In the world of protein supplements, whey has been the long-reigning king. The milk-based protein is a fan favorite because it contains all nine essential amino acids the body needs to maintain and build muscle—and it’s pretty easy for the body to digest. But there’s another type of milk-based protein fitness fanatics should have on their radar: casein.
You know the nursery rhyme about curds and whey? Well, casein is the ‘curds.’ “When you expose cow’s milk to heat or acid, it separates into its two major proteins: whey and casein,” says Jim White, R.D., owner of Jim White Fitness and Nutrition Studios. That casein actually makes up about 80 percent of the protein in milk and other dairy products.
Like whey, casein is a ‘complete protein,’ meaning it contains all nine of the essential amino acids our body needs and can only get from food. The major difference between casein and whey is that whey digests quickly, while casein breaks down more slowly, says White.
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Plus, casein has a higher concentration of an amino acid called glutamine. Glutamine is essential for preventing over-training and muscle breakdown, especially when you’re sick or under stress, explains White. (One scoop of your average casein protein contains about half White’s recommended five grams of glutamine per day.)
Another major perk of casein: It may keep you full for longer than whey. One study published in Advances in Nutrition, for example, found casein to be more satiating than whey throughout a period of about six hours.
Because of casein’s slow digestion and absorption into your system, and its high levels of glutamine, it’s the perfect protein source to consume before bed. “Casein is great at night because it helps prevent protein and muscle breakdown,” says White. So while you may want whey’s quick delivery of amino acids before or after a workout, casein’s slow and steady release can help to sustain that hard-earned muscle throughout hours of fasting while you sleep.
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Given that, casein can be a great protein for those looking to lose weight without sacrificing muscle mass. Overweight police officers who supplemented with casein throughout a 12-week diet and strength-training program lost more body fat and saw greater strength gains than those who supplemented with whey, according to a study published in the Annals of Nutrition and Metabolism.
You may also see casein as one of a few types of protein in protein blend supplements. With a blend, you reap the benefits of both whey and casein, says White. While casein is higher in glutamine, whey is higher in another amino, called leucine, which is key for stimulating protein synthesis and muscle growth. These casein-containing supps can be a good, satiating option for post-workout, he says.
Related: Find the perfect protein supplement for your needs.