Yes, when the body is in a caloric deficit in enters a catabolic state where it will consume existing tissue, i.e. fat and muscle, for energy. For me it was definitely noticeable after just 3 days of not eating. Exercising might reduce the loss of muscle mass to some extent, but most people will still lose some or a lot.
"Our results suggest that an intermittent fasting program in which all calories are consumed in an 8-h window each day, in conjunction with resistance training, could improve some health-related biomarkers, decrease fat mass, and maintain muscle mass in resistance-trained males."
