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How to Lose Weight: The Science Behind the Numbers
Weight loss comes down to one fundamental principle: a sustained calorie deficit. When you consistently consume fewer calories than your body burns, it draws on stored body fat for energy. However, it's not quite as simple as "calories in, calories out" — as you lose weight, your metabolism adapts, burning fewer calories at rest. This is called metabolic adaptation, and it's why weight loss often slows after the first few weeks.
Understanding Your Numbers
| Term | What It Means |
|---|---|
| BMR (Basal Metabolic Rate) | Calories burned at rest — just to keep your organs functioning |
| TDEE (Total Daily Energy Expenditure) | BMR × activity factor — total calories your body burns per day |
| Calorie deficit | Eating less than TDEE — forces the body to use stored fat |
| ~7,700 cal = 1kg of fat | A 500 cal/day deficit = ~0.5kg lost per week |
Safe Rate of Weight Loss
Health Australia guidelines and international evidence recommend losing no more than 0.5–1kg per week for sustainable results. Faster loss often leads to muscle loss, nutrient deficiencies, metabolic adaptation and rebound weight gain. Slow, consistent progress — supported by adequate protein (1.6–2.2g per kg body weight) — preserves muscle while burning fat.
The Role of Exercise
Exercise contributes to weight loss in two ways: direct calorie burn during activity, and improved insulin sensitivity and metabolic health over time. However, exercise alone rarely produces significant weight loss without dietary changes — exercise is most powerful when combined with a moderate calorie deficit. Resistance training is particularly valuable as it preserves muscle mass during a deficit, maintaining your metabolic rate.
This calculator provides general educational information. Weight loss outcomes are highly individual. Consult a GP or registered dietitian before making significant changes to your diet or exercise habits, particularly if you have any health conditions.