Understanding Metabolism, Calories & Energy Expenditure | Triage Thoughts Ep. 250

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What metabolism actually is

When someone says they have a fast or a slow metabolism, what are they really referring to? Most people will tell us it has something to do with how quickly they burn calories, and that is roughly in the right ballpark, but the actual definition is broader and more useful than that. In bioscience terms, metabolism is the sum total of every chemical reaction happening in the body. Your heart beating, your enzymes working, hormones being made, food being digested, cells being repaired, every bit of it runs on chemical reactions, and every chemical reaction requires energy. Just like a television needs to be plugged into the mains to work, your body needs energy to do anything at all, including the apparently passive act of lying down doing nothing.

Within that sum total, two broad processes are running in parallel. Catabolism is the breaking down of molecules, and anabolism is the building up of molecules. Both require energy, and both are happening continuously. Worth holding onto that, because there is a common assumption that “breaking things down” is the only side of the equation that costs calories.

Food is energy, building blocks, signals and waste

So if metabolism is just chemical reactions requiring energy, where does that energy come from? Food. But food is not only fuel. When we eat protein, we digest it and break it down to its constituent amino acids, and those amino acids can either be burned for energy or used as building blocks to construct other proteins in the body, muscle being the classic example. The same goes for lipids, which end up forming things like cell membranes, and for carbohydrates. There is a constant continuum between using a nutrient for energy and using it as a building block, and the balance shifts with context. Importantly, in a starvation setting where energy is scarce, even protein you eat will be diverted away from building blocks and pushed into the energy pathway, because keeping you alive trumps adding to your muscles.

There is a third thing food does, which tends to get less attention. Certain components of food act as signalling molecules. The amino acid leucine, found in protein, signals via mTOR to drive muscle building. Saturated fat, in the team’s view, reduces LDL receptor expression, which is itself a signalling effect even though we do not usually frame it that way. Even those signalling processes are chemical reactions, so they too feed into the overall metabolic load.

And then there is the fourth piece, which is the breakdown and elimination of metabolic waste. When you break down protein, you end up with by-products such as ammonia, which is toxic and has to be cleared. Storing nutrients, mobilising them again later, and getting rid of the waste all cost energy. Metabolism is genuinely the sum total of all of that, not just one slice of it.

Why all of this converges on calories

That whole picture is fascinating if you are a biochemist, but it can lead people astray in practice. There is a familiar pattern in the health and fitness world: someone learns a little bit about metabolic pathways, then concludes that calories must not really matter because there are hormones and enzymes and mechanisms doing all sorts of things in the background. The problem with that move is that it forgets to zoom back out.

All of these chemical reactions, however they are routed, converge on the same currency: energy. We call that energy calories. Calories are not a thing you can hold in your hand, in the same way that a kilogram is not a thing you can hold; they are a unit of measurement. But every diet you can name, whether it is keto, carnivore, vegan, low fat or high carb, all of them work, when they work, by changing the relationship between calories coming in and calories going out. The metabolic pathways are real, but they are already baked into the calories in, calories out equation. So when we talk about understanding metabolism, the goal is to dig deep, then come back up with conclusions that actually help in real life.

The four components of energy expenditure

When people talk casually about their metabolism, they usually mean the calories out side of the ledger, even if they do not say it that way. The energy you burn over a day, your total daily energy expenditure, or TDEE, breaks down into four broad components.

The first is your basal metabolic rate, or BMR. This is the energy your body burns just to stay alive: heart beating, brain ticking over, organs maintaining themselves, no movement required. It is roughly fixed on any given day, although, as we will come to, it is not immutable.

The second is non-exercise activity thermogenesis, or NEAT. This is every calorie you burn doing things that are not formal exercise. Fidgeting, tapping your toes, gesturing while you talk, walking from one room to another, climbing the stairs to grab your keys. In practice, your daily step count is a decent proxy for it, although it does not capture absolutely everything you do.

The third is exercise activity thermogenesis, or EAT, which is the calorie burn from your formal training. Resistance work, sprints, long-distance running, anything you have set aside time to do as exercise.

The fourth is the thermic effect of food, sometimes called the thermic effect of feeding, or TEF. This is the energy your body has to spend just to digest, assimilate and process what you eat. Protein in particular has a relatively high thermic effect, which is why people sometimes notice they feel a touch warmer after a protein-heavy meal. Every macronutrient produces some thermic effect, but they are not equal.

Add those four together and you have your TDEE. That is the number calculators try to estimate online, and it is the number the calories in, calories out conversation hangs on.

