Terms like “fast” and “slow” metabolism are carelessly thrown around in weight loss and fitness communities. You can categorize yourself as someone who is either able to eat a an excessive amount of food without gaining weight, or easily pack on a pound by consuming just one extra french fry. This terminology can become a cruel way to compare bodies and shame eating habits, and even at an age well past our adolescent anxieties, people still find blame, or pride, in whatever speed they determine their metabolic rate to be. But despite speaking about metabolism so easily, we rarely think about it in the right way. Tune in to find out what it really means.
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Credit to MonoSheep for the theme music.
Terms like “fast” and “slow” metabolism are carelessly thrown around in weight loss and fitness communities. You can categorize yourself as someone who is either able to eat a an excessive amount of food without gaining weight, or easily pack on a pound by consuming just one extra french fry. This terminology can become a cruel way to compare bodies and shame eating habits, and even at an age well past our adolescent anxieties, people still find blame, or pride, in whatever speed they determine their metabolic rate to be. But despite speaking about metabolism so easily, we rarely think about it in the right way. Tune in to find out what it really means.
Want to recommend an episode topic? Send me a message on Instagram @nutritionbitespodcast
Credit to MonoSheep for the theme music.
Welcome to Nutrition Bites. The no nonsense podcast where you get the truth about food so you can eat what you want, and be healthy. I’m your host Maggie and welcome to Episode 22 of this series! Today we’re focusing on a subject regularly discussed within the fitness community and weight loss circles. It’s talked about in fairly simple terms but the reality is a lot more complicated. On the menu today, metabolism.
If you have ever been concerned about “getting shredded” like an Abercrombie mannequin, or losing weight to look like a Victoria’s Secret Angel, then you’re probably familiar with the term “metabolism”. I know that the Abercrombie and VS references are dated, but back when I first learned about metabolism, these were cool things, which says a lot about my age. The topic of metabolism is often first discovered during our teenage years, when young girls begin comparing their bodies against unrealistic standards, and young boys are face first in a pile of food to combat their never-ending hunger. Terms like “fast” and “slow” metabolism are learned, and eventually you categorize yourself as someone who either can eat a seemingly excessive amount of food without gaining weight, or pack on a pound by consuming just one extra french fry. This terminology can become a cruel way to compare bodies and shame eating habits, and even at an age well past our adolescent anxieties, people still find blame, or pride, in whatever speed they determine their metabolic rate to be. But despite speaking about metabolism so easily, we rarely think about it in the right way. So let’s find out what it really means.
Metabolism is a LOT more complicated than how the influencer communities talk about it. In essence, metabolism covers a huge variety of biochemical reactions revolving around the creation and destruction of molecules in order to keep us alive. How’s that for a definition? You can also think about it this way. To sustain life for any biological organism, energy is required, and the primary source of energy for humans is food. So metabolism can also be described as a collection of reactions that transform food into energy that our body can use. All 3 macronutrients, carbs, fat, and protein, provide energy when broken down. And when their pieces are later reassembled into new molecules, for example amino acids being built into different proteins, this process consumes energy. Keeping all of these reactions in balance is key to our health, and when they are consistently disrupted it can lead to disease. Diabetes is a great example. This metabolic disorder is a result of disruption to glucose metabolism, where glucose is not being broken down efficiently, thereby causing too much sugar to be circulating in the bloodstream. The breakdown and rebuilding of carbs, protein and fat, occur thousands of times over in our body, alongside other biochemical reactions that also release and consume energy. And metabolism covers it all.
One of the reasons metabolism is commonly associated with weight is because our metabolic rate dictates the exact amount of energy required for our body to function. If we ingest too little food, we die, and if we ingest too much food, we also die...just in a different way. And when we discuss energy in this context we’re really talking about calories. Despite being wrongly synonymous with the moral value of food, a calorie, or kilocalorie if we’re being real specific, is a unit of energy. So when you see a magazine article, or God forbid Goop newsletter discussing metabolism, it’s often written about as the amount of calories our body burns. While this is true, the way it’s framed is focused on weight, when really the rhetoric should be switched. Our metabolic rate is the number of calories we need to live. So how much is that number?
