this process is used to form the high-energy compounds ATP and NADH.

Glycolysis is a process for glucose only. Amino acids and fatty acids come in just before or during the Krebs cycle.

The basic point of glycolysis is to break down glucose into what’s called pyruvate. For every single molecule of glucose that goes through this process, we get two molecules of pyruvate (don’t worry if you’ve never heard of it).

The backbone of a glucose molecule has six carbons. When pyruvate is created each carries forward three carbons like this:

Glucose(C+C+C+C+C+C) + Glycolysis = Pyruvate(C+C+C) + Pyruvate(C+C+C)

For one molecule of glucose to complete glycolysis, our body must expend 2 ATPs. By the time it’s finished, however, we end up with 4 ATPs. That’s already a 100% return on ATP investment, and this is only the first step of cellular respiration.

Carbohydrates > Glucose > Pyruvate

*If you’d like to get deeper into the biochemistry on any of these topics, check out the resource list at the end.

Now that we have our pyruvate it’s time to go to the next step.

Stage 2: The Krebs Cycle

Scientific definition: The citric acid cycle – also known as the tricarboxylic acid cycle (TCA cycle), or the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats and proteins into carbon dioxide and  chemical energy in the form of adenosine triphosphate (ATP).

 the krebs cycle

The point of the Krebs cycle is to create more ATPs but also to set up the right chemical conditions for stage three – the electron transport chain.


What happens to pyruvate is not officially part of the Krebs cycle, it’s actually a preparatory stage called pyruvate oxidization, but we’ll include it here to keep things simple.

So as you can probably guess, in order for pyruvate to get ready for the Kreb cycle, it needs to be oxidised. When this happens, pyruvate becomes a compound called Acetyl CoA. This compound is right at the centre of all of the action in the Krebbs cycle.

If you remember, each pyruvate has a three-carbon backbone. In order for it to become Acetyl CoA, it must lose one of those carbons:

Pyruvate(C+C+C) + Oxygen = Acetyl CoA(C+C)

When a carbon molecule pops off, it leaves the system by combing with some oxygen can turn into carbon dioxide. Cellular respiration is the reason we need to breathe…

Note: In the absence of oxygen (i.e muscle cells fatiguing during anaerobic exercise), those two pyruvates get reduced into lactate as part of a process called lactic acid fermentation – you know that burning feeling? This is essentially a faster and simpler alternative to the full cellular respiration process. It allows us to put to immediate use the 2 ATPs produced from glycolysis, albeit at the cost of getting the potential 38 ATPs if oxygen were present.

All of the Kreb cycle is catalysed by enzymes. If you’re not sure what enzymes are, they’re just proteins that bring together the constituent things that need to react, so they do react.

AcetylCoA then merges with oxaloacetic acid to create citric acid, the stuff in orange juice.

Acetyl CoA(C+C) + Oxaloacetic Acid(C+C+C+C) = Citric Acid(C+C+C+C+C+C)

This is why the cycle is sometimes referred to as The Citric Acid Cycle. But Hans Adolf Krebs was the name of the super smart guy who discovered it, and I like to pay credit where credit’s due.

From here the citric acid goes around the cycle bouncing and reacting with high energy electron carriers called NADH and FADH2 – we’ll get to them in a moment.

By the time the cycle comes to its end the citric acid loses two carbon molecules and becomes oxaloacetic acid again.

Citric Acid(C+C+C+C+C+C) – (C+C) = Oxaloacetic Acid(C+C+C+C)

The Krebs cycle starts again by merging oxaloacetic acid with a new batch of acetyl CoA. Life goes on.

The cycle goes around twice for every molecule of glucose because for every molecule of glucose we get two pyruvates.

After two Krebs cycles, we get a net total of 4 ATPs. If you remember in the beginning I told you that for every molecule of glucose we get a total of 38 ATPs. This occurs in stage 3 of cellular respiration (see below).

Amino Acids

Amino acids don’t go through glycolysis but can be broken down into Acetyl CoA or pyruvate, depending on the amino acid, and enter directly into the Krebs cycle this way.

