A protein intake of 1.5 grams per kilogram of body weight during severe caloric restriction preserves lean body mass.

Study: Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial.

When people eat less food but lots of protein and exercise hard, they keep more muscle. This study showed that eating more protein (2.4g per kg) helped people gain muscle even while dieting, compared to eating less protein (1.2g per kg). So eating 1.5g per kg — in between — likely helps too.

Study: Higher protein intake during caloric restriction improves diet quality and attenuates loss of lean body mass

When people ate more protein while dieting, they lost less muscle than those who ate less protein. This suggests that eating more protein helps keep your muscles during weight loss, even if the study didn’t test exactly 1.5 grams per kilogram.

Study: Short-term intermittent fasting and energy restriction do not impair rates of muscle protein synthesis: A randomised, controlled dietary intervention.

When people ate fewer calories but got enough protein, their muscles didn't break down any more than when they ate normally. This suggests that eating 1.5 grams of protein per kilogram of body weight while dieting likely helps keep muscle mass intact.