Muscle does not convert itself into fat when people step away from the gym, according to Prof Leigh Breen, a specialist in muscle physiology at the University of Leicester.
“This is not true,” Breen said. He added that the claim is one he hears frequently, particularly from people who have taken a long break from exercise, who often describe their muscles as having “softened” or as no longer feeling toned.
Breen said no biochemistry connects the two tissue types, which are distinct. “There’s no known process in the human body that would support muscle turning into fat, or vice versa,” he said.
He attributes the confusion to what happens once training stops, especially resistance training. Some of the muscle built through exercise may be lost, and if a person keeps eating the same amount while moving less, an energy surplus can develop and body fat can rise.
“This combination of a reduction in muscle mass and potentially an increase in fat mass can give the impression that one is turning into the other,” Breen said. “Which isn’t the case.”
The size of the shift depends on the length and nature of the break, he said. A couple of weeks away from a usual routine while staying relatively active is likely to bring fewer changes than being completely immobilised for an extended period because of an injury.
Diet during a break also influences how much muscle and fat mass is gained or lost. Breen recommends keeping protein intake high while slightly reducing calorie intake.
Bodies are constantly changing, he said, and small fluctuations in muscle and fat mass during breaks in training are generally normal. “Slight fluctuations are generally nothing to worry about, as long as over the course of months and years consistent exercise and dietary habits have been maintained,” Breen said.
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