Smoothies are great, that's no secret. But when they pack protein, they're nothing short of magical. Why? Protein does it all: it keeps cravings at bay, boosts energy, revs your metabolism, and builds muscle. So why would you ever sip a smoothie sans protein? Here, Keri Gans, R.D., author of The Small Change Diet, shares her favorite smoothie mix-ins for pumping up the protein.