Nutrition glossary
What is a calorie deficit?
A calorie deficit means you take in fewer calories from food and drink than your body uses over a period of time. To cover the gap, your body draws on stored energy, mostly fat, which is why a steady deficit leads to weight loss.
How a calorie deficit works
Your body burns energy all day. Most of it goes on simply keeping you alive: breathing, pumping blood, keeping warm. The rest goes on digesting food and moving, from walking to the bus to a workout. Add those together and you get your maintenance calories, the amount that keeps your weight stable.
Eat below that number and you are in a deficit. Eat above it and you are in a surplus. Every diet that leads to weight loss, from keto to intermittent fasting, works by creating a deficit in one way or another. They just differ in how they get you there and how easy they are to stick with.
A worked example
Imagine your maintenance is about 2,200 kcal a day. A normal day for you might look like this:
| Meal | Usual day | Deficit day |
|---|---|---|
| Breakfast | Two slices of toast with butter and jam, latte (about 450 kcal) | Porridge with berries, black coffee (about 300 kcal) |
| Lunch | Meal deal: sandwich, crisps, fizzy drink (about 750 kcal) | Chicken and bean salad wrap, apple, water (about 500 kcal) |
| Dinner | Large pasta bowl with garlic bread (about 800 kcal) | Smaller pasta portion with extra vegetables (about 650 kcal) |
| Snacks | Biscuits (about 200 kcal) | Yogurt (about 250 kcal) |
| Total | About 2,200 kcal | About 1,700 kcal |
That is a deficit of roughly 500 kcal a day, made from small swaps rather than skipped meals.
Why "500 a day = 1 lb a week" is only a rough rule
The old rule says a pound of fat holds about 3,500 kcal, so 500 kcal a day for seven days loses one pound. It's a handy starting point, but it is an oversimplification, and your real results will drift from it:
- In the first week or two you lose extra weight as water, especially if you cut carbs, so the scale drops faster than the maths predicts.
- As you get lighter, you burn fewer calories, so the same intake gives a smaller deficit over time.
- Some of what you lose is muscle, not fat, especially if you eat little protein and don't do any strength work.
- Calorie counts on labels and in apps are estimates, and most of us underestimate portions.
Expect weight loss to slow over the months, even when you are doing everything right. The NIH Body Weight Planner uses a more realistic model if you want a better estimate.
How big a deficit is safe?
The NHS suggests aiming to lose about 0.5 to 1 kg (1 to 2 lb) a week. For most adults that means a daily deficit somewhere around 500 kcal, give or take. Larger people can often manage a bigger gap; smaller, lighter people may need less.
Very low calorie diets, around 800 kcal a day or less, should only be done with medical supervision. If you have diabetes and take insulin or other medicines that lower blood sugar, are pregnant or breastfeeding, or have a history of an eating disorder, talk to your doctor before you start cutting calories. See what a safe weight-loss program looks like.
Signs your deficit is too big
- You feel hungry nearly all the time and think about food constantly.
- You are tired, cold, irritable or dizzy, or your sleep gets worse.
- Your workouts fall apart and you feel weak lifting things you managed before.
- Your hair thins or your periods become irregular or stop.
- You keep losing more than about 1 kg (2 lb) a week after the first few weeks.
If several of these sound familiar, eat a bit more. A smaller deficit you can keep going for months beats a big one that falls apart after three weeks.
Common questions
What does being in a calorie deficit mean?
It means that over a day or a week, you are eating and drinking fewer calories than your body burns. Your body makes up the difference from stored energy, so your weight goes down over time.
Can you be in a calorie deficit and not lose weight?
Over a few days, yes, because water shifts, salt, a big meal or your menstrual cycle can hide fat loss on the scale. Over several weeks, if the scale hasn't moved at all, the usual reason is that you are eating more than you think. Try weighing portions for a week.
How do I work out my maintenance calories?
Online calculators give a rough starting figure from your age, sex, height, weight and activity. Treat it as a guess, then adjust: if your weight is steady for two or three weeks at a certain intake, that intake is roughly your maintenance.
Related
A deficit tells you how much you eat, not how well. Pair it with diet quality and enough protein from the macronutrients entry. For eating patterns that make a deficit easier, see time-restricted eating and the weight-loss hub. Not sure where to start? Read how to choose a diet and how long to try a diet.