Exercise

If you stop exercising, your muscles will turn to fat

Aaron recently went on an exercise and diet program called P90X. With great zeal, he followed this workout routine and diet for ninety intense days, during which he sculpted his pale, soft body into a much more buff version. And it worked! At the pool, friends admired his newly defined muscles. We have been debunking a lot of health myths, but there are some good health truths to remember. Among the more important is that exercising and building up your muscles are great ways to help your body stay healthy.

Now that Aaron has his bulging P90X muscles, what will happen to them when he stops his crazy workout routine (which he did while writing this book)? Will his muscles soften into pudgy fat? Aaron is great at following diet and exercise plans, but even he might have difficulty finding a way to maintain these muscles.

Aaron can take comfort in one thing, though—it is a myth that muscle will turn into fat when you stop exercising. It is just not true. Fat cells and muscle cells are different things, and one cannot convert into the other. Cells are the smallest functional units in your body, the building blocks of how you are put together as a living creature. You have fat cells, muscle cells, blood cells, bone cells, and so on. These cells do not convert from one kind into another. Muscle cells and fat cells look very different and work in different ways. Muscle cells are mostly a bundle of fibers or filaments that are attached to each other and contract when electricity from the body’s nerves come into the cell. Muscle cells are like tiny ropes, powered by small engines and connected together to do the work of pulling your bones around. In contrast, fat cells do not seem like they do very much. Under a microscope, they look like motionless globs. These cells are focused on storing fat and making fat from things like sugar. Fat cells do have jobs; they provide your body with emergency food, they insulate your body, and they help facilitate hormonal activity in your body.

When Aaron stops his P90X program, his muscle cells will not go away. If he stops doing his pull-ups and starts eating food that has a bit more fat in it, he will not have any fewer muscle cells; however, Aaron’s muscle cells will get smaller and thinner. He will not develop any more fat cells, but the fat cells that his body has will get bigger and bigger as they store more fat inside them and use less. The muscle and fat cells will not change in number, and the muscle cells will not become fat cells. Instead, the muscle cells will be getting smaller while the fat cells are getting bigger. As the fat cells get bigger, it might look like Aaron’s muscles are turning into fat.

There are a few other dimensions to this idea of muscle turning into fat. If a person is really desperate for energy, such as a person who is starving and cannot get enough food, the body can break down some of your muscle fibers to use them for energy. If the muscle fibers are used for energy (by breaking them down through a process called catabolism into the body’s sugar source of glucose), and if this happens to such an extent that there is extra sugar left over that the body does not need, then the body will store that sugar as fat. This is pretty rare, as it requires you to need to use parts of your body as an energy source in the place of having food, and yet you need to do this to an extent where you have some leftover to store as fat. The scenario where you let your muscles get small and wasted (from not exercising them) while your fat cells store up more fat (because you are eating more calories than you burn up) is much more common. Interestingly enough, the number of fat cells in your body remains almost constant throughout your life. Some of the fat cells die, and others replace them, but you do not grow brand-new fat cells when you get fat. Instead, your fat cells get bigger and bigger as they store more and more fat.