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Calories to lose fat
without losing muscle
The short answer: there is no number that works for everyone. What the studies support is to estimate what you burn, subtract a moderate deficit (in people who do strength training, a meta-analysis suggests not going beyond about 500 kcal a day to keep lean mass), do strength training, eat enough protein and adjust according to how your weight changes, because the formulas can be quite wrong for each individual. Below 1,200 kcal, UK guidelines already speak of diets for a specialist service.
Every figure links to its study and says how strong the evidence is.
1. What you burn: an estimate, not a measurement
- Weak evidence · observational, model or experts The Mifflin-St Jeor equation was built with 498 adults measured in a lab and explains 71% of the variation in resting energy expenditure (Mifflin 1990). It is the most accurate of the usual ones, but with notable errors for a given person (Frankenfield 2005), and it is the one the US Academy of Nutrition and Dietetics recommends when it cannot be measured (AND).
- Weak evidence · observational, model or experts Multiplying by activity adds more error: total energy expenditure equations fell short, with considerable individual errors (Prado-Nóvoa 2024).
- Moderate evidence · trials Your weight is the best correction. With repeated weigh-ins, a model estimated real expenditure with an average error of less than 40 kcal a day, although about 215 kcal for each individual (Sanghvi 2015). In practice: if after a few weeks your weight does not move as you expected, the starting number was wrong, not you.
How our calculator does it: it uses Mifflin-St Jeor and a factor based on the days you train; the gentler option takes off 20% and the faster one 30%, and it does not go below 1,200 kcal for women or 1,500 for men: the low end of the ranges in section 7. Work out yours here.
2. Why «7,700 kcal = 1 kilo» fails
- Weak evidence · observational, model or experts What it takes to lose a kilo depends on how much fat you have: the rule fits people with a lot, and exaggerates in leaner people (Hall 2008).
- Weak evidence · observational, model or experts The body changes as you lose weight. In the NIH model, half the loss from a permanent cut arrives at around one year, and the fixed rule can predict twice what happens in that first year (Hall, Lancet 2011). Compared with real experiments, the rule overestimates the loss (Thomas 2013). The NIH has a public simulator with this model (Body Weight Planner).
3. How big a deficit, if you want to keep muscle
- Strong evidence · meta-analyses Up to about 500 kcal a day. In strength trials done in a deficit, less lean mass was gained than without a deficit (strength was gained just the same), and from a deficit of about 500 kcal a day it was no longer gained (Murphy and Koehler 2022, small studies).
- Moderate evidence · trials Slower preserved it better. In 24 elite athletes, losing 0.7% of body weight per week raised lean mass by 2.1%; losing 1.4% did not change it (Garthe 2011). Losing the same amount over 15 weeks left a better body composition than over 5 (Ashtary-Larky 2017). With a very low diet, of 500 kcal, 8.8% of fat-free mass was lost compared with 1.3% with 1,250 kcal (Vink 2016).
- Weak evidence · observational, model or experts The recommended rates: 0.5–1% of body weight per week for people who do strength training (Helms 2014); 0.45–0.9 kg (1–2 lb) per week (CDC); or 5–10% of body weight in 6 months (NIDDK). The 2013 US obesity guideline proposes a deficit of 500 to 750 kcal a day (AHA/ACC/TOS 2013).
- Moderate evidence · trials Fast or slow, in the long run the same amount is regained: three years later, about 70% in both groups (Purcell 2014, 200 people; and also in Vink 2016).
4. Strength training and eating protein in the meantime
- Strong evidence · meta-analyses Exercising in a deficit preserves almost half the lean mass that would otherwise be lost: 0.87 kg on average across 34 trials; with strength training, 0.83 kg; with strength and aerobic training, 1.2 kg (Deller 2026; some of the trials are at risk of bias).
- Strong evidence · meta-analyses In older adults with obesity, strength training prevented almost all the lean mass loss from the diet (Sardeli 2018, 6 trials).
- Weak evidence · observational, model or experts With diet alone, it was usual for at least 15% of what was lost to be fat-free mass (Weinheimer 2010).
- Protein has its own guide: how much protein per day.
