Muscle loss is starting to be an issue for me too. However I am certain I get more than enough protein. The issue for me I think is that as my life is slowing down, I simply don't demand as much as often of my muscles than when I was younger and working daily.
Cycling, IMO, only strengthens one muscle and that is your heart. So to slow or stop my muscle loss, I think a regular visit to the gym will be necessary so I can work out a wide range of muscle groups.