Cavemen were generally larger and stronger I believe until grains and processed foods were introduced. You will not lose muscle mass as long as you consume an adequate amount of nutrients. Once your body is adjusted. Try switching to leaner and leaner cuts of meat keeping a healthy amount of protein to encourage use of body fat. Maybe every week cut out around 500 fat calories, just keep that protein up (not a ridiculous amount I mean). You keep that protein up you'll only lose fat. I did this diet a couple years ago for 3 months and got really cut, my bench went up from 195 to about 215 I dropped from 175 to 165 pounds in weight. Then I went to school and said screw it, now I regret that, as I climbed up to 200 pounds and got weaker.
