Protein comprises 3.3 percent of milk content, according to Cornell University. Milk contains two main types of protein: casein and whey. Casein is the more abundant form, comprising approximately 82 percent of the protein content, the Cornell website states. Milk protein isolate is a filtered product designed to concentrate the protein in milk. Bodybuilders and athletes may use either milk protein isolate or casein as an over-the-counter dietary supplement, mainly in powder form.
Casein, which forms the curd when manufacturers separate milk, contains nearly pure protein in the form of casein, while whole milk contains fats and sugars in addition to casein and whey. Milk protein isolate, like whole milk, contains both casein and whey. Milk protein isolate has a protein content of 90 percent or more, as opposed to casein, which is 100 percent protein. Because milk contains more casein than whey, milk protein isolate will also contain a higher percentage of casein than whey. Casein digests more slowly than whey, so it will stay in your stomach longer. Whey digests more quickly but doesn't digest as completely as casein.
As many as 30 million Americans have some degree of lactose intolerance, according to PubMed Health. Casein contains just 0.1 percent lactose, C. R. Southward of the New Zealand Dairy Research Institute reports. Milk protein isolate still contains between 1 to 2 percent lactose, according to the Idaho Milk Products website. If you have lactose intolerance, casein will not cause a reaction, while milk protein isolate might, even though the amount of lactose it contains is lower than the amount found in milk.
Milk allergy differs from lactose intolerance. Proteins in milk cause allergy symptoms. You can have an allergy to either casein or whey; if you're allergic to whey, casein supplements can boost your protein intake without the risk of an allergic reaction. Whey supplements would benefit you if you have a casein allergy. Since milk protein isolate contains both types of protein, it would not be a useful source of additional protein if you have an allergy to either protein.
Bodybuilders and athletes take both casein and milk protein isolate supplements, as well as pure whey protein supplements. Whey costs less and also digests more quickly, making it more useful as a quick source of pre or post-workout protein. Take supplemental casein at bedtime or during the day, but not for rapid amino acid absorption. Whey will absorb within around 45 minutes, while casein will take up to seven hours to absorb, which helps with tissue building overnight, the Muscle and Strength website explains.