Caloric deficit

A caloric deficit (also known as calorie deficit, in British English calorific deficit) is any shortage in the number of calories consumed relative to the number of calories needed for maintenance of current body weight (energy homeostasis).

A deficit can be created by decreasing calories consumed by lower food intake, such as by swapping high-calorie foods for lower calorie options or by reducing portion sizes. A deficit can also be created by increasing output (burning calories) without a corresponding increase in input. Increased output is created by increasing physical activity, from increased caloric requirements necessary to heal an injury, or from growth. There are also some substances, including caffeine, which can create a small (3-5%) increase in caloric expenditure, via a variety of pathways that include increasing physical activity levels and increasing thermogenesis (heat output), and/or by reducing caloric input via appetite suppression. Drugs and herbal treatments creating a more extreme metabolic effect exist; however, they cause extreme increases of heart rate and thermogenesis that can cause death in even very healthy and athletic individuals, and these drugs are not widely sold.

As the calories required for energy homeostasis decreases as the organism's mass decreases, if a moderate deficit is maintained eventually a new (lower) weight will be reached and maintained, and the organism will no longer be at caloric deficit. A permanent severe deficit, on the other hand, which contains too few calories to maintain a healthy weight level, will eventually result in starvation and death.

To reduce 1 kg (2.2 lbs) of weight, about 7000 kcal deficit is required.

How to calculate a calorie deficit?
Start by finding your basal metabolic rate (BMR), which is the amount of calories your body burns at rest. There are several formulas to help you calculate your BMR based on your age, gender, weight, and height. Add the estimated number of calories you burn during physical activity to your BMR. You can use various online calculators to help you estimate caloric expenditure depending on the type of activity and duration. Then subtract the number of calories you want to take in per day from this total caloric expenditure. It is usually recommended to choose a caloric deficit between 500 and 1000 calories per day for mild to moderate weight loss. This will achieve an average weight loss of about 0.5 to 1 kg per week.