What is a calorie deficit?
A calorie deficit is the situation in which daily energy intake is below expenditure (TDEE), forcing the body to draw on its reserves — that is the physiological principle of any weight loss. A moderate deficit of 300 to 500 kcal per day leads to a loss of about 0.3 to 0.5 kg per week, considered sustainable by most health professionals.
How to calculate it in practice
First step: estimate TDEE, meaning total daily calorie expenditure. The Mifflin-St Jeor formula (1990) remains the reference: for a woman, BMR = 10×weight + 6.25×height − 5×age − 161; for a man, BMR = 10×weight + 6.25×height − 5×age + 5. That BMR is then multiplied by a physical activity factor ranging from 1.2 (sedentary) to 1.9 (very active).
Second step: subtract 300 to 500 kcal to get the target. A more aggressive deficit (700 to 1000 kcal) is sustainable for a few weeks but often triggers muscle loss, chronic fatigue, and a rebound effect. Conversely, a deficit that is too light (under 200 kcal) is mathematically valid but gets lost in the noise of daily variation and often discourages the user for lack of visible results.
Third step, often skipped: keep protein intake high during the deficit, ideally 1.6 to 2.2 g per kg of body weight. That precaution keeps weight loss from being made of water and muscle rather than fat mass.
Why 500 kcal and not more
The empirical 500 kcal rule comes from a simple line of reasoning: 1 kg of fat mass equals about 7700 kcal stored. A deficit of 500 kcal × 7 days = 3500 weekly kcal, or ~0.45 kg per week. That is fast enough to stay motivating and slow enough to preserve muscle mass with adequate protein intake.
Beyond that, the body triggers metabolic adaptation mechanisms: a 5 to 15% drop in TDEE, fatigue, lower libido, sleep problems, stronger cravings. Those effects have been documented since the Keys studies (1944) on semi-prolonged fasting and confirmed since by dozens of modern studies.
An overly aggressive deficit is also the leading factor in long-term failure: 80 to 95% of restrictive diets end in weight regain within 5 years, often above the starting weight.
Common mistakes to avoid
The first mistake is not counting drinks and sauces. A soda, a glass of wine, two spoonfuls of mayonnaise can double the calorie intake of a meal that still feels light. Those liquid or added calories are massively underestimated in manual food diaries.
The second mistake is forgetting weekends. A 500 kcal deficit Monday through Friday cancelled by a Saturday night at +1500 kcal gives a net weekly deficit of 1000 kcal — not the 2500 you expected. The balance is calculated over seven days, not five.
The third mistake is aiming for a deficit without tracking protein. Without that anchor, weight loss contains a disproportionate share of muscle, which lowers basal metabolism and makes regain easier.
How it connects to Daizu
Daizu automatically sizes the nutrition plan to the calorie target that matches the goal entered in the profile — loss, cut, maintenance, muscle gain, sport, or healthy eating. The deficit or surplus is applied by default, with protein intake preserved when that is relevant. Deviations (restaurants, unplanned dishes) are tracked by photo, which avoids the classic error of manual underreporting.
