Find out exactly how many calories to eat to lose weight safely.
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A calorie deficit is when you consume fewer calories than your body burns in a day. Your body needs energy to survive — when it doesn't get enough from food, it turns to stored body fat. That's how fat loss works. No deficit, no fat loss — it really is that simple at the physics level.
Your body burns calories in three ways: your basal metabolic rate (BMR — the energy needed just to keep you alive), the thermic effect of food (digesting what you eat), and your activity level. Add these together and you get your Total Daily Energy Expenditure (TDEE) — your maintenance calories. Eat less than this and you lose weight. Eat more and you gain.
This is the most important decision in a fat loss diet — and most people get it wrong. The temptation is to cut aggressively ("I'll lose faster") but the evidence says otherwise:
Easiest to maintain. Minimal hunger, minimal muscle loss. Best if you're already lean or have a lot of life stress.
The gold standard for most people. Fast enough to see real progress, small enough to keep energy, muscle, and mood intact.
Effective but demanding. Hunger increases, training performance may drop. Works well for heavier individuals with more fat to lose.
Aggressive. Risk of muscle loss, fatigue, and metabolic adaptation increases. Only for those with significant excess weight under medical supervision.
The NHS and most sports nutrition bodies recommend losing no more than 0.5–1 kg per week for sustainable fat loss. At the lower end of this (0.5 kg/week, a 500 kcal daily deficit), you preserve more muscle mass, maintain better training performance, and are far less likely to experience metabolic adaptation — the slowdown that happens when your body detects prolonged energy restriction.
Aggressive deficits also tend to fail in practice. The hunger and fatigue they cause leads to overeating events that wipe out days of deficit in a single sitting. Slow and steady isn't just a cliché — it's what the data shows actually works long-term.
When you're in a calorie deficit, protein is the most important macro to protect. Your body is more likely to break down muscle for energy when calories are low, and adequate protein is the main defence against this. The macro split shown above prioritises protein (at ~2g/kg body weight) then balances carbs and fat across the remaining calories.
Carbohydrates fuel your workouts and keep energy stable — cutting them too aggressively undermines training. Fat supports hormonal health and helps you feel satisfied. The split above is a solid baseline; you can adjust carbs and fat to suit your preferences without meaningfully affecting fat loss outcomes.
Your body adapts to a sustained calorie deficit over weeks and months. It reduces your BMR, lowers non-exercise activity (unconscious fidgeting, posture shifts), and becomes more efficient at digesting food. This is often called a "plateau" but it's actually a predictable physiological response.
The solution isn't to cut more calories — that accelerates adaptation and muscle loss. Diet breaks and refeeds (periodically returning to maintenance for 1–2 weeks) have been shown to slow adaptation and improve long-term fat loss outcomes. PrepSmart's meal plans are built around this — cycling between deficit and maintenance phases to keep metabolism responsive.
A safe calorie deficit is typically 300–500 kcal below your maintenance calories (TDEE). This produces gradual weight loss of 0.3–0.5 kg per week and preserves muscle mass. Larger deficits (500–1,000 kcal) can work but increase the risk of muscle loss, hunger, and metabolic adaptation. Deficits larger than 1,000 kcal per day are generally not recommended without medical supervision.
To lose 1kg of body weight per week, you need a daily calorie deficit of approximately 1,000 kcal (since 1kg of fat stores roughly 7,700 kcal). For most people this is aggressive and hard to sustain. A more manageable target is 0.5kg per week, which requires a 500 kcal/day deficit. Starting smaller and being consistent over months produces better long-term results.
Yes, to some extent — a phenomenon called metabolic adaptation. As you lose weight and eat less, your body burns slightly fewer calories at rest. The effect is modest (typically 100–200 kcal/day) and can be minimised by keeping protein intake high, doing resistance training, and avoiding extremely large deficits. Periodic diet breaks or refeeds also help prevent significant metabolic slowdown.