How many calories you consume and how many calories you burn determine whether you lose weight or gain weight. For weight loss, the math is simple: you must burn more calories than you consume or eat fewer calories than you burn. In short, you have to create a calorie deficit.
When you create a calorie deficit, there is not enough calories immediately available from the food you eat and you force your body to tap into your fat stores – your excess fat – for energy. This is when weight loss occurs. You can also get the same effect by increasing your physical activity, so that your body will need more energy.
One pound of fat is approximately equivalent to 3,500 calories. To lose about 1 to 2 lbs per week, you’ll have to reduce your calorie intake by 3,500 to 7,000 calories each week.