As a 90s kid, there was one guy who seemed to know it all when it came to science: Bill Nye the Science Guy. Each week on his show, Bill would cover everything from geology, biology, physics, electricity, zoology, and more. He made science way cooler than your boring school teacher Ms. Bumfuddle. But Bill Nye isn’t the first “science guy.” That title belongs to the great Aristotle.

The ‘Stotle.

The Original Science Guy

Aristotle did more than hang out with Plato and whip up the next few thousand years of Western Philosophy. His writings covered a plethora of different scientific fields. And his ideas were taught for thousands of years in colleges around the world. Some of Ari’s theories were considered correct up until the beginning of the 20th Century. Some theories, like his observations on marine biology, have been proven to be true by modern scientists.

Somewhere around 350 BCE, Aristotle wrote one of his major treatises, “On the Soul.” It was part psychology and part biology. But it was in this book that he first mentions a word that everyone loves to talk about when they’re trying to lose weight: metabolism.

Like many words in the English language, and in the medical profession, metabolism has its roots in Ancient Greek. In Greek, metabolé, means “change”, or “overthrow.” And now that I think about it, “overthrow” kinds of makes sense when I think about what Taco Bell does to my insides.

Anywho, Aristotle defined metabolism like this:

“a process, where animals take in matter, change its qualities, and redistribute these to grow, live, and reproduce.”

Our modern scientific definition leaves it at, “life-sustaining chemical reactions in an organism.”

That means your metabolism isn’t some ephemeral force floating around your body and laughing at you while you run on the treadmill. It’s the process your body goes through that keeps you living and breathing, or what you would call, not being dead.

Unlike the gas you put into your car, your metabolism doesn’t need to be refilled. It also doesn’t shut down when you go a few hours without eating. It’s always working. Even when you sleep. Metabolism is just another way to say, “there are chemical reactions happening inside your body.”

That’s right my friend: you’re a giant walking, talking, farting chemistry lab.

Magical farts are the best farts.

The Two Functions of Your Metabolism

So, since you now know that “metabolism” refers to the chemical processes that happen in your body and cells, let’s get Bill Nye nerdy real quick. Your metabolism does two things: builds and destroys. And yes, there are fancy pants science words for building and destroying.

Anabolism means building.
Catabolism means destroy.

(Which, by the way, sounds like the most metal term ever. If Dethklok ever did a live album they’d call it “Suffering Catabolism.”)

Catabolism is the process that breaks down large molecules into smaller molecules. Chewing your food is the first catabolic process of your digestive system. Then your food enters your stomach where it’s broken down further via enzymes and hydrochloric acid before it enters your small intestine.

And it’s in your small intestine where the majority of chemical reactions happen, and where most of the nutrients you eat are absorbed into the bloodstream.

Once in the bloodstream, your cells will then catabolize the simple sugars, amino acids, and free fatty acids into energy for your cells to use. Then your cells can use that energy to get to work on building more cells or repairing damaged cells.

Anabolism is the process of building larger molecules from smaller molecules. Basically, this is what happens when you build muscle: you break it down (catabolism) and then rebuild more of it (anabolism) so that it comes back larger and stronger. Both of these processes are fueled by the food you eat. And those processes are what keep you alive.

Of course, at this point I could get super duper nerdy. There’s a whole host of nerdtastic things I could dive into here about your body, your organs, and energy use.

But here’s what you need to really know:

  • Your brain cannot store fuel and needs a constant source (it prefers glucose but can use ketones if carbohydrates are limited); it consumes roughly 120g of glucose everyday [~420 calories]
  • Muscle can store glycogen (about three-quarters of all your body’s glycogen stores are in your muscle)
  • Your kidneys produce urine, which is used as a vehicle for excreting metabolic wastes; but your kidneys also filter your blood plasma (nearly 60 times a day) and it reabsorbs water soluble materials out of your blood, like glucose and water, to prevent wasteful loss; this requires a ton of energy
  • Your liver is the metabolic hub of your body; weighing between 2 to 4% of your body weight it processes fuel for your brain, muscles, and other organs.
Organs, yo.

Fun fact: Your organs comprise about 6% of your body weight. But combined, they require 60 to 70% of your daily energy needs.

Suffice it to say, humanity is not Energy Star compliant. We use a ton of energy every day. And that energy is created by your metabolism.

Your metabolism is always on; it never turns “off.” Your body constantly goes through catabolic and anabolic processes every second of the day.

Meaning: you’re burning fat, building muscle, storing fat, eating muscle, burning glycogen, storing glycogen, even if all that happens for a few seconds, your body, and your metabolism, are chugging away. Wanna know when it does turn off? When you die. And even then some of your cells keep living for a bit.

