The belief that muscle turns into fat when a person stops exercising is a common misconception, according to experts in muscle physiology. Professor Leigh Breen of the University of Leicester, who specializes in the field, explains that muscle and fat are distinct tissue types and there is no biological process through which one transforms into the other.

This misunderstanding often arises when individuals take a break from resistance training or reduce their activity levels. During such periods, muscle mass can decrease due to lack of use, and if dietary intake remains unchanged or excessive, fat accumulation may increase. The simultaneous reduction in muscle and rise in fat can create the impression that muscle has converted to fat, but these changes occur independently.

Professor Breen emphasizes that the human body does not support muscle-to-fat conversion or vice versa. Instead, changes in muscle and fat are linked to activity levels and nutritional habits. For example, if someone stops exercising but maintains their usual calorie intake without adjusting for reduced energy expenditure, it may lead to a net energy surplus that promotes fat gain.

The extent of these changes varies depending on the level of activity during a break. Short breaks from exercise—lasting a couple of weeks—while remaining somewhat active, generally result in minor alterations. However, periods of complete inactivity, such as immobilization following injury, tend to cause more pronounced muscle loss.

Diet also plays a significant role in managing body composition during exercise breaks. Maintaining a higher protein intake while reducing overall calories can help mitigate muscle loss and limit fat gain, according to Breen.

Overall, fluctuations in muscle and fat mass during temporary breaks from training are normal and typically not cause for concern. Sustained patterns of regular exercise and balanced nutrition over months and years are more important for long-term body composition and health outcomes.