This is simply not true and as long as you perform of heavy resistance training 2 - 3 times per week, then you shouldn't fear about losing any muscle mass. (fitmole.org)
If you have trouble gaining muscle mass, consider adding extra calories to your diet while consuming larger amounts of more good foods, and continue with heavy resistance in your training. (muscleandfitnesstips.org)
The time course for elevated muscle protein synthesis following heavy resistance exercise. (completehumanperformance.com)