Calorie deficit, explained simply.

What is a calorie deficit?

Your body uses energy all day: to keep your heart beating and organs working, to digest food, and to move. That total is often called your maintenance calories or total daily energy expenditure. When you take in less energy from food and drink than your body uses, you are in a calorie deficit, and over time your body draws on stored energy to make up the difference. That is what leads to weight loss.

The idea is simple. Putting it into practice is where the nuance comes in.

How maintenance calories are estimated

Daily energy use is made up of three main parts:

• Basal metabolic rate (BMR): the energy your body uses at rest. It is usually the largest part and depends on size, age, sex and body composition.
• Activity: everything from exercise to walking, standing and fidgeting. This varies the most between people and between days.
• Digesting food: a smaller amount used to break down and process what you eat. Protein takes a little more energy to digest than carbohydrate or fat.

Apps and online calculators estimate maintenance using formulas based on age, sex, height, weight and activity level. These are useful starting points, but they are estimates: two people with the same numbers can differ by a few hundred calories a day. The most reliable approach is to start with an estimate, track your intake and weight for a few weeks, and adjust based on what actually happens.

How big should a deficit be?

A moderate deficit is generally more sustainable than an aggressive one. A commonly cited starting point is around 500 calories a day below maintenance, which has traditionally been linked to losing around 0.5 kg (about 1 lb) a week. In reality, the rate tends to slow over time as your body gets smaller and adapts, so treat that as a rough guide. Public health bodies such as the NHS generally describe a gradual loss of around 0.5 to 1 kg a week as a sensible, safe rate for most adults.

Very low-calorie diets can cause fatigue, nutrient gaps and loss of muscle, and are only appropriate under medical supervision. If you are pregnant, breastfeeding, under 18, have a medical condition or a history of disordered eating, do not try to create a deficit without advice from a doctor or registered dietitian.

Why the scale does not move in a straight line

Day-to-day weight can swing by a kilogram or more because of water, salt, carbohydrate intake, digestion and hormonal cycles. A salty dinner or a hard workout can make the scale go up overnight, even in a deficit. That is why a weekly average or trend line is far more useful than any single weigh-in. If the trend over three or four weeks is not moving as expected, that is a better signal to adjust than one disappointing morning.

Keeping a deficit sustainable

Prioritise protein

Eating enough protein is linked to holding on to more muscle while losing weight, and protein-rich meals may help with fullness. See how much protein you need.

Fill up on high-volume foods

Vegetables, fruit, soups, legumes and whole grains give a lot of food for relatively few calories, and the fibre may help you feel satisfied.

Keep moving

Physical activity adds to the deficit, but it also supports heart health, mood and muscle. Strength training is especially useful for maintaining muscle.

Watch liquid calories

Sugary drinks, juices, alcohol and large milky coffees add energy without much fullness. Cutting back is often one of the easiest changes.

Sleep and stress

Poor sleep is linked to increased hunger and cravings for many people. Looking after sleep makes sticking to a plan easier.

Tracking helps, but perfection is not required

Logging food, even roughly, is one of the most practical ways to see where your calories come from. You do not need to be exact; consistent estimates are enough to spot patterns and adjust. Photo logging is a quick way to keep the habit going; see counting calories from a photo.