5. Does your metabolism slow down?
- Moderate evidence · trials A little, on a normal diet. With 15% fewer calories for two years, energy expenditure was about 80–120 kcal a day lower than expected (CALERIE, Redman 2018).
- Weak evidence · observational, model or experts And it shrinks after the deficit ends: −92 kcal a day during the diet, −38 when weight stabilized and almost nothing after a year (Martins 2020), with a lot of variation between people.
- Weak evidence · observational, model or experts The famous TV case is extreme: 14 contestants who lost 58 kg were still burning about 500 kcal less than expected six years later (Fothergill 2016). It does not represent a moderate diet.
6. Do diet breaks work?
- Moderate evidence · trials For hunger, it seems so; for muscle, it is not clear. In people who do strength training, one week off every three left the same fat and the same lean mass, with less hunger (ICECAP, Peos 2021). In men with obesity, alternating two weeks of diet and two of maintenance lost more fat and slowed expenditure less, but lean mass was lost just the same (MATADOR, Byrne 2018).
7. The floor not to go below
- There is no single official minimum. The ranges of 1,200 to 1,500 kcal for women and 1,500 to 1,800 for men come from the 2013 US guideline as a way to prescribe the diet, adjusted for body weight, not as a safety limit (AHA/ACC/TOS 2013).
- Diets of 800 to 1,200 kcal, and those below 800, only within a specialist service according to the UK guideline; those below 800, only with a clinical need, under supervision and for 12 weeks at most (NICE NG246, 2025).
- Weak evidence · observational, model or experts If you train a lot and eat very little, low energy availability can affect health and performance; the International Olympic Committee consensus advises against relying on a single threshold (IOC 2023).
- Weak evidence · observational, model or experts Losing weight very fast increases the risk of gallstones: with 500 kcal diets, about 3 times more than with 1,200–1,500 kcal (Johansson 2014, observational).
8. And then, keeping it off
- Strong evidence · meta-analyses A good part is regained: after 5 years, people were on average still a little over 3 kg lighter, more than 3% of their starting weight (Anderson 2001, 29 studies).
- Weak evidence · observational, model or experts People who keep it off tend to be active for around an hour a day, keep an eye on their weight and eat at regular times, weekends included (National Weight Control Registry, Wing and Phelan 2005, observational).
Frequently asked questions
How many calories do I need to eat to lose fat?
There is no number that works for everyone. You start from your estimated energy expenditure (for example with the Mifflin-St Jeor equation and your activity) and subtract a moderate deficit: a meta-analysis in people who do strength training suggests not going beyond about 500 kcal a day if you want to keep your lean mass. Then you adjust by watching how your weight changes over a few weeks, because the equations can be quite wrong for each individual.
How big a calorie deficit to avoid losing muscle?
A 2022 meta-analysis (Murphy and Koehler) found that less lean mass is gained in a deficit, and that from a deficit of about 500 kcal a day it was no longer gained. In athletes, losing 0.7% of body weight per week preserved lean mass better than losing 1.4%. Strength training and eating enough protein also help.
Is it true that 7,700 kcal equal one kilo?
Only at the start and roughly. The body changes as you lose weight, and NIH models show that this rule can predict twice the loss that actually happens in a year.
Does dieting slow down your metabolism?
Energy expenditure drops a little more than the weight lost explains, but in most studies the difference is small: between about 50 and 120 kcal a day, with a lot of variation between people. Extreme cases, such as the contestants of a TV show who lost 58 kg, do not represent a normal diet.
Is it better to lose weight fast or slowly?
In the long run the same amount is regained: in a trial with 200 people, three years later about 70% had been regained both by those who lost it fast and by those who lost it gradually. But losing very fast took off more lean mass in several trials.
What is the minimum number of calories per day?
There is no single official minimum. The ranges of 1,200 to 1,500 kcal for women and 1,500 to 1,800 for men come from a 2013 US guideline as a way to prescribe the diet, adjusted for body weight, not as a safety limit. In the United Kingdom, NICE considers that diets of 800 to 1,200 kcal and those below 800 should be done within a specialist service.
Keep count and see your weight line
Reverfit Diet is a tool to log what you eat and see your calories, your protein and how your weight changes week by week, which is what lets you adjust the starting number.
Diet
