The short answer is “no.” Muscle and fat are two fundamentally different types of tissue, so muscle never degenerates into fat, and fat cannot be transformed into muscle.
The components your body uses to make each are not the same, so while it can appear that one is morphing into the other, there are actually two processes at work.
So, what’s actually happening when we witness dramatic changes in our body composition over time?
The prime factor in how much, or how little fat you have is food. The determining factor in the amount of muscle you have is stress (like exercise) on your body.
Much more than a band of excess tissue around your waist or on your thighs, fat is made of thousands of unique cells that have the ability to hold onto fluid. Fat cells respond to conditions in the body in three ways—shrink when there is less food energy available, stay the same when there are adequate calories, or grow if there is excess.
Food energy does not have to begin as fat to be stored as fat. Your body has to break everything you eat into pieces small enough to get into individual cells, which means carbohydrates go through multiple processes to become glucose, protein ultimately gets broken into amino acids and fat will wind up as fatty acids.
Your body does not typically lose fat cells unless you surgically remove them, and your body will not usually make more individual cells past puberty, although if you gain a tremendous amount of weight as an adult, your body will adapt by inciting cells to divide.
Muscle cells also become more efficient through improved signaling and coordination in the body, which happens in response to challenges that force the muscle beyond its “comfort zone.” Muscles hypertrophy (grow bigger) by experiencing stress: only the muscles being taxed have to undergo modification. Muscles atrophy when they’re no longer subjected to the same demands under which they were constructed. This means that muscle building can be location-specific, whereas fat storage and loss are systemic in nature.
While your body will not physically transform fat to muscle, or let muscle become fat, your choices and habits dictate how much fat or muscle you maintain. Remember that your decisions have the most impact on your fitness and health goals: be strategic and live actively!