“Conquer your cravings,” says the box of Fiber One Protein Caramel Nut Chewy Bars. “Go strong,” says the bag of Special K Protein Bites. Getting enough protein matters. Eat too little and you’ll lose muscle. But does extra protein help you eat less, stay trim, and build muscle? Or are companies just using protein to sell cookies, bars, shakes, and other junk foods?