Calorie deficit: how big it should be, and how to keep it without strain

What a calorie deficit actually is

The NHS puts it simply: to stay at about the same weight, the calories your body uses should match the calories you eat and drink. Eat and drink fewer than you use, and your body uses its stored fat for energy. That gap is the deficit.

As a rough guide, the NHS gives 2,500 kilocalories a day for an average man and 2,000 for an average woman. Your own figure depends on your age, weight, height and how much you move, which is why an individual estimate is more useful than an average.

Step one: estimate your maintenance

The calculator on this page applies the Mifflin-St Jeor equation, published in 1990 from measurements of 498 healthy adults. A systematic review by Frankenfield and colleagues in 2005 found it the most reliable of the common equations: it predicted resting expenditure within 10% of the measured value in more people, of normal weight and with obesity, than any other.

Take an illustrative case. Sara is 35, weighs 68 kilograms, measures 167 centimetres and trains three to five times a week. The equation gives about 1,390 kilocalories at rest, and about 2,150 for the whole day once activity is counted.

That figure is an estimate. The guide to calorie needs explains how to choose your activity factor honestly and how to check the estimate against three weeks of real life.

Step two: choose the size of the deficit

The 2013 guideline of the American Heart Association, the American College of Cardiology and The Obesity Society lists the approaches it considers effective:

• A fixed deficit of 500 or 750 kilocalories a day below your estimated needs.
• A fixed intake of about 1,200 to 1,500 kilocalories a day for women and 1,500 to 1,800 for men, adjusted to body weight.
• A diet pattern that removes or limits certain foods, which creates a deficit without counting, provided intake really falls.

For Sara, a 500 kilocalorie deficit means eating about 1,650 a day. No method was declared the winner: the guideline lists several evidence-based diets, and what matters is that the deficit is real and sustained.

The NHS frames the target as a pace rather than a number: aim to lose 0.5 to 1 kilogram a week. If your weekly average falls faster than that for several weeks, your deficit is probably larger than it needs to be. The guide to the safe weight loss rate explains why the gentler pace is usually the wiser one.

Why the 3,500 kcal rule misleads you

You will often read that a deficit of 3,500 kilocalories equals one pound of fat, so 500 a day means a pound a week, forever. Thomas and colleagues tested that rule against seven weight-loss experiments with closely supervised or measured food intake, and concluded that it grossly overestimates actual weight loss over time.

The reason is that your needs fall as you lose weight. A smaller body uses less energy, so the same intake gives a shrinking deficit. Hall and colleagues, in a modelling study published in the Lancet in 2011, proposed a better rule of thumb for an average overweight adult: each lasting change of 100 kilojoules a day, about 24 kilocalories, leads eventually to a weight change of about 1 kilogram, with half of it reached in about a year and 95% in about three years. Weight responds to a change in eating slowly, and it keeps responding for a long time.

Two practical consequences follow. Expect the pace to slow over the months, which is normal rather than a failure; see weight loss plateau. And recalculate your needs with the calculator every few kilograms.

Keep the deficit, keep the muscle

A deficit takes weight from fat and, partly, from muscle. Three levers shift the balance towards fat.

1. Strength training. The ACSM position stand on weight loss found that resistance training may increase fat-free mass and the loss of fat mass, even though it does not add to the loss on the scale.
2. Protein. A meta-analysis of 24 trials by Wycherley and colleagues found that higher-protein diets, at the same calorie level, led to slightly more fat loss and better preservation of lean mass. The details are in protein for weight loss.
3. Sleep. In a controlled study by Nedeltcheva and colleagues, people on the same diet lost 55% less of their weight as fat when they slept 5.5 hours rather than 8.5. The guide to sleep for executives covers the habits that protect the night.

Making it livable

A deficit that is too harsh tends to break at the first business dinner. A few habits make a moderate one hold:

• Build each meal around a protein source and vegetables, then add starch according to the day.
• Notice drinks: they carry calories without filling you up. The trade-offs of evenings out are covered in alcohol and training.
• Keep one or two flexible meals a week rather than a rigid plan with no exits.
• Weigh yourself several mornings a week and compare weekly averages, not single days.

A calorie deficit is not suitable for everyone. If you are pregnant or breastfeeding, have diabetes, kidney disease or a history of disordered eating, or are underweight, talk to your doctor or a registered dietitian before reducing what you eat.

Exercise makes the deficit easier to hold

You can create a deficit with food alone, but training changes how it feels and what it produces. It protects muscle, improves health markers even when the scale is slow, and, as the 2013 guideline notes, high levels of physical activity of about 200 to 300 minutes a week help maintain lost weight in the long term. The guide to exercise to lose weight shows how much training to plan around your deficit.

Turn the deficit into a routine with Never Skip Club

A deficit works when the rest of the week supports it. Create your free account on Never Skip Club, launch guided sessions of 20, 30 or 45 minutes built for your level and your equipment, and start a crew with three to seven friends: you set a rhythm together, and the ranking rewards the sessions you keep, not extra ones. If you would like a professional to review your plan, find a coach from our network, or sign in if you already have an account.