Calorie deficit
Fat loss requires eating fewer calories than you burn for weeks to months; every diet that works does this, whatever it is called. The body pushes back as you lose weight, so progress is slower than simple arithmetic predicts and the deficit needs adjusting along the way.
What the science says
- A sustained energy deficit is what drives weight loss Strong evidence
In the POUNDS Lost trial (Sacks et al., 2009), 811 adults with overweight were randomized to four reduced-calorie diets with very different fat, protein and carbohydrate shares. All groups lost about 6 kg at 6 months and ended with similar losses at 2 years, because the calorie reduction, not the macro mix, did the work.
- Weight responds slowly and the body adapts Strong evidence
Hall et al.'s 2011 Lancet model of human metabolism showed that the weight response to a change in intake is slow, with a half-time of about a year, because energy expenditure falls as weight drops. People with more body fat lose more weight for the same calorie cut.
- Weight loss is resisted by counterbalancing physiology Moderate evidence
Hall and Guo's 2017 review explains that intake and expenditure are linked: as weight falls, expenditure drops and appetite rises, which favors a set-point view of body-weight regulation. 'Eat less, move more' is correct physics but incomplete advice; plans have to expect this pushback.
How to apply it
Estimate your maintenance intake, eat a moderate amount below it, and adjust every few weeks based on the scale trend, not single weigh-ins.
Who should be careful
A moderate deficit is safe for healthy adults; very large deficits raise the risk of muscle loss, fatigue, nutrient gaps and gallstones. Anyone with diabetes, high blood pressure or another obesity-related condition, a history of eating disorders, who is pregnant or breastfeeding, or who is considering weight-loss medication should plan weight loss with a physician or registered dietitian; this page gives no drug advice.
Frequently asked questions
Does it matter which diet I use to create the deficit?
Much less than people think. In POUNDS Lost, four diets with very different fat, protein and carbohydrate shares produced similar weight loss at 2 years; pick the one you can keep.
Why is my weight loss slower than the calorie math predicts?
Because energy expenditure falls as you lose weight and appetite rises. Hall's model shows the weight response to a calorie change is slow, with a half-time of about a year.
How big should my deficit be?
Start around 300–500 kcal/day below maintenance and aim to lose about 0.5–1% of your body weight per week, adjusting if the weekly average stalls for 2–3 weeks.
Scientific references
More on weight loss
- Rate of weight loss Moderate evidence
For people who lift, losing about 0.
- High-protein diet while dieting Strong evidence
Eating more protein during a calorie deficit helps keep lean mass and gives a small extra fat loss compared with a standard-protein diet of the same calories.
- Resistance training while dieting Strong evidence
Lifting weights during a diet makes much more of the weight you lose come from fat instead of muscle, and it keeps or raises strength.
- Cardio for weight loss Moderate evidence
Cardio helps fat loss, but at typical doses it produces only a couple of kilograms on its own; clinically meaningful loss takes high volumes (more than about 250 minutes a week) or a calorie deficit alongside it.
- Diet breaks and refeeds Moderate evidence
Planned periods at maintenance calories in the middle of a diet are at least as effective as dieting continuously and may make the diet easier to tolerate.
- Intermittent fasting Moderate evidence
Intermittent fasting (5:2, alternate-day fasting or time-restricted eating) works for weight loss about as well as an ordinary calorie-controlled diet, but not better.