Lentils should be your secret mass-building Opens a New Window. weapon. One cup of cooked lentils contains 18 grams of protein and 40 grams of slow-digesting quality carbohydrates. They are also very inexpensive and have a long shelf life. They cook up in just 10 minutes and can be added mixed in with brown rice, sprinkled over a salad, or eaten as a standalone side dish.