The Last 2,000 Years

Alright, back to Ancient Greece for a second. The majority of what we understand about science and health today is built upon the writings and observations of a few ancient Greeks. Hippocrates, who came before Aristotle, is considered the “Father of Medicine.” He believed that disease wasn’t a punishment from the gods, but a product of environmental factors, diet, and day-to-day habits.

Then Aristotle burst on the scene and laid the foundations for physics, biology, zoology, and more, all while tutoring Colin Farrell.

Arguably the most famous and impactful Greek physician in history, Galen of Pergamon, lived during the time of the Roman Empire. As a researcher, he furthered the works of his ancestors and helped create or improve the studies of anatomy, physiology, pathology, pharmacology, and neurology—he was the first person to describe agonist and antagonist muscles, which makes him a bro in my book.

Galen didn’t study metabolism directly. But he was the first person to theorize the act that would later be called “insensible respiration.” However, there were no further attempts to study metabolism for more than 2,000 years.

However, the earliest reference of the concept of metabolism as you and I know it, came from Ibn al-Nafis, an Arab physician living in Damascus in the 13th Century. But like those who came before him, al-Nafis declined to push the study of metabolism any further. Not a big deal. I mean al-Nafis was a bit too busy defining the process of pulmonary circulation.

It wasn’t until the 16th Century that a young Venetian took Galen’s theory and expanded on it to become the “godfather of metabolism.” So let’s hop back into my hot tub time machine and head to 16th Century Venice.

Rub a dub.

Renaissance Venice

16th Century Venice was quite the place. You could call it the Silicon Valley of its day. And its greatest college, one you could compare to Harvard or Oxford, was the University of Padua.

Padua U produced Copernicus, the salacious Casanova, as well as some of the most groundbreaking doctors of the Renaissance:
  • Gabriele Falloppio (the guy who first described the fallopian tube);
  • Andreas Vesalius; and,
  • William Harvey

It also happened to be the same university where Galileo taught. Most of those names are names you know. But one you’ve probably never heard of is Sanctorio Sanctorius.

Sanctorio was no slouch. In fact, he’s often credited with creating the first thermometer as you and I know it. One of his greatest inventions, the pulsilogium, allowed him to measure the pulse of the heart. Which subsequently led to Sanctorio discovering the circadian rhythm of the heart.

But it was his invention of a static weighing chair in his Sanctorius Sanctorium that allowed him to study human metabolism. This chair allowed Sanctorio to weigh himself continuously throughout the day.

Sitting for science.

He weighed himself:

  • before and after eating
  • while working on his inventions
  • during sleep
  • after taking a dump or pissing
  • and purportedly after sex as well

Everything Sanctorio could do in a day, he did. Then he weighed himself to see the difference. What did he discover?

Sanctorio noticed that what his body eliminated was much smaller than the volume of what he took in via food or drink.


Sanctorio concluded that the weight difference must have been lost through his skin and mouth during respiration; what he called ”insensible respiration.” And he theorized that the amount of “insensible respiration” would change depending on internal and external factors.

Meaning: Sanctorio hypothesized that (bodily) changes occurred based on what he ate, his physical activity, and environmental factors like cold and heat.

But that was about as far the good doctor pushed his studies on metabolism. It wasn’t until the late 18th Century in the totally-not-about-to-have-a-massive-revolution France that Antoine Lavoisier picked up where Sanctorio left off and continued the study of metabolism.

Wine, Cheese, Guinea Pigs, & the First Metabolism Measuring Machine

Antoine Lavoisier was the first scientist to create an apparatus that used heat from respiration to melt ice. He called his device a “calorimeter.” How did it work?

Well, for his heat source, he chose guinea pigs. He placed the little porkers in a cage that he then slipped into a double-walled container. Which looked kind of like this.

Chill out, bro.

The guinea pigs hung out in the innermost chamber. Then he placed ice in the middle and outer chambers. The body heat the guinea pigs produced melted the ice of the interior wall. And at the bottom of his calorimeter was a dish that collected the melted ice.

With this data, Lavoisier theorized that heat and gas are excreted by animals and thus relates to the energy they burn. This experiment backed up Sanctorio’s claim about respiration. Luckily, before the French Revolutionaries went all Ned Stark on Antoine’s head, Lavoisier tested this theory out on a human subject, his assistant Armand Seguin.

Lavoisier measured the oxygen consumption of his assistant at rest. Then, he compared that to the air consumed when his assistant performed some sort of physical activity or when Seguin ate food.

What Lavoisier discovered was that both physical activity and digestion increased the amount of oxygen Seguin sucked into his lungsMeasuring the amount of oxygen inhaled and the CO2 exhaled is still how modern metabolic chambers operate.

