Why Your Body Fights Back When You Diet — or Try to Outrun It

If you’ve ever lost weight only to see it creep right back, you’re not broken — you’re built for survival.

For years, I lived by the “energy in = energy out” equation, just as we’d all been taught. According to this theory, I had a choice: create a calorie deficit by eating less or exercising more.

As a runner and food lover, I chose to exercise more. I wasn’t overweight, but who doesn’t like to drop a dress size?

So I upped my running. I’d proudly finish long training runs, check my Strava, and smile at the calorie tally. Those numbers indicated a job well done — I could either enjoy the food credit or revel in my well-earned calorie deficit. Job done!

My first marathon — probably the fittest I’ve ever been, yet despite the huge calorie burn, the stubborn belly fat stayed put ( hidden here!).

But when I trained for my first marathon, something didn’t add up. I was burning hundreds of calories a week, yet I didn’t shift a bit of the stubborn fat we women in our 40’s hold on to. Instead, I felt tired and run-down. I was eating healthily, but not enough to cover the expenditure, and yet my body didn’t respond as I thought it would. 

Retaining my wobble seemed unfair!

That experience sent me searching for answers — and it took a lot of reading, questioning, and connecting the dots to piece it all together. It’s not rocket science, but it’s an inconvenient truth for the diet industry and the beliefs we’ve held on to for decades.

So today, I want to shed a little light on:

• How and why your body defends weight loss

• The “hidden thermostat” that controls your weight

• The hormones you really need to understand: leptin, insulin, and ghrelin

  
  

Because it turns out, your body isn’t a simple maths equation — it’s a thermostat.

The Myth of the Simple Deficit

We know that to lose weight we need a calorie deficit — consuming less energy than we use. In simple maths terms, it works: fewer calories in than out, and the scale moves.

But that equation only tells half the story. It doesn’t account for how the body responds to that deficit.

As energy drops, the brain quietly adjusts the dials: hunger rises, metabolism slows, and movement decreases.

The weight loss that worked for a few weeks now stalls, not because of failure or lack of willpower, but because biology adapts to protect you.

The Body’s Hidden Thermostat

Deep in the brain sits a tiny region called the hypothalamus. Among many other things, it acts like your body’s thermostat for weight. It keeps tabs on how much fat you’re carrying and adjusts hunger, metabolism, and movement accordingly.

When you diet, you’re effectively turning that thermostat down. At first, you lose a few kilos, but then your body senses danger. It doesn’t know you’ve simply decided to lose weight — it senses famine and immediately goes into energy-protection mode.

Think of your phone when it switches to battery-saving mode: it still works, but quietly shuts down background functions to preserve power. That’s your body on a diet.

Your body on a diet? Think “battery-saver mode.” It conserves energy, reduces output, and preserves fat — your built-in insurance policy against famine.

Here’s the crux of the problem: your body doesn’t want to lose fat, even though your conscious brain does. The two aren’t wired to negotiate. Your body’s instincts are ancient and built purely for survival, overruling your best intentions every time.

In times of famine — perceived or real — the body holds on to fat. Hormones shift, hunger increases, and energy output falls. It’s not sabotage; it’s survival. Your body is doing exactly what it was designed to do: keep you alive.

And for those of us diligently chasing calorie deficits, every round of yo-yo dieting reinforces the same message:

“Food isn’t guaranteed — hold on to your reserves.”

So it does —by storing more fat. Gah - cutting calories has the opposite effect in the longer-term!

Meet the Hormones Who Take Charge

There’s an invisible conversation happening inside us — a trio of hormones constantly negotiating energy, hunger, and fat storage on our behalf.

Most of us have heard of insulin, the one that manages blood sugar. But its co-stars, leptin and ghrelin, deserve a proper introduction too.

Leptin — The Fuel Gauge

Leptin is made by your fat cells. The more fat you have, the more leptin circulates in your bloodstream.

In a perfect system, it acts like a fuel gauge. When fat stores are sufficient, leptin levels rise and signal to the brain: “We’re well stocked — famine isn’t a threat.” So appetite decreases, metabolism stays steady, and the body feels safe enough to rest and repair.

Ghrelin — The Hunger Alarm

Ghrelin works in the opposite way. Produced mainly in the stomach, it’s the “I’m hungry” hormone. Levels rise when your stomach is empty or when you’ve gone too long without eating. In a healthy rhythm, ghrelin reminds you to refuel — then quiets down once you’ve eaten.

Together, leptin and ghrelin form a perfect push-pull system: one quietens appetite, the other sparks it.

