The simple fact of the matter is - if you consume more calories than you burn, you WILL gain weight, whereas if you consume less calories than you burn you WILL lose weight.
So not to undervalue the importance of the many other factors which will be covered, calories are the first part of the jigsaw to put in place.
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.
Generally, when looking to lose weight, we aim to work at a 10 - 20% deficit. For most people this will be enough to elicit fat loss. A greater deficit may likely result in compromised energy and performance and extreme calorie deficits can lead to many health complications.