The idea that muscle turns into fat if you stop exercising is not true, according to Professor Leigh Breen, a specialist in muscle physiology at the University of Leicester. He notes this is a common misconception, particularly among those who have taken a long break from physical activity.
Professor Breen explains that muscle and fat are distinct tissue types, and there is no known biochemical process in the human body that supports one converting into the other. The confusion likely stems from the changes that occur when exercise, especially resistance training, ceases.
When individuals stop exercising, they may lose muscle mass. If they also maintain their previous calorie intake while being less active, this can lead to an energy surplus and an increase in body fat. This combination of reduced muscle and increased fat can give the impression of conversion, but it is not the case.
Professor Breen suggests that keeping protein intake high and slightly reducing calorie intake during exercise breaks can help manage changes in muscle and fat mass. He adds that small fluctuations in body composition during breaks are generally normal, provided consistent exercise and dietary habits are maintained over time.