“Muscle turns into fat when you stop exercising”

The idea that muscle transforms into fat when you stop exercising is a widespread misconception, but it's important to understand that muscle and fat are entirely different types of tissue within the body. Scientifically, muscle tissue is composed of densely packed protein fibers and has a high water content, making it denser. In contrast (Review), fat tissue, also known as adipose tissue, is primarily made up of stored triglycerides and has a lower water content. These distinct cellular structures mean that one cannot physiologically convert into the other, much like an orange cannot turn into an apple.

This common belief likely originates from the noticeable changes in body composition that occur when a person stops regular physical activity. Without the stimulus of exercise, muscles begin to atrophy, meaning they shrink in size and lose tone. This process can start within a few weeks of inactivity. Simultaneously, if an individual continues to consume the same amount of calories they did when active, or even maintains a surplus, the reduced energy expenditure leads to the body storing these excess calories as fat.

The visual effect of shrinking (Review) muscles and accumulating fat can create the illusion that muscle has somehow "turned into" fat. People often observe their bodies appearing "softer" or "flabbier," which reinforces the myth. Furthermore, muscle tissue is more metabolically active than fat tissue, burning more calories at rest. When muscle mass decreases, the body's resting metabolic rate slows down, making it easier to gain fat if dietary habits are not adjusted to match the lower energy expenditure. Therefore, while the appearance of one tissue replacing the other is compelling, it is simply two separate biological processes occurring concurrently.