Calorie deficit: how much should you subtract?

What deficit do guidelines recommend?

The most common recommendation is a deficit of 500 to 750 calories a day. A 2013 guideline for adults with overweight or obesity, from the American Heart Association, the American College of Cardiology and The Obesity Society, describes several ways to set it. You can aim for a deficit of 500 or 750 calories a day, cut about 30 percent, or eat a fixed 1,200 to 1,500 calories a day for women and 1,500 to 1,800 for men, adjusted for body weight and activity.

The three approaches give different targets depending on where you start:

For a smaller or less active person, a 750-calorie deficit can land below the 1,200 to 1,500 range the guideline typically uses for women. The same guideline says very-low-calorie diets, under 800 calories a day, should be used only in limited cases, by trained practitioners in a medical setting, because of the rapid weight loss and the risk of complications.

How fast will a deficit make you lose weight?

More slowly than the old rule suggests, and more slowly as time goes on. The old rule says a pound of weight equals 3,500 calories, so a 500-calorie daily deficit should take off a pound a week, week after week. A 2011 analysis in The Lancet called that static rule wrong because it ignores how the body adapts as weight changes.

Its authors proposed a rough rule of thumb for an average adult with overweight instead. Each lasting change of 10 calories a day leads to an eventual weight change of about a pound, with half of it arriving in about a year and 95 percent in about 3 years. In their model, a sedentary man weighing 100 kg (220 lb) who cut 480 calories a day was predicted to level off near 75 kg (165 lb), reaching half of that loss in roughly a year.

Body composition matters too. A 2008 analysis found that the 3,500-calorie rule roughly fits people who start with more than 30 kg of body fat but overstates the deficit per pound for leaner people. Leaner people are predicted to lose a larger share of lean tissue, which holds fewer calories per pound than fat.

The 2013 guideline reports the same pattern in trials. Average weight loss peaked at about 6 months, at 4 to 12 kg, and was 4 to 10 kg at 1 year and 3 to 4 kg at 2 years as some weight returned.

Why not make the deficit bigger?

Faster isn't better for most people. The CDC notes that people who lose about 1 to 2 pounds a week are more likely to keep the weight off than people who lose it faster, and that goals such as losing 20 pounds in 2 weeks can leave you feeling defeated and frustrated.

Your body also pushes back. A 2002 review found that everyday movement, like walking, typing and fidgeting, falls when people eat less, which shrinks the deficit you think you have. And because weight is part of the resting-energy formula, each kilogram you lose lowers your maintenance number.

Protecting muscle takes more care in a steep deficit. A 2017 position stand from the International Society of Sports Nutrition found that people who strength train may need 2.3 to 3.1 grams of protein per kilogram a day to hold on to as much lean mass as possible while eating below maintenance.

Who should get individual advice?

Some people need a plan from a clinician rather than a formula. That includes anyone who is pregnant or breastfeeding, teenagers, people with a medical condition or on medication that affects weight, and anyone with a history of disordered eating. The CDC lists medicines, medical conditions, hormones and age among the factors that affect weight management.