Every weight-loss method — keto, intermittent fasting, low-carb, slimming clubs, GLP-1 injections — works the same way underneath: by putting you in a calorie deficit. Understand the deficit, and you understand why some plans work and others don't. Here's how it actually works.
What "calorie deficit" means
A calorie deficit is when you eat fewer calories than your body burns. Your body makes up the difference by using stored fat (and a bit of stored muscle, which is why protein matters in a diet).
That's it. There's no clever metabolic trick — every diet that produces fat loss does so by creating a deficit, even if it doesn't talk in those terms.
The maths
A pound of body fat contains roughly 3,500 calories of stored energy.
| Daily deficit | Weekly loss | Notes |
|---|---|---|
| 250 kcal/day | ~0.5 lb/week | Very gentle, often used for maintenance phases |
| 500 kcal/day | ~1 lb/week | NHS-recommended starting point |
| 600 kcal/day | ~1.2 lb/week | NHS upper-end for general use |
| 1,000 kcal/day | ~2 lb/week | Aggressive — only short term, hard to sustain |
Why 500–600 kcal is the sweet spot
The NHS, NICE, the British Dietetic Association and most credible UK weight-loss services all recommend a deficit of 500–600 kcal per day. Here's why:
- It's sustainable. Not so big that you're miserable, hungry and ready to binge by Friday.
- It protects muscle. Bigger deficits force the body to break down muscle for fuel.
- It minimises metabolic adaptation. Aggressive deficits cause your TDEE to drop more than expected.
- It produces a steady ~1 lb/week. Boring, but reliable. After 6 months that's 26 lb / nearly 2 stone.
Why bigger deficits backfire
People often assume that if 500 kcal/day = 1 lb/week, then 1,500 kcal/day = 3 lb/week. The body doesn't work that linearly.
What actually happens with very aggressive deficits:
- Your body adapts. NEAT (fidgeting, walking, general movement) drops. You burn less without realising.
- Hunger hormones spike. Ghrelin (hunger) goes up, leptin (fullness) goes down. Cravings get intense.
- You lose more muscle. Without enough protein and resistance training, up to 25% of weight lost can be muscle.
- It's miserable. Energy crashes, sleep suffers, mood drops. Most people quit.
- Rebound is brutal. When eating returns to normal, the lower TDEE means weight comes back fast.
How to find your deficit number
Step 1: Find your TDEE
Use the calorie calculator to estimate your Total Daily Energy Expenditure (TDEE). For most UK adults this is 1,800–2,800 kcal.
Step 2: Subtract 500 kcal
That's your starting target.
Step 3: Track honestly for 2 weeks
Weigh yourself once a week, same day, same time, before breakfast. If you're losing 0.5–2 lb/week, you've got it right. If not, the issue is almost always under-tracking, not metabolism.
Step 4: Adjust as you lose
Every stone lost reduces your TDEE by ~50–80 kcal. After losing 2 stone, you'll need to recalculate or progress will stall.
Floors you shouldn't go below
The NHS and most UK dietitians warn against eating below:
- 1,200 kcal/day for women
- 1,500 kcal/day for men
Below these floors, it's hard to get all essential nutrients (calcium, iron, B vitamins, fibre) and almost impossible to sustain. Any "very low calorie diet" (under 800 kcal) should only be done under medical supervision — these are clinical interventions, not DIY.
Common deficit mistakes
1. Eating back exercise calories
Your fitness watch says you burned 400 kcal on a run. You "earn" a 400-kcal pastry. Net deficit: zero. Either don't track exercise calories at all, or eat back only half — wearables overestimate by 30–50%.
2. Weekend resets
Mon–Fri 1,500 kcal, Sat–Sun 3,000 kcal. Average: 1,930 kcal. You're at maintenance. Weekend eating undoes weekday discipline more than people realise.
3. Liquid calories
Two glasses of wine (~250 kcal), a latte (~150 kcal), a smoothie (~300 kcal) = 700 kcal that don't fill you up. Easy to wipe out a deficit without "eating" anything.
4. Underestimating portions
UK research suggests people typically underestimate intake by 20–30%. Weighing food for the first week of a diet is humbling but useful.
5. Quitting too soon
Weight loss isn't linear. Hormonal cycles, water retention, salt and exercise can mask real fat loss for 1–2 weeks at a time. Trust the process — the trend over 4 weeks is what matters.
The bottom line
A 500–600 kcal daily deficit is the sustainable sweet spot for most UK adults. Bigger deficits aren't faster — they just fail more dramatically. Track honestly, recalculate as you lose, and don't dip below 1,200 (women) or 1,500 (men) without medical input.
Sources: NHS Live Well, NICE clinical guideline CG189, British Dietetic Association, NHS Weight Loss Plan. Always consult your GP before starting a weight-loss programme if you have health conditions.