Casein protein is the primary protein found in milk, and is essential for human growth and development. It contains all of the essential amino acids your body needs and cannot synthesize on its own. According to the Protein Digestibility-Corrected Amino Acid Score system created by the World Health Organization, casein is one of the best proteins you can consume in terms of digestibility and nutritional value.
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The micellar structure of casein protein is different from many other proteins in that when you disrupt it from its suspension in milk, it will clot together. This process occurs in cheese-making and in your intestines after you consume milk. The clotting of casein protein enables it to break down slowly in your body, releasing amino acids continuously into your bloodstream over several hours. Because of slow breakdown, your body retains a high amount of the nitrogen from the casein.
Muscle Protein Synthesis
Casein protein is popular as a supplement with athletes and bodybuilders because it is effective at increasing muscle growth when you use it in conjunction with a resistance training program. According to Jay Hoffman, Ph.D., consuming casein protein has been shown to increase your muscle protein synthesis rate by 31 percent after ingestion, and these increases may last up to seven hours or more. This will help your muscles grow at a faster rate than if you engaged in weight training without additional protein supplementation.
One potentially harmful effect of casein protein is kidney damage. Casein is generally safe to consume if you have normally functioning kidneys, even at levels as high as 2.8 grams per kilogram of bodyweight according to some research. If you already have existing kidney damage, however, the additional renal load from casein’s nitrogen byproducts can exacerbate your condition. A 2004 study examining the effects of casein protein on rats with kidney disease found that a diet of just 20 percent casein protein caused the rats to develop renal insufficiency, while a 20 percent soy protein diet actually improved kidney function.
For some individuals, the casein protein in milk can trigger allergies. This negative side effect crops up when your body identifies casein protein as harmful and releases histamine to neutralize it. The Allergy Society of South Africa notes that both types of protein found in milk -- whey and casein -- can trigger an allergic reaction. Speak to your doctor if you notice allergic symptoms like hives, runny nose, itchy eyes or rashes after consuming casein protein.