To be more specific, it is a loss of skeletal muscle mass in combination with fat mass gain happening at approximately the same rate and the same time. (fitnessandpower.com)
The smaller and lighter you get, especially if there's a large drop in skeletal muscle mass, the fewer calories you need. (fitwatch.com)
The present study shows that 3 mo of resistance - type exercise training increases skeletal muscle mass, strength, and muscle fiber size in young men. (academic.oup.com)