By
Daniel Cheung, MPharm
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6 min read
Punch your stats into any calorie calculator — including ours — and you'll get a confident-looking number. Total Daily Energy Expenditure: 2,579 kcal. It feels like a fact about you, like your height.
It isn't. It's a snapshot of a moving target — and the act of dieting is precisely what moves the target.
Your body fights the maths
When you eat in a deficit, your body doesn't passively surrender its energy reserves. It adapts, on several fronts at once:
You get lighter. A smaller body simply costs less to run. Every kilo lost trims your daily expenditure.
You move less without noticing. Non-exercise activity — fidgeting, pacing, taking the stairs — quietly drops in a deficit. This is often the single biggest adaptation, and it's almost entirely unconscious.
Your metabolism becomes more efficient. Beyond what weight loss alone predicts, energy expenditure falls further — a phenomenon called adaptive thermogenesis.
Hunger hormones shift. Leptin falls, ghrelin rises. The deficit gets harder to hold at exactly the moment it's delivering less.
The result: the 500-kcal deficit you set in week one might be a 200-kcal deficit by week six — without you changing a thing.
This is why plateaus feel so unfair
You're doing everything right. Same meals, same logging, same discipline. But you're aiming at where the target was. Most people respond by blaming themselves, cutting harder, or quitting. All three are the wrong lesson — the plan failed, not the person.
The weekly trend — the number that actually tells the truth.
A better formula isn't the fix. A process is.
No equation can predict how your body adapts — the individual variation is too large. What works is measurement and correction: track the weekly trend (not the daily noise), compare it against the intended rate of change, and adjust the target to where your body is now. Then repeat, every week, for as long as you're dieting.
You can run that loop manually with a spreadsheet and some discipline. It's what good coaches charge hundreds a month to do. It's also, precisely, what Fettle's Engine automates — with the added honesty of showing you each decision and its reasoning.
Fettle recalculates your targets every week from your real trend — then turns the updated plan into your grocery basket.
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A better formula isn't the fix. A process is.
No equation can predict how your body adapts — the individual variation is too large. What works is measurement and correction: track the weekly trend (not the daily noise), compare it against the intended rate of change, and adjust the target to where your body is now. Then repeat, every week, for as long as you're dieting.
You can run that loop manually with a spreadsheet and some discipline. It's what good coaches charge hundreds a month to do. It's also, precisely, what Fettle's Engine automates — with the added honesty of showing you each decision and its reasoning.
Notes & further reading
Key topics to cite for the curious: Mifflin-St Jeor equation validation studies; adaptive thermogenesis literature (e.g. Rosenbaum & Leibel); NEAT research (Levine).
This article is general information, not medical advice. If you have a medical condition or take medication affecting weight or appetite, speak to your healthcare professional before changing your diet. See our health disclaimer.
