If I eat below my daily claoric needs, but still train quite heavy, is it even possible that when my body uses its stored fat to provide for my body that it can even build new muscle or do I always need to be eating a lot more than my basic needs?
Fat can provide energy once it’s broken down in the body, but it’s not going to help you build muscle.
You build muscle by breaking it down through training and then having the body repair itself by growing more muscle.
You can grow muscle if not eating a surplus of calories. However, you will need to be eating a certain amount of protein in order to help the body repair the muscle.
There are studies showing people on a very calorie restricted liquid diet (800-900 calories) who lost fat and gained muscle. I’m not going to say this will always be possible as it does depend on your training age, the type of training you are doing, etc.
However, it does prove that muscle can still be built provided the correct stimulus is in place even if on a low-calorie diet.
Nate is (as expected) right… being fat does not per se help, but if restricting calories then as you suggest calories from bodyfat more readily substitute for dietary caloric deficit in meeting the body’s energy needs than is the case when lean, where energy may well come from catabolizing muscle as well.
So while a lean person finds it harder than does a person with substantial excess bodyfat to gain muscle on a sustained basis while on a caloric-deficit diet, or impossible, it’s not unusual to have moderate muscle gains with more novice individuals who have excess bodyfat despite being below maintenance calories.
If at maintenance calories or above, then the excess bodyfat itself provides no direct muscle building advantage.