1. Protein and muscle gain. Morton RW, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med, 2018. 49 studies, 1,863 participants. Benefits plateaued near 1.62 g/kg/day, upper confidence interval ~2.2 g/kg/day.
2. Protein while dieting. Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr, 2014. Suggests 2.3-3.1 g/kg of lean body mass, and weight loss of ~0.5-1% of bodyweight per week to maximise muscle retention.
3. Sleep and body composition. Nedeltcheva AV, Kilkus JM, Imperial J, Schoeller DA, Penev PD. Insufficient sleep undermines dietary efforts to reduce adiposity. Ann Intern Med, 2010;153:435-441. Randomised crossover, 10 adults, 14 days. Equal weight loss, but short sleep cut the proportion lost as fat by 55% and increased fat-free mass loss by 60%.
4. Training volume. Schoenfeld BJ, Ogborn D, Krieger JW. Dose-response relationship between weekly resistance training volume and increases in muscle mass. J Sports Sci, 2017;35(11):1073-82. Graded dose-response, with roughly 10+ weekly sets per muscle group used as a common benchmark.
5. Training frequency. Schoenfeld BJ, Grgic J, Krieger J. How many times per week should a muscle be trained to maximize muscle hypertrophy? J Sports Sci, 2019;37(11):1286-95. 25 studies. No significant difference between higher and lower frequency once weekly volume was equated.
6. Muscle memory. Rahmati M, et al. Myonuclear permanence in skeletal muscle memory: a systematic review and meta-analysis. J Cachexia Sarcopenia Muscle, 2022. Found myonuclei retained in rodents but not in humans, so claims that muscle returns faster after a layoff remain contested.

General guidance, not medical or individual advice. Consult your physician before starting any training or nutrition programme.