Just my experience, but about a year and half ago pushing 51, I decided to get "healthy". I've put on nine pounds of muscle and lost 10% body fat with four more pounds of muscle to go and 3% more body fat to lose.

Move weights, eat well (this comes and goes), and cardio (at some level to not take from your muscle gains) has been key to me.

Most important in diet is complete protein, protein with the full amino acids.

I'm no doctor but this is what has worked for me.