What is a calorie deficit, and how big should it be?

How to set a calorie deficit

1. Estimate maintenanceWork out your TDEE with an equation and an activity factor, or better, from two to three weeks of logged intake and weigh-ins.
2. Pick a paceChoose a weekly loss of about 0.5–1% of your body weight. The slower end is easier to sustain and protects muscle better.
3. Turn the pace into caloriesMultiply the weekly kilograms by 7,700 and divide by 7 to get the daily deficit. Subtract it from maintenance.
4. Keep protein high and keep liftingAim for roughly 1.6–2.4 g of protein per kg of body weight, and keep resistance training in your week.
5. Review every two to three weeksCompare your weekly average weight with the pace you chose. If it is off, adjust intake by 100–200 kcal and review again.

Every fat-loss plan, whatever it is called, works by creating a calorie deficit. Keto, fasting, cutting out snacks, counting points: each is a different route to eating less energy than you burn. The deficit is the mechanism. The details decide whether you can keep it up, and whether what you lose is mostly fat.

How a deficit works

Your body burns a certain amount of energy each day, your TDEE. When food supplies less than that, the body makes up the difference from stores: glycogen in muscle and liver, fat tissue, and some protein. Over weeks, a steady deficit shows up as a falling weight trend.

The planning figure is about 7,700 kcal per kilogram of body fat, or 3,500 kcal per pound. Fat tissue is not pure fat. It also holds water and some protein, which is why the figure is lower than 9 kcal × 1,000 g. A daily deficit of 550 kcal adds up to 3,850 kcal a week, which the rule turns into about 0.5 kg.

Why 7,700 kcal per kg is only an approximation

• Water and glycogen: The first week of a diet often shows a drop of a kilogram or more. Much of it is glycogen, which is stored with roughly three grams of water per gram, plus water from eating less salt and less food sitting in the gut. It is real weight, but it is not fat.
• A shrinking target: As you get lighter, your TDEE falls, so the same intake makes a smaller deficit. Progress slows even when you change nothing.
• Adaptation: In a sustained deficit, many people move a little less without noticing, and the body becomes slightly more economical. Both shrink the gap further.
• Logging error: The deficit you think you have depends on how well you count. Cooking oil, drinks and weekend meals are where most of it leaks.

Kevin Hall’s model, published in The Lancet in 2011, puts numbers on this. For an average adult with overweight, each sustained cut of about 100 kJ (24 kcal) a day eventually moves body weight by about 1 kg, with half of that change in the first year and nearly all of it within three years. The static 7,700 rule predicts much faster loss than that over a year. That is not a reason to cut harder. It is a reason to expect a slower second half and to judge by the trend, not the plan.

How big a deficit to choose

A sensible pace for most people is about 0.5–1% of body weight per week. Helms, Aragon and Fitschen recommended this range in 2014 for natural bodybuilders preparing for a contest, because faster loss tends to cost more muscle. Larger bodies lose more kilograms at the same percentage. Leaner people should sit at the slower end.

Worked through: an 80 kg person with a maintenance of about 2,500 kcal who picks 0.5% a week needs a deficit of 440 kcal a day (0.4 × 7,700 ÷ 7), so a target of about 2,060 kcal. At 1% a week the deficit is 880 kcal and the target about 1,620. The calorie deficit calculator runs this for your own body and pace.

Many guidelines treat about 1,200 kcal a day for women and 1,500 for men as a floor below which you should diet only with a doctor or dietitian involved. Those are rules of thumb, not physiology, but they are a sensible line. If you are pregnant, have diabetes or kidney disease, take medication that affects appetite or blood sugar, or have a history of disordered eating, get personal advice before you start a deficit.

Keep the muscle

Weight lost in a deficit is not all fat. How much muscle goes with it depends mostly on things you control.

• Lift: Resistance training tells the body the muscle is still needed. Keep the heavy work in, even if you trim volume when energy is low.
• Eat enough protein: The ISSN position stand on protein (2017) puts most people who exercise at 1.4–2.0 g per kg of body weight a day and notes that lifters in a deficit may need more. Many aim for about 1.8–2.4 g/kg while dieting. The protein calculator gives a range for your weight.
• Go slow: A smaller deficit is easier to keep, keeps training quality up, and loses a larger share of fat.
• Sleep: In a 2010 study by Nedeltcheva and colleagues, dieters given 5.5 hours in bed lost less fat and more lean mass than the same people given 8.5 hours, on the same diet.

Common mistakes

• Cutting too hard at the start: A big early deficit feels productive and is usually the first thing abandoned. Start moderate.
• Reading daily weigh-ins: The scale swings with salt, carbs and water. Use a weekly average, compared over two to three weeks.
• Dropping protein with calories: Protein should hold steady as the deficit grows. Trim fat and carbohydrate first.
• Never adjusting: Maintenance falls as you lose weight. Recheck your numbers after every few kilograms.

How FORM uses it

When you choose the lose weight goal, FORM starts from Mifflin-St Jeor times an activity factor and sets a target below it, with the weekly pace capped relative to your body weight. Every target is editable, including protein. Meals logged by photo, barcode, menu or text count against the target in the macro tracker, and the weight tracker charts the trend you judge it by.

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