If protein is a building block for muscle, then you will never gain fat from overeating protein, right? Sorry, Timmy, but the weight can definitely creep on if your caloric equation shifts toward a surplus. While it is harder for protein to convert into fat than its other macronutrient buddies, if you eat way more than your body needs—no matter what it is—the excess could go into fat deposits. Plus, as your protein intake goes up, protein oxidation increases, which means you’ll burn fewer carbs or fats for fuel.
Beyond that, protein quality is measured in a variety of ways, including biological value, net protein utilization, and the Protein Digestibility Corrected Amino Acids Score (PDCAAS). The PDCAAS test is the FDA’s preferred method of determining protein quality. A PDCAAS value of 1 is the best possible score. Casein, whey, egg white, and soy protein all fall into this category, so they’re all stellar protein sources.