Back in the 1930s scientists were keen to answer that question. In particular, they wanted to know the minimum amount of energy the human body required to survive. A formula was created that determined exactly this - how many calories the body burned when at complete rest. This function, called The Basal Metabolic Rate, or BMR, allowed scientists to understand the energy requirements to keep vital functions operating - things like breathing, pumping blood and staying warm. The unfortunate thing about the BMR though is that it’s a pretty useless calculation. Every time you make even the slightest movement, from scratching a back itch to reaching across your bed for the phone charger, you utilize energy. So in place of BMR, another formula was created which encompasses those daily micro movements - the RMR, or Resting Metabolic Rate. The output of the RMR is the same - a number of calories an individual needs, but it accounts for the extra energy required for basic movement. To get your true RMR you have to be examined in a clinical lab, but a rough estimate can be worked out through online calculators. All you need to do is plug in 4 key inputs: your height, weight, sex, and age.
Your height and weight have a big influence on your daily caloric needs for a fairly obvious reason. The bigger you are, the more energy that is required to feed all your cells, pump all your blood, and keep that rockin’ bod alive. Your biological sex is also influential, mostly because there’s an association with sex and body size, with males averaging larger. Interestingly, a woman's RMR actually changes on a regular basis, fluctuating with the menstrual cycle, because...everything in our bodies fluctuates with the damn menstrual cycle. In particular, a woman's RMR increases after ovulation, in the latter half of the cycle, which may partially explain the infamous appetite spike around this time. Lastly, age has a massive impact on our metabolism because our energy needs are directly related to stages of growth. From birth to around age 20, our size-adjusted resting metabolic rate is the highest it gets, which makes sense because our bodies require a ton of energy for the biological building spree we undergo. From age 20 to 60 our RMR is relatively stable, with an eventual decline in our older years as our bodies enter...a retirement of sorts. Now the RMR is a great starting point to the metabolism conversation, but remember that it only represents our caloric requirements for basic functioning - about 70% of the total energy we need. So let’s talk about the other missing pieces of the metabolism puzzle.
Around 10% of our daily caloric needs comes from digesting food. I know this may seem backwards, because the purpose of eating is to take in energy, but the process of digestion is actually really energetically demanding. I mean, you can literally feel your stomach and intestines move as they work through a big meal. And if you are lucky enough to suffer from indigestion, it’s like a never ending dance party. The energy required to digest food can fluctuate slightly depending on what you eat, but in the big picture of metabolism, it’s not wildly influential. The much more powerful portion of your daily energy requirement is your physical activity level. How often and how much you move, including both traditional exercise and less strenuous movements like walking and cleaning, make up around 20% of your energy requirements.
So, to get a more holistic reflection of your daily energy needs, we simply combine your RMR with the energy required to digest food, and your physical activity level. This value is called your Total Daily Energy Expenditure - aka how many calories your body requires to live its life. But despite having a simple formula at our fingertips to determine this number, there is still individual variability. After all, you can have a group of people of the same sex and age, with the same height and weight, who eat the same diet and work out the same amount, but each can still have slightly different metabolism - so what gives?
Genetics is thought to be one of the reasons that metabolic rates vary between people, but it will likely be a while before scientists can determine which combination of genes are most influential. In spite of this unsolved genetic mystery, there is a factor well known for causing differences in our metabolism - body composition. Body composition refers to the percentage of your body made up of fat vs. fat-free mass. Fat-free mass includes muscles, organs, bones, connective tissue - essentially everything apart from fat. The percentage of fat-free mass you have is important because your muscles and organs in particular, consume a lot of energy. You’ve probably heard that muscular people burn more energy just chilling than those with less muscle mass, and that’s completely true. At rest, 1 lb of muscle burns triple the amount of calories than an equal amount of fat. This means that the more muscle mass you have, the more calories your body requires to survive, and therefore the higher your metabolic rate. Similarly the size of your organs, particularly the highly energetic ones like your brain, liver, and heart, also can affect your caloric needs. So despite a group of people looking the same on the surface, what their body composition is like on the inside, will affect their metabolic rate.
