Why Your Body Fights Back When You Diet

(and what your hormones are trying to tell you)

Ever felt like you’re doing everything you’ve been told: eating less, moving more —and yet you still can’t keep the weight off?

For decades, those have been the guiding principles of weight loss. But beyond a brief burst of success, they rarely work for long. It’s every dieter’s greatest frustration. So in this blog, I want to reassure you: you’re not broken; you’re built for survival.

Eating well brings countless benefits, but let’s be honest, weight loss is the number one reason most people want to change their diet. The link between food and body weight is direct, and the promise of a slimmer body is what nudges many of us to make a change.

But most of us are still caught in an outdated, over-simplified belief that we can calorie-cut our way to long-term weight loss. It’s not true — we prove it to ourselves with every diet cycle that fails, and end up feeling betrayed by our own bodies.

Here’s the truth: when it comes to weight loss, your body isn’t a calculator.

It’s a thermostat.

The Myth of Simple Maths

The calorie in/out model — eat less, burn more — sounds logical but when energy intake drops, your brain quietly adjusts some dials. Hunger rises, metabolism slows, and you unconsciously move less. The weight loss that worked for a few weeks stalls - not because of lack of willpower - but because biology adapts.

This clever defence mechanism has a name: adaptive thermogenesis — your body’s built-in famine protection system.

To your body, a calorie deficit doesn’t look like healthy discipline; it looks like danger. So it acts accordingly:

• Metabolism slows — you burn fewer calories just keeping the lights on.

• Hunger hormones rise — food suddenly dominates your thoughts.

• Energy dips — workouts feel harder, and the sofa starts calling.

  
  

If your diet stopped working, it’s not because you failed. It’s because your body has proudly succeeded at keeping you alive.

Your weight-loss agenda simply can’t negotiate with your evolutionary hard-wiring. So your body does what it’s built to do: it defends what you’re trying to lose — your fat reserves. Darn.

So how exactly does your body pull this off? The answer lies deep within your brain…

Your Body’s Hidden Thermostat

Deep in your brain sits a small but powerful control centre: the hypothalamus. Among its many jobs, it acts as your body’s weight thermostat.

When fat stores fall, the hypothalamus senses the drop and quietly turns the dials: hunger increases, fullness signals fade, and metabolism slows.

Think of your phone in low-power mode — it still works, but it quietly shuts down background processes to conserve energy. In your body, that means less spontaneous movement (you fidget less, walk slower, sit more), reduced thyroid and sex-hormone output, and fewer resources directed to non-essential repair tasks like hair growth, skin renewal, and even immune defence.

That’s your body on a diet — still running, but conserving every watt it can.

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

Research shows that after calorie-restriction weight loss, metabolism can remain 10–15% lower than expected for months, even years. That means your body burns fewer calories than someone of the same size who’s never dieted.

This is where yo-yo dieting makes things even trickier. Each time you lose weight and regain it, your body remembers the perceived famine. It becomes even quicker to slow metabolism and more efficient at storing fat — a kind of metabolic “muscle memory.” Every new attempt to diet feels harder, progress is slower, and the rebound faster.

It’s frustrating, but it’s also proof that your body is brilliantly protective. It’s not trying to sabotage you; it’s trying to save you.

Meet the Hormones Running the Show

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

Behind that metabolic slowdown is a hormonal conversation you can’t see — one that quietly dictates how hungry you feel, how much energy you burn, and how easily you store fat.

There’s an invisible trio leading the discussion: three key messengers that regulate hunger, energy, and fat storage.

Leptin — the Fuel Gauge

Made by your fat cells, leptin signals to the brain that energy stores are sufficient: “We’re well stocked — you can relax.”When you lose fat, leptin levels fall. The brain reads that as an energy crisis, raising appetite and slowing metabolism.

Ghrelin — the Hunger Alarm

Produced in the stomach, ghrelin is the “feed me” hormone. Levels rise when you’re hungry and drop after eating.During calorie restriction, ghrelin stays high — keeping the hunger alarm ringing.

Insulin — the Energy Manager

Released whenever you eat carbohydrates or sugar, insulin moves glucose from your blood into cells for use or storage.When it’s balanced, your body easily switches between storing and burning energy. But modern diets and constant snacking keep insulin chronically elevated — priming the body to store fat, not burn it.

The Evolutionary Logic: Why the Body Fights Back

To understand why our bodies defend fat so fiercely, we need to look way back.

Humans are unusually good at storing fat. While most mammals hover around 3–8% body fat, healthy humans carry 15–25% — and that’s no accident.

Fat is our biological insurance policy. It fuels our large, energy-hungry brains, supports fertility, stabilises hormones, and provides a backup energy supply when food disappears.

In an unpredictable, feast-or-famine world, that ability to store fat was a superpower. The problem is, our physiology hasn’t caught up with our food environment.

When you diet, your body can’t tell the difference between conscious calorie restriction and a prehistoric famine. So when hunger and fatigue eventually drive you to eat again - and food is now everywhere, so you can - your body naturally refills its reserves with interest - extra fat.

Every crash diet reinforces the same message: “Food isn’t guaranteed — hold on to your reserves.”

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.

The Modern Mismatch

We no longer live in a world of feast and famine: food is everywhere, and the modern landscape confuses this ancient system.

Ultra-processed foods, constant snacking, poor sleep, and stress keep our hormones on high alert, tricking the brain into thinking scarcity is just around the corner.

When you couple that with our constant urge to diet, tone, and chase the “body beautiful” ideal - you have the perfect storm. Our biology is built to protect us, yet our culture keeps telling us to fight it.

Understanding this mismatch between our Stone Age biology and our modern environment is the first step toward real change.

Changing Beliefs, Not Behaviour

This blog’s purpose is to change beliefs, not behaviour.

Once you understand that your biology is built for protection, not sabotage, the struggle softens. The goal stops being control and starts becoming cooperation.

When you start understanding your body, you listen with compassion. How to make dietary changes comes next, but eating whole foods really is the biggest win to gaining your body’s trust again.

In my next post, I’ll explore diet and how our modern food environment causes biological confusion which can spiral into leptin resistance — the state where your brain stops hearing the “I’m full” signal.

This is the mechanism behind the “I really don’t eat much but keep putting on weight” scenario — and it’s real. Your body isn’t ignoring your efforts; it’s responding exactly as it was designed to, doing its best to protect you in a world your biology still thinks is unpredictable.

References & Further Reading

1. Rosenbaum, M., & Leibel, R. L. (2010). Adaptive thermogenesis in humans. International Journal of Obesity,34(Suppl 1), S47–S55.

   – Describes how the body defends weight loss through metabolic and hormonal adaptations.

2. Sumithran, P., et al. (2011). Long-term persistence of hormonal adaptations to weight loss. New England Journal of Medicine, 365(17), 1597–1604.

   – Shows that hunger hormones and metabolic rate remain altered long after dieting stops.

3. Pontzer, H. (2021). Burn: The Misunderstood Science of Metabolism. Penguin Random House.

   – Demonstrates that total energy expenditure plateaus over time — we can’t “out-burn” biology.

4. Hopkins, M., Blundell, J. E., & King, N. A. (2014). Individual variability in compensatory eating following exercise. Obesity Reviews, 15(S4), 57–66.