Muscle loss is the breakdown of muscle tissue, which in bodybuilding usually shows up as an unwanted side effect of dieting. In a deficit your body does not only lose fat, and muscle that is not being used is readily broken down.
What raises the risk
Three factors matter most. First, the rate: for strength athletes, 0.5 to 1 percent of body weight per week is considered a good range (Helms et al. 2014), and above that muscle loss becomes more likely. Second, your starting point: the leaner you are, the higher the risk. Third, the size of the deficit: according to a meta-analysis by Murphy and Koehler 2022, a deficit of about 500 kcal a day or more prevented gains in lean mass.
What helps
Heavy training is the signal to keep muscle. Keep your loads and effort close to failure, and you can trim volume a little. Protein is the second lever: the International Society of Sports Nutrition cites 2.3 to 3.1 g per kg of body weight as an amount that may be needed for resistance-trained people in a deficit. In a trial by Longland et al. 2016, the higher-protein group did clearly better for both lean mass and fat loss.
How to spot it
The scale alone cannot tell fat from muscle. Signs pointing toward muscle loss are a trend dropping much faster than planned, a waist that barely shrinks even though your weight falls, and strength that declines clearly over several weeks. No single sign proves anything. Water and glycogen often make muscles look flatter in a deficit without any tissue being lost.