How body weight, body composition and movement change the picture

If metabolism is influenced by the amount of food we eat, because more food means more digestion and more building, it is also influenced by the number of cells we have. None of us has counted our cells, so we use body weight as a rough proxy. More mass, broadly speaking, means more cells doing things, which means a higher baseline metabolism.

But the picture refines further, because not all cells are equally metabolically active. A muscle cell does more chemical work than a fat cell, even at rest, and so it burns more energy just sitting there maintaining itself. Fat cells are not metabolically silent, but they are far less demanding. So if two people weigh the same and one of them carries a lot more lean mass, that person will tend to have the faster metabolism, even at rest. Build out more lean tissue and you genuinely shift the floor of your daily calorie burn.

There is a further wrinkle that catches a lot of people out. If you weigh more, you also burn more energy moving. Carry a hundred kilos up a flight of stairs and you will burn more than someone carrying fifty-five kilos up the same stairs. The simplest way to feel this in your own body is to put on a twenty or thirty kilo weighted vest and try to go about your normal day. You will be sweating before you have finished the washing up. So heavier individuals, far from having a slow metabolism in any literal sense, are usually doing more work just to exist, particularly the moment they start moving.

This also explains the common online comparison that drives people mad. Someone says, “I am seventy kilos and I am stuck at fifteen hundred calories, but I follow a woman online who is also seventy kilos and eats twenty-five hundred.” The likely answer is that the seventy kilo woman online is jacked, with a lot of lean mass, while you may be the same weight at a much higher body fat percentage. The number on the scale is the same; the metabolically active tissue is not.

Heat loss is not a bug

Some heat is always lost in chemical reactions; that is just how energy conversion works. Your body is no exception, and you will often notice that food makes you feel warmer through exactly this mechanism. It might look like a flaw, but it is one of the more useful features of how human metabolism works. The ability to generate heat by eating, and to hold body temperature steady, is part of why humans can live in cold climates at all. It is also why severe starvation can lead to hypothermia and death from exposure: it is not only that the body is weakened, it is that there is not enough fuel coming in to keep the thermostat going. There do appear to be differences between populations whose ancestors evolved in very different climates, and in the team’s view that is worth acknowledging even if we do not dwell on it.

Edge cases that sharpen the principles

The principles get clearer when you look at outliers. A newborn at around seven kilos has a metabolic rate of roughly a hundred calories per kilogram for the first ten kilos of life. If a hundred kilo adult ran at that rate, they would need ten thousand calories a day. The difference is what that energy is being used for: a baby is in an enormously anabolic state, growing organs, bones and muscles at a pace no adult could match. Metabolic requirements are not arbitrary, they are literally programmed into what the body is being asked to do.

Severe burn victims sit at the opposite extreme. Energy demand goes through the roof, partly because the body has to break down vast amounts of damaged tissue (catabolism that itself costs energy), partly because it then has to rebuild what has been lost (anabolism, also expensive), and partly because the burnt skin can no longer regulate heat properly. This is one of the reasons burn patients are sometimes put on anabolic drugs such as Anavar, in an effort to tip the balance away from catabolic loss and towards anabolic rebuilding.

The aerobic versus anaerobic distinction is also worth mentioning. The aerobic energy system is slower but more efficient, extracting more ATP, the body’s energy currency, from each unit of food. The anaerobic system is faster but less efficient, producing less ATP for the same substrate. People with poor aerobic fitness, including those with certain lung diseases, tend to rely more on anaerobic metabolism at rest, which shows up as a higher resting lactate. It looks paradoxical at first: an inefficient system “burns more calories” to do the same job. But the practical reality is that those people are also more fatigued, less inclined to move, and end up doing less work over the day. So the lesson is not to chase inefficiency; it is that improving aerobic fitness gives you more usable energy across the day, which is why the counterintuitive advice to do some cardio when you feel constantly tired tends to be correct.

Pregnancy pulls several of these threads together. Energy requirements rise because there is another human being built from scratch, but they also rise because the mother gains body mass, which itself raises baseline expenditure. Several variables move at once, all consistent with the same underlying picture.

Fast and slow metabolism are usually the wrong way round

Once the components are in place, the colloquial use of “fast” and “slow” metabolism starts to look strange. A heavier person, all else equal, has a higher BMR, burns more energy moving around, and processes more food. In a literal accounting sense, that is a faster metabolism, not a slower one. Someone who is naturally lean and complains they cannot gain weight, when you actually map out their day, almost always has a lower baseline rate; they are just more active, often without realising it. They fidget more, walk more, and as soon as they are in a surplus they spontaneously do more of all of it. So the labels people use to explain why they cannot lose weight or cannot gain weight are usually inverted relative to the physiology.