The Krebs cycle is always the same, regardless of the origin of the fuel source. 

When the amino acid makes this transition amine is given off and becomes one of the waste products in our urine. The rest is used for ATP.


triglyceride structure

Fats are made up of a glycerol backbone and long chains of fatty acids. The fatty acids have a two carbon backbone and enter the Krebs cycle at the level of acetyl-CoA, the glycerol has a three-carbon backbone and enters the cycle at the level of pyruvate.

ATP production continues like normal.

Note: Stored fatty acids can be turned into glucose in the liver and go through glycolysis before entering the Krebs cycle. But technically speaking, this is indirect and still glucose.

Stage 3: Electron Transport Chain

Scientific Definition: An electron transport chain is a series of compounds that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with the transfer of protons across a membrane.

The electron transport chain is by far the most complex part of cellular respiration to get to grips with. It’s also at the cutting edge of biochemistry, and not everything about it is entirely known.

To understand exactly how it’s explained in textbooks I’d have to introduce many more pieces to the puzzle such as the anatomy of a cell, positively charged atoms, electrons, protein complexes and even a thing called the matrix… really.

You don’t need to know all of this but if you’re curious I’ve provided a big list of resources at the end of the article.

Back to the big picture…

So far, we’ve made a total of 4 ATPs from glycolysis and Krebs cycle combined. That’s not much compared to the promised 38. So how do we get there?

The first two stages of cellular respiration manufactured some ATP sure, but they also manufactured two other compounds called NADH and FADH2.

These are sometimes referred to as ‘high energy carriers’ and it’s the oxidisation of these high energy carriers that indirectly creates the extra 34 ATPs.

In total, by the end of the Krebs cycle we have on top of our 4 ATPs:

10 NADH (1 NADH = 3 ATPs)

2 FADH2 (1 FADH2 = 2 ATPs)

NADH is more highly charged than FADH2 and so can indirectly create more ATP.

This is how it works:

As the high energy carriers get oxidised, they lose electrons before finally being converted to H2O (water). It is this ‘transport’ of electron charges that indirectly creates ATP.

Every time an electron goes from a higher energy state (NADH & FADH2) to a lower energy state (H2O) it releases energy. And this energy pumps positively charged hydrogens into the outside compartment of the cell membrane.

This is where it gets strange…

In order to get back into the cell membrane they have to go through a special ‘gate’ called ATP synthase, and as they pass through this gate they cause it’s inner workings to rotate sort of like a turbine.

In this spinning turbine-like structure ADPs (A+P+P) collide into floating phosphate molecules and adjoin, creating ATP (A+P+P+P).

So the electron transport chain indirectly creates ATP by powering a mechanism that slams ADP into a phosphate molecule.

In ideal circumstances the 10 NADHs can create 30 ATPs, the 2 FADH2s can create 4 ATPs which adds up to a grand total of 38 ATPs when you add the 4 ATPs garnered from the  Krebs cycle.

If you’re still reading… well done.

Here’s an easier to understand explanation of the process:

Highly charged compounds bounce down a biochemical ‘staircase’ called the electron transport chain. As they go down this staircase, they release energy that make the right molecules collide necessary for the creation of ATP.


Scientific definition: Anabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy.

house being built like an anabolic reaction

This house is going through an anabolic reaction.

On top of what I’ve already mentioned about the anabolic reaction, another good way to think of it is like the reverse of internal catabolism (breaking down stored fuel).

Catabolism and anabolism are not either/or switches in the body. In truth, they’re going on all of the time simultaneously.

If you get wounded, for example, the actual healing of that wound will depend on anabolism (biosynthesis), but the reconfiguration of proteins in your diet and the production of the ATP needed to fuel biosynthesis depend on catabolism.

You don’t suddenly stop healing and wait for your metabolism to stopcatabolizing your last meal into ATP before you resume healing again. It happens simultaneously, albeit in different pathways.

Structural Anabolism

Structural anabolism covers everything the body needs. 

