What Is a Calorie Deficit? (And Why It's the Only Way Diets Work)
Keto, fasting, low-fat — every diet that works, works the same way. Here's the mechanism.
Strip the branding off any successful diet — keto, intermittent fasting, paleo, low-fat — and you find the same engine underneath: a calorie deficit, eating less energy than your body uses.
The energy budget
Your body spends energy constantly: about 60–70% on basic life support (BMR), ~10% digesting food, and the rest on movement. When food supplies less than that total, the difference comes out of storage — mostly body fat. About 3,500 kcal of cumulative deficit equals one pound of fat.
So why do diets feel so different?
Because they create the deficit by different routes. Fasting compresses the eating window so you eat less. Keto and high-protein diets blunt appetite. Low-fat cuts the most calorie-dense nutrient. All roads lead to the same place: calories in below calories out.
Building a deficit you can live with
- Size it honestly: 250–500 kcal/day is sustainable for most people. Use the deficit calculator to match it to a deadline.
- Spend it on food you enjoy: adherence beats optimization every time.
- Move more, cut less: a 300 kcal walk plus a 300 kcal food cut feels far easier than a 600 kcal food cut.
- Expect adaptation: metabolism drifts down slightly as you lose — recalculate as you go.
Start by finding your maintenance number with the calorie calculator — everything else builds on it.