Ask any bodybuilder what the most important macronutrient for building muscle is, and the easy answer is protein. But dig a little deeper, and you'll find that protein isn't just protein. That macronutrient can come from milk, eggs, beef, chicken, fish, elk, moose, deer, bison, and even plant-based sources. As for the milk derived proteins, the nursery rhyme Little Miss Muffet is rather important:
Little Miss Muffet
Sat on her tuffet
Eating her curds and whey
There came a big spider
Who sat down beside her
And frightened Miss Muffet away.
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Putting aside the question, "What is tuffet?" and ignoring Miss Muffet's arachnophobic reaction, this little rhyme is muscle building gold: Miss Muffet was eating her curds and whey--milk proteins. The curds--like cottage cheese--were rich in casein; and the whey was, well, whey. So what are these milk proteins and how can they help us conquer our own dark mountain and achieve new heights in our physiques and in physical performance?
Whey is the fluid that is left after milk has been processed into cheese or yogurt. It is a fast absorbing protein in our gut; meaning, that upon ingestion, it is quickly broken down into amino acids--the building blocks of all proteins--that are swiftly absorbed into the bloodstream and delivered to the cells that will make use of those amino acids. Casein, on the other hand, is the protein that makes up cheese and yogurt. It's much thicker than whey and takes time to digest. As a result, it is not quickly absorbed upon ingestion.
Why would we care whether a protein is fast or slow acting? Isn't protein, protein? While on the surface, it might be true that protein is protein--a long chain of individual amino acids--we can get different benefits based on whether the protein is fast or slow acting. When we work out, really tax the muscles to stimulate new growth, we damage the muscle cells. At that point, the muscle filaments are primed to start repair work--building thicker and stronger muscle strands. But the repairs can only be done if the necessary materials are available for the repair. Just as you can't build a house without bricks; so too you can't repair and build muscle without amino acids. They are the bricks. So right after working out, we want to supply the damaged muscle tissue with the amino acids necessary for them to start the repair work.
Enter whey. As a fast digesting and absorbing protein, whey supplies our body with the amino acids that will aid in repairing and building the muscle tissue. Add some simple sugars to the mix and we create an anabolic environment that is perfect for muscle repair and growth. (But that addition of simple sugars to maximize the effect of whey supplementation after working out is a topic for another time.) Therefore, supplementing with whey protein right after a workout is an important step in helping us build and strengthen the muscle.
"Alright, that makes sense," I hear you say. "But then why would you ever need or want a slow digesting protein?" I'm glad you asked. The process of muscle repair and building is not confined to the hours right after working out. The body is constantly going through repair processes and is really either in a state of anabolism (building up) or catabolism (breaking down). While our bodies start the repair work right after we workout, the best time for our bodies to really repair us, is at night when we are asleep. For several hours we lie in our beds dreaming about our next work out, that next PR, the next hunt, or the next hike. And during that sleeping time, our body enters a fasted state with no food going into the system. Without the bricks to repair the muscle, the body can't keep building and repairing.
Enter casein. As a slow digesting protein, casein is a great protein supplement right before bed. Its slow digestion and its slow, but steady, release of amino acids into the bloodstream helps to keep the bricks moving to the job site--the muscle repair site. Casein thus helps us to stay anabolic through most of the night, providing the amino acids for the muscle repairs that will happen while we sleep.
So there you have it. The benefits of both whey and casein--the two milk-based proteins. Whey is fast-acting and helps kickstart and fuel the muscle repair right after a grueling workout. Casein is slow-acting, keeping the muscle building and repair going through the night while we sleep and rest up for the upcoming workout when we kick our butts again the next day. Like little Miss Muffet, we need to be eating our curds and whey, if we're serious about defeating our own dark mountain.