The amount of energy in any food or drink that we consume is measured in calories. Any unused calories are stored as fat in the body, to be used later. We refer to the number of calories that we require when at rest as our BMR (basal metabolic rate) and the number of calories we expend to support our lifestyle each day as our TDEE (total daily energy expenditure). This is the number of calories that we require to maintain our weight.
Consuming a higher number of calories than our TDEE is known as a calorie surplus. Regularly being in a calorie surplus will result in weight gain. This is because our body will continue to store these extra calories as body fat for later use.
Consuming fewer calories than our TDEE is known as a calorie deficit. Regularly being in a calorie deficit will result in weight loss. This is because our body will begin tapping into those fat stores for energy.