It is possible to gain lean muscle mass while in a significant energy deficit (up to 40% below requirements), provided that a high protein intake is combined with intense training.

noli's read

This advice is based on a randomized controlled trial (RCT) published in the American Journal of Clinical Nutrition in 2016 by Longland et al., conducted under the supervision of Stuart Phillips. The researchers demonstrated that in overweight young men following a 40% energy deficit, a high protein intake (2.4 g/kg/day) combined with resistance and high-intensity interval training effectively allowed for a gain in lean mass, unlike a group consuming less protein. This is not a generalization that is easy to achieve: the participants followed a very demanding supervised training program (6 days out of 7). Although this result is documented, it is important to note that gaining muscle mass in a caloric deficit is not a universal phenomenon and depends heavily on training volume, initial fitness level, and protein intake. Meta-analyses further suggest that a prolonged energy deficit can hinder long-term muscle mass gains, even if strength may continue to progress.

the sources

1. 1Morton RW, Murphy KT, McKellar SR, et al. 2018
2. 2Leidy HJ, Clifton PM, Astrup A, et al. 2015