Insulin — The Energy Manager

Then there’s insulin, released whenever we eat carbohydrates and sugar. Its job is to move glucose from the bloodstream into cells for use or storage as glycogen or fat.

When the system works properly, insulin rises after a meal and puts our bodies in ‘store mode’. When it falls between meals and at night the body can switch to burn mode, and our fat stores begin to reduce.

The three key players in your body’s energy conversation — leptin (the fuel gauge), insulin (the energy manager), and ghrelin (the hunger alarm).

When the System Breaks

Here’s where things start to unravel.

Our modern food environment and habits are no match for the world our genes evolved for. True famine is rare these days — if anything, we’re in a healthy-food famine, but that’s another story.

Instead, we’re surrounded by ultra-processed, carbohydrate-rich, and sugary foods. We’ve also been taught to snack constantly, so there’s a near-continuous conveyor belt of food for ourselves and our children.

This constant feeding means insulin levels stay high for most of the day, which promotes fat storage. Crucially, high insulin also blocks leptin’s message to the brain. Even though leptin levels are high (because fat stores are full), the brain doesn’t hear the “we’re safe” signal — so it doesn’t stop us from foraging for food. Back to the biscuit tin we go...

This is called leptin resistance.

When the leptin signal isn’t heard, the brain believes we’re running out of energy and responds exactly as it would during famine: it ramps up hunger, slows metabolism, and reduces movement to conserve fuel. We eat more, move less — and gain weight.

Then, when we logically try to reverse the problem through dieting or restriction, ghrelin, the hunger alarm, steps in. As calories drop, ghrelin rises — adding yet another powerful cue to eat.

Together, these hormones create the perfect storm: we feel hungry, tired, and driven to eat — even when our energy stores are overflowing.

It’s not a willpower issue; it’s the biology of survival playing out in a modern environment of abundance. The same system that once protected us from starvation now quietly pushes us towards weight gain.

The Evolutionary Logic: Why the Body Fights Back

To understand why our bodies react this way, we need to look way back.

Humans are unusual among mammals for carrying relatively high levels of body fat. While most wild animals hover around 3–8% body fat, healthy humans typically carry 15–25%. That’s not a flaw; it’s an evolutionary masterpiece.

Fat is our biological insurance policy - it fuels our large, energy-hungry brain, supports reproduction, keeps hormones stable, and acts as an emergency energy bank when food disappears.

In an ancient world of feast and famine, our ability to store energy as fat was a superpower.

The problem is, our bodies don’t distinguish between a modern weight-loss plan and a prehistoric famine.

So in the case of calorie-restrictive diets, when slowing, shifting hormones and rising hunger finally drive us to eat again, our bodies refill their reserves with interest — adding a little extra “just in case” for next time. That’s why many people end up heavier after each dieting cycle.

The feast and famine cycle drives us to retain weight as our insurance policy. With each cycle we get a little bigger, which completely goes against what we set out to achieve.

Eating to satisfy hunger and cravings is not a flaw in your character; it’s a feature of your physiology — one that was once essential for survival but now works against us in a world of constant abundance.

Our modern western diet keeps that famine signal switched on. When insulin levels stay high — driven by constant snacking, refined carbohydrates, and sugary foods — leptin’s “I’m full” message can’t get through to the brain. We keep eating, not out of weakness, but because the signal to stop never arrives.

The solution isn’t to eat less; it’s to eat differently. Real food — the kind your body recognises — helps lower insulin, re-sensitise leptin, and quieten ghrelin’s constant chatter.

It’s not calories that count most — it’s chemistry: the molecules within food and how they interact with our bodies.

When we fuel with slow-burning, unprocessed, fibre-rich foods, we send the message our biology has been waiting for all along: “You’re safe. You can let go.”

The story doesn’t end here…

Next time, we’ll look at how to earn your body’s trust — how nourishment, rest, rhythm, and real food can help you reset your thermostat and rebuild a relationship of safety with your body.

The Science Behind This Post

The science behind this post fascinated me — if you’d like to read more, I’ve linked some of the research that helped it all click for me below.

References & Further Reading

1. Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. International Journal of Obesity. 2010; 34(Suppl 1): S47–S55.

2. Myers MG Jr, Cowley MA, Münzberg H. Mechanisms of leptin action and leptin resistance. Physiological Reviews. 2008; 88(2): 381–404.

3. Sumithran P et al. Long-term persistence of hormonal adaptations to weight loss.

   New England Journal of Medicine. 2011; 365: 1597–1604.

4. Pontzer H. Burn: The Misunderstood Science of Metabolism. Penguin Random House, 2021.