I have had a chat with someone who recently heard I am doing a degree in nutritional therapy and she had one urgent question to ask: ‘so tell me - Is it calories in, calories out or hormones? I can’t figure out who is right, I hear scientists, doctors, clear experts in their field claiming one or the other with great conviction - who is right? I need to know!’
If you go by social media, you will have no choice but to think there is a debate going on between two camps on the topic of what really drives weight loss. Is it ‘eat less, move more’, also known as calories in calories out (the claim that weight loss comes down to thermodynamics: eat fewer calories than you burn and you will lose weight), or is it out of balance hormones which we just need to…optimise?
If you too are puzzled about how serious experts, people who clearly know their field, can disagree so completely, and you are struggling to figure out the ‘right’ answer - this article is for you.
There is no weight loss without a calorie deficit
To lose fat, you need to burn more energy than you take in. There is no debate around that. A calorie is nothing more than a unit of energy, a way to measure how much goes into the body and how much the body spends to keep us alive, moving, thinking and breathing. When you exercise, you burn more calories than when you are at rest. Hence, the more you move, the more energy you require, the more calories you burn.
When we’re in a calorie deficit, i.e. we spend more energy than we generate from eating food, that spent energy has to come from somewhere. If there is not enough food eaten to generate the energy needed, our body starts generating energy from our own reserves. The body breaks down fat, and sometimes muscle, to make up the difference. That is what weight loss physically is.
This part isn’t really controversial. It’s physics. If the body isn’t fed enough to cover its energy needs, it draws on what it has stored, and we lose weight. Ideally from fat rather than muscle, though that depends a great deal on how we go about the weight loss process.
So why do people say it’s not about calories?
You’ll also hear, just as loudly, that weight loss isn’t about calories at all. It’s about hormones. Inflammation. Insulin. All the machinery running quietly in the background that decides whether we store fat or burn it.
And that’s also true. Our body is not a fixed engine. It does not have fixed inputs and outputs. Our engine is constantly changing and transforming, reacting to our environment and to other changing signals in our body. So how much we absorb and use from food and how much we extract or burn for energy - fluctuates, all the time.
Let’s take a woman in midlife for example. When oestrogen falls in perimenopause, insulin sensitivity goes down. When the cells become less responsive to insulin, blood sugar stays elevated longer because insulin isn’t doing its job efficiently (opening the cells up to take in sugar).
Our body always takes the easiest fuel source available. With high blood sugar sitting there, the body will burn that sugar first. Meanwhile, elevated insulin is also signaling the body to store fat rather than burn it.
So our theoretical midlife woman ends up in a situation where sugar is more abundant and thus being burned for any needed energy, but any excess is being stored as fat, because of the higher insulin, and fat burning is not needed and not happening.
In that situation the body has a reduced ability to flexibly switch between fuel sources. When you’re insulin sensitive, your body can easily flip between “burn the carbs I just ate” and “tap into stored fat.” With insulin resistance, that switch gets sticky. The dial moves more into burning available sugar and holding onto stored fat.
It’s what’s often called a loss of metabolic flexibility, the ability to move smoothly between fuels depending on what’s available.
The practical result: fat is harder to lose even if the ingested calories are the same - because they are not used or burned the same way. It’s also why you will hear women in midlife suddenly saying: ‘I am eating and exercising exactly the same as I used to, but I am suddenly gaining weight.’
‘Calories in, calories out’ still holds true, but the engine mechanics have suddenly changed and now, despite doing the exact same ‘exercise’ you are burning less. The engine is not as efficient.
So the equation holds, but what drives each side of it has changed. And that nuance isn’t explained when people say it’s not the calories.
If we picture fat loss as an equation, energy in minus energy out, the calories in, calories out crowd are right that the equation holds. What most thermodynamics thinking misses is that it treats the body like an engine, a fixed machine that burns a set amount of fuel to produce a set amount of work.
But we are not fixed engines. Our whole biology is a web of feedback loops, constantly reading our environment and adjusting to keep us alive and safe. Both sides of that equation move. And for women with our special fluctuating hormonal picture, they move even more.
Let’s walk through another example.
When you lose fat, your fat cells shrink. And fat cells aren’t just storage - they’re constantly sending out a hormone called leptin. Leptin is essentially our body’s fuel gauge. When fat stores are full, leptin is high, and it tells your brain “we have plenty, everything’s fine.” When fat stores shrink, leptin drops, and your brain reads that drop as “danger - we don’t have enough reserves.”
Your brain doesn’t know you’re dieting on purpose and doesn’t care that your jeans don’t button up. It sees the fuel gauge dropping and an ancient part of your brain interprets a falling fuel gauge the same way it always has: we are heading for a possible famine. There are hard times ahead and I need to do what I need to do so that we can survive.
What it does is respond on two fronts at once:
It turns hunger up: Ghrelin, the hormone that drives appetite, rises. Food becomes more interesting, more rewarding, harder to ignore. Your brain is doing exactly what it’s designed to do, pushing you to refuel.
It turns spending down: Your body starts running the same tasks on less energy. Thyroid hormone (T3) dips, which lowers your resting metabolism (how many calories you burn at rest). Your nervous system quietens the systems that burn energy through heat and movement. You also tend to fidget less and move less without noticing. Every one of these is your body trying to conserve fuel, by burning less calories.
Put those two together and you get a gap. You’re hungrier than the math says you should be, and you’re burning less than the math says you should be. So holding onto that lower weight suddenly requires eating even less than you’d expect, while feeling hungrier the whole time.
The thermodynamics equation holds: if you are consuming less calories than you are burning, you will lose weight. But consuming less and burning the same or more, becomes so much harder.
To make it even more fun, some of these changes don’t fully switch off when you stop dieting. Your metabolism can temporarily stay lowered even after you’ve regained the weight. So the next time you try, you’re often starting from a body that’s already running a bit more efficiently than before - meaning the same deficit does a little less work than it used to.
So from the outside it can look as though calories in, calories out has stopped working. It hasn’t. One side of the equation changed while we carried on eating and moving as though nothing had. The maths still holds. The numbers on each side of the equation are changing.
Why it matters
Does the debate on social media matter? I will argue it does, because of the story we’ve been told about weight.
Yes, calories in, calories out is true. But the implication underneath it is that it’s a simple math equation. Losing weight is simple: you eat less than you burn and all will be well. And if you can’t manage that, then you must be eating too much, or moving too little, or you simply don’t want it badly enough. It’s the will power argument and it feeds years and years of moral implications directed at overweight individuals. Why won’t you just move more or eat less?
That framing badly underestimates what we’re actually up against, which is millions of years of evolution built to hold on to body fat and a modern food environment engineered to manipulate our biology to over consume. Top that up with a biology that adjusts both sides of the equation the moment we try to make a change.
None of this means that change isn’t possible. It absolutely is. But it does mean this was never the simple sum we were sold and that ‘eat less, move more’, while true, is a gross simplification of the story.
Because once we understand the real forces at work, the ones managing hunger, fuel and energy in the background, we can stop reading every setback as a personal failing and start working with our body instead of against it.
Once you understand how the mechanics work, and what levers you can shift to make a change, there is much less room for beating yourself up or for self hatred. You can approach it from a place of compassion instead. That’s a very different place to begin from, and a far more hopeful one.
I know this and yet feel like a failure anyway when I ate stuff that we all know I shouldnt have……