What happens if I don't eat enough calories?

If you're not eating enough calories, you're going to run into some serious problems. Don't be afraid to eat more, you'll need all the energy you can get. Your body will be weak, and you might even get sick. If you're still not convinced, let's look at this in a slightly less serious way. If you don't eat enough calories, the result will be your body having to take the energy you need from somewhere. It will take that energy from the fat in your cells and turn your body's muscles into jelly. That won't be good, you don't want to be weak and sick and floppy!
On average, women need around 2000 calories a day and men need around 2500. If you don't eat enough of these calories, you'll see symptoms of malnourishment. The first and most common symptom of malnourishment is fatigue. This is because your body is running out of energy and needs to focus more on basic functions. You might even feel dizzy! Eventually your body will start to focus on more vital functions, like breathing and heart rate. If you don't eat enough calories, your body will begin to consume muscles and bones. This can cause your hair and fingernails to grow slower, you might have trouble breathing, your endurance will be lowered, your heart and kidneys will be affected, and your immune system will be compromised and you'll need to eat more calories to avoid death!
When you don't eat enough calories, this can cause your metabolism to slow down. That's why eating enough calories is important but it's not just a matter of counting calories! There are plenty of nutrients you need in your diet. If you're not getting enough of these, that can cause all sorts of problems.