Modern Metabolism Measuring Chambers

Metabolic chambers are kind of like the Hyperbolic Time Chamber from Dragonball Z. Except time doesn’t slow down, gravity remains the same, the temperature’s regulated, and the air doesn’t get denser the deeper you go.

Can we talk about how Krillin is the real hero of DBZ?

Alright, so I guess it’s nothing like the Hyperbolic Time Chamber at all; but it is a chamber you live in for 24 hours, so let’s just say they’re the same.

Metabolic chambers, or whole room calorimeters, have been around since the mid-1860s. Wilbur Atwater built the first one in the United States in the 1890s. And today, the National Institute of Health has three.

These chambers are almost 100% airtight. And inside is a fixed amount (volume) of oxygen and carbon dioxide. This allows researchers to measure the oxygen a subject inhales and the CO2 they exhale for every minute they’re in the chamber.

Why are those measurements vital?

Every cell in your body runs off of ATP (adenosine triphosphate). Your body creates ATP by breaking down fats, carbs, and protein from the food you eat. But oxygen’s needed to “unlock” the energy in these macronutrients — a process known as oxidization. The product of that process is CO2, which you then exhale.

This is how metabolic chambers measure how many calories a subject burns in a day:

air’s sucked out of the chamber and through pipes where gas analyzers measure the amount of oxygen consumed and CO2 produced; this data is sent to a computer where researchers can calculate the number of calories a subject has burned.

These machines are super accurate. So accurate, in fact, that they can calculate the changes in your metabolism over 24-hours within 1.2-2%.

Sequestering yourself off into one of these chambers isn’t cheap. Vanderbilt University has one, but it costs around $800 for commercial use (it’s a little cheaper for academics but not much). Unless you’re a super nerd, you’re not going to take a day out of your life to measure the calories you burn in a day.

For 800 bucks, though, you can buy like four FitBit Versas or at least two iWatches.

Wearable Technology and Measuring Your Metabolism

Wearable technology that tracks your activity and calories burned is a growing worldwide market. In the 2nd quarter of 2018, sales of these devices grew 5.5% worldwide, according to the International Data CorporationAnd in June of 2019, the IDC predicted that the wearable market will continue to grow at an estimated rate of 7.9% every year between now and 2023.

Millions of people already rely on these devices to track their activity and to tell them how many calories they’ve burned exercising. But here’s the shocking truth about these devices: they’re egregiously inaccurate. Hell, blind Stormtroopers are more accurate than these things.

A 2017 study at Stanford found that these devices are off by anywhere from 27.5% to 93% when it comes to how much energy is burned during exercise.

If I threw down my hard-earned money for a device that I was told could calculate how many calories I burned lifting or running on the treadmill, and found out later that it was 30-90% inaccurate in what it reported, I’d be MEGA pissed.

There is one upside to these devices: they’re super freaking accurate when it comes to measuring heart rate; 6 out of 7 of the devices tested measured heart rate within 5%. So at least they’re not a complete bust.

All of this stuff is super cool to know. But at the end of the day, here’s what you need to remember: your metabolism isn’t a math equation.

Yes, you can use formulas or cloister yourself off like Goku in a metabolic chamber, but your metabolism isn’t linear. You won’t burn the same calories seven days a week. Some days you’ll burn more, some days less.

When I give my online coaching clients their nutrition goals, there’s nothing magical about the calorie/macro numbers I give them. The same goes for you if you use an online calculator. Those numbers are simply a starting point. If nothing changes with my clients after a couple of weeks, then I tweak things, and we reassess progress a couple of weeks later.

At the end of the day, what you burn in the gym doesn’t mean a damn if you’re stuffing your face with more calories than you need.

What You Should Do from Here

Your metabolism is continuously changing, it’s never the same day in and day out. And scientists “can” measure it in a metabolic chamber at a pretty accurate rate, which is pretty freaking coolBut fitness/activity trackers are pretty much lying to you about the calories they tell you that you’ve burned.

If your goal is to lose body fat, focus on nutrition and don’t worry about how many calories you’re burning in the gym. Make your initial focus on understanding how much food you’re eating in a day.

First, download MyFitnessPal and track everything you eat and drink that isn’t black coffee, water, or green leafy veggies for a couple of weeks. And make sure to turn off “exercise calories” in the goals section of MFP.

Second, read this post to better understand how to set-up MFP and know how many calories you should be eating.

Third, attempt to get 7-8 hours of sleep per night. That’s one of the most important things you can do for fat loss, period.

Fourth, be patient. You didn’t get fat overnight and you won’t get rid of belly fat in two weeks either. This takes time.

And finally, because I know you’re going to ask about it, yes, there are ways to boost your metabolism. But not in the way supplement companies or some dumb Pop Sugar or Men’s Health article claims. Read this to discover how to increase your metabolism without buying bogus supplements.

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