The scientists investigated the effect of either a high carbohydrate (CHO; 65% carbohydrates, 15% protein and 20% fats), a "high protein" (PRO; 40% carbohydrates, 30% protein and 30%) and a non-restricted (Control) diet on body composition and weight of 30 active, military males (age=25 ± 4 yr, body fat=15 ± 7%):
Control and PRO group increased FFM, 1.7±1.2 kg and 0.8±1.5 kg, respectively. CHO group lost -0.2±3.8 kg FFM. PRO and CHO groups lost 1.0±1.0 kg and 1.0±1.8 kg of FM, respectively. Control group lost 0.7±0.7 kg FM.Although, judged by a bodybuilders standard, for example, the "PRO"-Diet was not really "high protein" in the sense that it still had 40% carbs and thus more carbs than protein, it was exceptionally effective not only to reduce muscle loss, but also to induce muscle growth (i.e. an increase in fat free mass = FFM). So, here you have another very good reason to mirror your carb-/protein intake carefully and to supplement additional protein if necessary.