Most of us assume that every calorie we eat goes directly to being metabolized by muscle and fat. Did you know that over half of the calories you burn are used to maintain your body temperature? When you severely restrict your calories, your body temperature drops. If you know anyone who is super lean, or anorexic, they are often wearing more clothing than others with regard to the ambient temperature, which illustrates this fact.
If you've been on a restrictive diet, and you are afraid to increase your calories, for fear you're going to gain weight, try this:
Take your body temperature. If it is less than 98.6 degrees, chances are, the main change you will see if you increase your calories, is an increase in your body temperature. It's usually best to increase your calories in 100 calorie increments at a time, and sit there for a week to evaluate the effect. If your weight stays stable or drops, but your temperature increases, your problem with weight may be that you're eating too little food. Try adding 100 calories a week until you get to 98.6 degrees and see where you land.
The only condition where this experiment may not work, is if you have a thyroid that is not working the way it should, as the thyroid directly affects your metabolism.
It's a very common sense tool to look at what you're doing vs. what you may need to be doing.
Try it. You may be surprised at the results.