Repairing and regenerating cells, wound healing, adapting to the environment are all structural functions. Things we’ve covered.

If there is a structural need the essential molecules will not go through the Krebs cycle, instead their component parts (amino acids, glucose, etc.) will be transported to the body sites in need.

i.e. After a weight training session our body will adapt to the stressor by taking amino acids and synthesizing muscle protein out of them. We get stronger and better muscular endurance from this anabolic reaction.

Storage Anabolism

Take a 70kg male. His daily caloric needs are 2000 calories per day, and he eats exactly 2000 calories per day, what happens?

Well, part of what makes up his daily caloric need is structural anabolism. The rest is used up in various forms of activity from walking about and fidgeting to the beating of his heart. (Check out the following article to learn more about the four ways calories are burned.)

But what happens if he eats 4000 calories? What if he eats 2000 calories more than his daily caloric needs? You can probably guess…

He gets fat.

The body is incredibly pragmatic. If your energy needs are met, but you have the building blocks to produce more ATP, rather than throw those building blocks away, your body will store them for later use.

Waste not, want not.

The stored form of protein is muscle, body fat for fat and glycogen for carbs:

Protein > Amino Acids > Lean Muscle Tissue

Fat > Fatty Acids > Adipose Tissue

Carbohydrates > Glucose > Glycogen

The most preferred system for storage in the human body out of these three is adipose tissue. You’d never have guessed…

At first it seems inefficient that the body would choose to store fat over lean muscle, surely muscle would be better? But not so.

The average 70kg male has stored in/on his body, roughly:

6,000 grams of lean muscle tissue (4 kCals x 6000 = 24,000 kCals)

460 grams of glycogen (4 kCals x 460 = 1,840 kCals)

12,000 grams of adipose tissue (9k Cals x 9000 = 108,000 kCals)

Look at the total amount of stored calories each of these produces. Fat clearly beats glycogen and protein for stored fuel. It contains more that twice the amount of calories per gram than the other two.

In theory, using the stats above, this is how long 70kg male could live, purely from his stored fuel sources with a daily caloric expenditure of 2000 kCals:

Lean muscle tissue = 12 days

Glycogen = 0.92 days

Adipose tissue = 54 days

“Muscle weighs more than fat” isn’t any more true than “feathers weigh less than stone”. Muscle takes up less space than fat per gram, sure, but gram for gram butter has more calories than tuna.

There is obviously some overlap between structural and storage anabolism. For example, a certain amount of adipose tissue on the body is healthy, it protects the organs, regulates our hormones and provides a failsafe in the event of food shortage. But anything above the needed levels would be classed as storage.

Overeating causes fat gain whatever the food source. Even a bodybuilder trying to grow stronger through structural anabolic reactions will still gain some fat in a calorie surplus. But weight training is one of the best ways to minimise fat storage in a calorie surplus and minimise muscle loss in a caloric deficit.

In a calorie deficit, the stored fuel sources are turned into ATP through glycolysis (for glycogen), the Krebs cycle and the electron transport chain.

That’s the end of your metabolism primer.

In future articles, I’m going to give more practical nutritional guides such as how to speed up your metabolism, the perfect cardio for weight loss and how to set up the ideal calorie deficit.

Let me know in the comment section if there’s anything you don’t understand, any feedback or ideas for things you’d like to see addressed in the future!


What Is Metabolism – Recommended Resources

Metabolism Overview

Basics of Metabolism by Kahn Academy

Introduction to Cellular Respiration by Kahn Academy

Overview of Metabolism – Anabolism and Catabolism by Kahn Academy

ATP & Cellular Respiration: Crash Course Biology

Biological Molecules – You Are What You Eat: Crash Course Biology

How Cells Obtain Energy

Metabolism Details

Glycolysis Made Easy by iMedical School

Glycolysis by Kahn Academy

Krebs / Citric Acid Cycle by Kahn Academy

Electron Transport Chain by Kahn Academy

Electron Transport Chain Animation by NDSU