In short, no. Muscle tissue is completely different from fat tissue and therefore it is impossible to convert one into the other.
It is a very common misconception within the fitness industry that muscle can be turned into fat, or conversely, fat can be turned into muscle. You may have heard someone say that they are toning up by "turning their fat into muscle". Alternatively, you may have heard comments that when an athlete ceases to train, their muscle turns into fat. Both of these scenario's are completely inaccurate.
Let's take a brief look at fat and skeletal muscle tissues:
Fat tissue, or adipose tissue, is primarily a storage tissue in adults. It's there as a survival mechanism for us just in case we are unable to eat. Glucose sugars are taken up from the blood stream and converted into triglycerides that are stored within the fat cell.
Muscle tissue is primarily required to generate force in one way or another. This may be to generate a heart beat, breath out, or to lift something. Skeletal muscle tissue is comprised of a series of fibres that run from one point to another and is connected to bones via connective tissues.
Of course, both forms of tissue have many other functions within the human body, but this is just a brief overview.
As you can see, fat and muscle have completely different functions. Not surprisingly, they also have completely different structures under a microscope. In fact, they almost look as similar as a wooden chair and a porcupine!
So with all this said and done, how on Earth could the misconception that "muscle turns into fat", or "fat turns into muscle" arise?
Well, let's take the two scenario's above:
Someone loses weight and tones up. Through exercise and nutrition, this person has managed to re-shape their body. But instead of turning their fat into muscle, what they managed to do is reduce the amount of fat they carry and increase the amount of muscle in their body. Thus, you can see how easy it is to misconceive this as "turning fat into muscle", when they have in fact lost fat and gained muscle.
An ex-athlete who used to be muscular, but gains weight after they cease training. This is a very common situation and can often turn people off the thought of weight training altogether. What often happens here is that the athlete is used to exercising for a number of hours each day. As a result, they expend a lot of calories and thus have to eat a lot of calories in order to fuel their bodies. As soon as they stop training, the amount of activity that they perform each day drops significantly. Therefore, they expend far less calories. But, if they do not modify your diet to accommodate for the lesser caloric expenditure, they will gain a substantial amount of fat tissue. With the decreased training frequency, their muscles will deteriorate from lack of use. Ultimately, they lose muscle mass and gain fat mass because of the changes in their lifestyle. Contrary to popular belief, muscle is not converted into fat.
Hopefully this clears up this very common misconception. If you are interested in learning more about weight loss, weight gain, nutrition and exercise, I highly recommend that you read our course entitled "Introduction to Physical Freedom".