A calorie deficit is the number of calories you consume in a day that is less than the number of calories your body burns.
To lose weight, you need to create a calorie deficit by eating fewer calories than your body needs. This forces your body to use its stored energy (fat) for fuel instead of constantly getting more from food. If you don’t eat enough calories, you’ll lose weight.
The exact amount of calories you need varies based on age, height and gender. But most people require between 1,000 and 2,000 calories per day to maintain their current weight — so if you want to lose weight, you’ll need to consume fewer calories than that each day or burn more through exercise.
A calorie deficit can also help with weight loss because it increases your metabolism (the rate at which your body burns calories). When you create an energy deficit by exercising more or reducing calorie intake, your body responds by burning fat for energy instead of carbohydrates or protein. This helps sustain muscle mass while losing weight.
When you’re in a calorie deficit, your body looks for other sources of energy to use. It starts by breaking down glycogen stores (the carbohydrates stored in muscles and liver) and converting the glucose molecules inside them into energy for use by your cells. When those are used up, the body turns to fat stores and converts those triglycerides into energy for use by cells. As long as there’s enough energy coming in from food or activity each day (not including any exercise), this process will continue until all of the excess calories have been converted into fat stores or used up through physical activity like exercise or daily living activities
What happens if I don’t have a calorie deficit?
If you don’t have a calorie deficit, then your weight won’t change much — if at all. The only way for your body to lose weight is through energy balance: taking in fewer calories than it burns over time. Unless something changes in this equation, no weight loss will occur.