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 | British Journal of Sports Medicinebjsm.bmj.com
"Protein intakes at amounts greater than ~1.6 g/kg/day do not further contribute RET-induced gains in FFM."
In a personal communication over Facebook, Eric Helms (one of the authors) added:
"We recommend 2.2 in the paper because it's the upper end of the confidence limit not just 1.6, I'd advise reading the full paper I'm sure you'll enjoy."
Confused about confidence intervals (CIs)? Peter Bond (another researcher added):
"1.62 g/kg bw/day was the mean value, the 95% confidence interval for that value stretches all the way up to 2.2 g/kg bw/day."
Still confused? A 95% CI means that you have 95% chance that the real population value is between the lower bound of the CI (not provided) and as high as 2.2 g / kg.
In short, if you want to play safe, intakes of 2.2 g/kg (1 g / lb) during muscle gain are still the way to go. Note that higher intakes are recommended during weight loss.