I have tried eating only organs in the past, and it seemed to work, at least temporarily. I don't know of anything that's present exclusively in muscle meat that you would need, but it is always best to try and eat as much of the animal as possible (various muscles, organs, fats, blood etc...). You can experiment with it and see how your body reacts. If problems do occur, you can always add some muscle meat as well. Heart is technically a muscle, although I can imagine there being certain benefits to different cuts of muscle as well.