Nothing beats pure protein when it comes to building muscle. The problem with most store-bought beef, however, is that the majority of cattle are fed grain, which gives their meat a relatively high ratio of omega-6 to omega-3 fatty acids. This, in turn, contributes to a host of problems. The fatty acids in grass-fed beef, on the other hand, are skewed toward the omega-3 variety. Such beef also contains conjugated linoleic acid (CLA), which studies have shown helps reduce belly fat and build lean muscle.