Modifying your body composition to increase your fat-free mass is one way to change your metabolism, or as click-bait headlines phrase this: “speed it up”. Incorporating heavy-duty resistance training, or High Intensity Interval Training, are two strategies you can use to help build muscle mass. Another way to increase your metabolic rate is to...be more active! Shocker, I know. The more physically active you are, the more energy your body will require, which means an increase in your metabolism. But here’s the catch. Alongside with your increased metabolic rate is an increase in appetite. After all, your body is designed to be in energetic balance, and this is it’s way to ensure it gets the energy it needs. So just because you may “speed up” your metabolism, it doesn’t necessarily lead to weight loss.
Apart from increasing your muscle mass and physical activity levels, there are really no other methods that will drastically ramp up your metabolic rate. Sure, there are wacky life hacks being neatly packaged in an Instagram reel on how to burn extra calories without putting in any work, but these are bogus. Tips like only drinking cold water or keeping your house frigid may help you burn a few extra calories, because you are fighting to stay warm, but it doesn’t change your metabolism. Same goes for any supplements, like green tea extract or capsaicin, or mysterious “thermogenic fat-burning” compounds. They usually don’t work and even if they do help increase your daily energy expenditure, it’s temporary. As soon as you stop taking the supplement, the effect disappears. Unfortunately, the same goes for physical activity and changes in body composition. As soon as you stop working out, or you lose muscle mass, your metabolic rate shifts. We just have to swallow the bitter pill that “speeding up” our metabolism is really difficult, and never permanent. In fact, the only permanent change we see in our metabolism is in the opposite direction - slowing it down.
Losing weight is the most effective way to decrease our metabolic rate. When our body isn’t supplied with enough energy, it adjusts by burning fewer calories throughout the day. This is a phenomenon known as “metabolic adaptation”. It’s a biological survival technique which attempts to preserve energy for vital functions. When people lose weight in a healthy way, which means slowly and gradually, the metabolic rate shifts down in proportion to the weight lost, which makes sense - a smaller body size requires less energy overall. But if someone loses weight quickly and drastically, like how many crash diets recommend, then the metabolic rate slows down much more than expected. What this means in practice is that if someone dropped a lot of weight quickly, their new metabolic rate will be extra slow. And in order to maintain their new, smaller body size, they will have to consume far fewer calories than what would’ve been required if they reached that same size in a more gradual way. One of the best examples of this extreme shift to a slow metabolism comes courtesy of “The Biggest Loser”.
If you didn't know, the mid 2000s was the golden era for competitive reality tv shows. We had Survivor, Big Brother, The Amazing Race, Amercia’s Next Top Model, Project Runway, and of course The Biggest Loser - an American tv show which featured obese contestants competing to win a prize by losing ridiculous volumes of weight in a matter of weeks. Competitors essentially starved themselves and worked out long hours every day, often dropping hundreds of pounds. But in 2016 a groundbreaking study revealed the dangers of this drastic weight loss experiment. Researchers followed a set of contestants from The Biggest Loser over 6 years to see if they maintained their extreme weight loss. What they found was that the average contestant actually regained 60% of the weight they lost, and a group of them were even heavier than when they entered the show. In addition, researchers discovered that the contestant's Resting Metabolic Rate was permanently damaged. Their RMR decreased so dramatically from the rapid weight loss, that they now burned 500 fewer calories per day than expected. By dropping so much volume so quickly, their bodies went into survival mode, radically slowing down their metabolism, and keeping it there. This in turn meant that their daily caloric intake was so extremely low, any increase above their new energy requirement, would result in weight gain. And that’s exactly what happened.
This discovery is a great reminder that our bodies are biological machines. Sure we can temporarily adjust a few levers that influence our metabolism, but our body will react accordingly to keep us in check. So instead of trying to hack our way towards a faster metabolic rate, let’s respect the fact that metabolism is a wildly complex process and it’s designed to keep us alive. Let’s stop fighting that.
That’s been the bite for today. Stay hungry.