If you wanna retain muscle mass, workout. There's no all-quick fix. You might want to look into creatine, which isn't a pre/postloading thing, but to be taken daily.
Andro is another muscle builder, but even I don't trust it, and I take every pill under the sun. Andro is converted into testosterone a short time after taken. I tried it once before lifting weights, I almost threw up at how hard I pushed myself. You can put up a couple more pounds, its some wicked stuff. Taking it regularly will keep your muscle mass up, and beyond, but you might stop producing testoterone naturally, and develop "bitch tits." Andro is one step away from steroids. Its even illegal in some countries. I don't recommend it.
I try to get a lot of protein into my diet, with shakes, or turkey, whatever. This helps develop muscle with a steady workout. In the past 3 months, I've lost about 30 pounds, but I can lift more with every muscle (of course I changed my diet, started a heavy workout schedule, take loads of supplements, and roll almost every weekend, so who knows what combo caused me to lose all that weight). So don't worry about weight loss, it might just be fat. If it is muscle, you should be eating better. Good luck!