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Marbleized fat, also called intramuscular fat, is fat that is stored within the muscle tissue.  The fat is streaked in a pattern of veins that resembles marble.  (You’ve seen it in steak.) Trained athletes have high levels of intramuscular fat but low levels of stored subcutaneous fat, whereas out-of-shape folks have an increased accumulation of both intramuscular and subcutaneous fat.   Exercise reduces intramuscular fat but diet alone does not.  Marbleized fat can supply up to 20% of energy during exercise.   Both weight-training and cardiovascular exercise can burn marbleized fat.