The calorie deficit, explained properly (and why yours might not be working)

Fat loss has one non-negotiable rule: over time, you need to take in less energy than your body uses. That's the calorie deficit. It's boring, it's unsexy, and it's the only mechanism that has ever worked for anyone.

But "a deficit" isn't a fixed number you set once and forget. Understanding why it drifts is the difference between people who lose weight and people who lose weight and keep it off.

What a deficit actually is

Your body burns energy in four main ways:

• Basal metabolic rate (BMR) — keeping you alive at rest. Usually 60–70% of the total.
• Non-exercise activity (NEAT) — walking, fidgeting, standing, housework. Hugely variable between people.
• Exercise — deliberate training. Smaller than most people think.
• Digestion (TEF) — around 10% of what you eat, higher for protein.

Add those together and you get your total daily energy expenditure (TDEE). Eat below it and you're in a deficit. Simple in theory.

Why your deficit shrinks over time

Here's the part that catches people out. As you lose weight, every one of those four numbers goes down:

1. A lighter body has a lower BMR.
2. Your body subtly reduces NEAT when energy is scarce — you fidget less, sit more, move slower without noticing.
3. Exercise burns fewer calories because you weigh less.
4. You're eating less food, so TEF drops too.

The deficit that worked at 90kg is often barely a deficit at 80kg. This is not a "broken metabolism" — it's physics — but it means plateaus are expected, not a sign of failure.

The three most common reasons a deficit "isn't working"

1. Untracked calories

Oils, sauces, "a bit" of cheese, the kids' leftovers, weekend drinks. Studies consistently show people under-report intake by 20–40%. You're not lying — you're human.

2. Water retention masking fat loss

Hard training, high sodium, stress, poor sleep and the menstrual cycle can all hold 1–3kg of water. Fat loss is happening underneath; the scale just can't see it yet. Track a weekly average, not a daily number.

3. The weekend effect

A 500-calorie deficit Monday to Friday is 2,500 for the week. Two relaxed weekend days at +1,250 each wipe it out entirely. You feel like you dieted all week and lost nothing — because, net, you didn't.

What to actually do

Treat your deficit as something you maintain, not something you set.

• Weigh yourself daily, but only compare weekly averages.
• If the average hasn't moved in 2–3 weeks, reduce intake by ~150 kcal or add 2,000 daily steps. Not both. Small levers.
• Prioritise protein (1.6–2.2g per kg of goal bodyweight) — it protects muscle and keeps you full.
• Take a planned maintenance week every 8–12 weeks. It resets your head as much as your body.

If you want this done for you, Macro Mastery includes a calculator and a plateau decision tree that walks you through exactly these adjustments.