Satiety also belongs in this conversation. People differ in how full they feel on the same number of calories, and that neurohormonal signalling means one person can comfortably hold a small deficit while another finds themselves drifting back to the kitchen at night. The friend who “eats so little but still gains weight” is usually compensating somewhere you cannot see, often after dinner when everyone else has gone to bed, and is also often less spontaneously active than they appear.

There is a real, individual difference layered on top of all of this. Some people, often described in the research as thrifty phenotypes, adapt strongly to a calorie deficit, downshifting expenditure in ways that protect their stored energy. Others, sometimes called spendthrift, dissipate excess energy fairly freely when overfed and do not downshift as readily when underfed. In practice this shows up in things like sweating, body temperature and apparent fidgetiness. One of us notices a clear pattern of sweating more, fidgeting more and feeling more restless in a surplus; the other adapts harder in a deficit and quietens down. Neither is a moral failing, and it is worth being aware of when a small deficit does not seem to produce the expected weight change. That is not proof that calories do not work, it is a sign that expenditure has adapted.

What you actually have control over

This is where the practical takeaway lives. Body mass, and particularly lean body mass, is modifiable. If you eat enough and train, you can build lean tissue, which raises your floor. NEAT is modifiable, particularly through your daily step count, which is the cleanest proxy we have. EAT is modifiable through your training schedule. TEF can be nudged a little by emphasising protein, although we would not lean on that as a strategy beyond getting protein into a sensible range in the first place.

What you do not really control, at least not directly, is your hormonal output. Thyroid is the one people reach for most often, but it tends to track in the opposite direction to what they assume. Thyroid output tends to rise with higher food intake, not fall, because the system is essentially signalling about energy availability. Bodybuilders dieting down to extremely low body fat levels often see their thyroid output drop into a range that looks clinically hypothyroid, with cold sensitivity and the rest of the classic picture; eating more food generally brings it back. Testosterone behaves similarly in the broad strokes: very low calorie intake suppresses it, and adequate energy intake helps keep it where it should be. Beyond that, you cannot will hormones to a different setpoint.

A related caveat: very low calorie intake also disrupts sex hormones more broadly, which is why low-calorie dieting is associated with menstrual cycle irregularities in women and with reduced libido and erectile function in men. The body, sensibly enough, treats sustained energy scarcity as a poor time to reproduce.

Metabolism adapts, but it does not break

When people say they have “broken” their metabolism, what has usually happened is that it has adapted. After a long stretch in a deficit, your TEF drops because you are eating less, your NEAT drops because you are moving less and fidgeting less, your BMR can come down a little if you have lost lean mass, and even your EAT drops because you have less energy to push hard in training. Add all of that up and your maintenance calories can sit noticeably below where a calculator predicts. That is not damage; it is the system doing what it is supposed to do. Eat a little more, gradually, and most of it comes back. The new number may not exactly match the old one because the body has genuinely changed, but the underlying physiology is intact.

It works the other way too. Push food back up and TEF rises, NEAT rises (more fidgeting, more spontaneous movement, more sweating), and lean mass and training output can climb with it. That is why heavier individuals often have, in absolute terms, a higher metabolism than leaner ones, not the lower one folklore would predict.

The constrained energy model and why steps usually solve it

There is one more piece worth understanding, which is that metabolism is not perfectly additive. You cannot keep stacking exercise and NEAT and expect calorie expenditure to climb forever. There is a ceiling, and the most compelling explanation we have come across is that the digestive and mitochondrial machinery simply has a maximum throughput per day. If you push exercise very hard, the body tends to compensate by pulling NEAT down elsewhere. You go and burn a couple of thousand calories swimming in cold water in the morning, and then you find yourself slumped on the sofa for the rest of the day, doing nothing. That is not a failure of willpower, it is the constrained energy model in action.

In practical terms, for almost everyone who is not a high-volume athlete, you can largely sidestep this by tracking your daily steps. For most people, somewhere between ten and fifteen thousand steps a day is a sensible, sustainable range. For someone in a sedentary office job, six thousand may be a realistic floor to work up from. Pushing into twenty, twenty-five, thirty thousand steps a day usually is not worth it, because you will be too wrecked to do anything else, and you will likely just claw the energy back somewhere else.

The honest summary is this: you cannot will your hormones to a different setpoint, and you cannot override how your body adapts. What you can control is how much you eat, how much you train, and how much you move outside of formal exercise. That is where the leverage is, and it is more than enough.