There’s a lot of confusion out there, so in the name of clarity, I’ll give you the basics on calorie intake, as simply as possible. A calorie is a unit of energy. Your body uses energy to fuel your daily activity. If you take in more energy than your body uses, it stores the excess, usually as fat, but if you’re lifting heavy and using your muscles it can and will use that excess energy to build muscle too. That’s what we want.
If you’re at a healthy weight and your goal is body recomposition, you need to eat AT LEAST as many calories as your body burns, preferably a little bit more. Your body needs raw materials to build muscle, and if it’s using all the calories you eat to fuel your activity there will not be any left to build muscle with. If you’re undereating at a healthy weight, your body will do whatever it can to burn fewer calories, so will slow down your metabolic processes and start burning muscle for fuel, because muscle requires more calories to maintain than fat. If you’re undereating, that muscle is taking up energy (calories) that your body would rather use to fuel your heartbeat and brain activity.
If you’re obese or overweight, you need to burn more calories than you consume to lose weight. Not too many more, I generally recommend keeping a 500 calorie deficit to keep your metabolism healthy. Alternately, you can use a calorie calculator to figure out how many calories your body will need to maintain your goal weight (including your activity), and eat that many calories. Your body will take care of the rest (that’s how I did it). If you keep your deficit relatively small and lift weights and get regular exercise, your body will tag your muscles as ‘in use’ and preserve them, burning stored fat to make up the deficit in energy in vs energy out. If you’re sedentary, your body will burn lean mass as well as fat to make up for that deficit.
NONE of this means that diets that don’t require calorie counting don’t work. It just means that, for some people, those diets help them to naturally and instinctively consume the number of calories that will support their goals. Different people can experience this with different diets. Some people (like me) don’t ever experience it (or at least I haven’t yet, and I’ve tried all the diets) and need to be at least somewhat aware of our energy requirements. And we need to eat enough to fuel our activity and support our goals. Restricting calories below that level is counterproductive and will ultimately lead to failure. For most people, including women over about 100 pounds, that means eating at least 2000 calories a day, much more for most of us (I eat about 3000 most days). Even to lose weight, very few of us will need to drop below that, and if we do we are setting ourselves up for a damaged metabolism and fat storage.
Here are two calorie calculators that I find realistic and helpful, neither of them will give you an unsustainably low calorie target, in fact you will likely be surprised by just how much you need to eat to stay healthy and support your goals.