A food is considered a good source of protein if it contains some or all of the nine essential amino acids that must come from the diet for the body to make the more than 10,000 different types of new proteins necessary for normal growth and maintenance. Skim milk and other animal products are good sources of complete protein, which means they provide all of the essential amino acids. The high-quality proteins found in skim milk are also isolated by food manufacturers for use in other products.
Skim milk is a nutrient-dense food, according to the Kansas State Research and Extension Family Nutrition Program. That means skim milk provides significant amounts of a variety of healthy nutrients for comparatively few calories. When compared with 2 percent milk, which contains 8 grams of protein in 1 cup and provides 120 calories, skim milk is a better source of protein because it provides 8 grams of protein in 1 cup, but only contains 90 calories.
The benefit of drinking skim milk as a source of protein is that it is a low-fat food compared with other animal sources of protein such as meat, whole milk and whole milk dairy products such as most regular varieties of cheese. In addition to protein, skim milk provides ample amounts of calcium, vitamin A and, in the case of fortified skim milk, vitamin D. Calcium and vitamin D are important for healthy bone growth and maintenance.
The protein in skim milk helps build and maintain lean muscle mass when combined with a program of resistance training, according to a Canadian study published in a 2007 edition of the American Journal of Clinical Nutrition. In this study, from McMaster University, researchers found that skim milk protein builds lean muscle mass more quickly than soy protein.
Milk contains many proteins, but the most significant are casein and the whey proteins known as beta-lactoglobulin and alpha-lactalbumin. Casein is easy to digest and, according to the University of Illinois, is considered an important human food because it is such a high-quality protein. Whey proteins, on the other hand, are not as easy to digest.
If not fully digested in the intestines, the proteins in skim milk, especially the whey proteins, can cause an immune system response, or allergic reaction. More common in babies than in adults, a milk protein allergy is not the same thing as lactose intolerance, which is an inability to digest lactose, which is the sugar in milk. For anyone with a milk protein allergy, skim milk is not a good source of protein.
Casein and whey proteins are isolated from milk and used in a variety of commercial food products, such as whipped toppings, nutritional drinks, processed meats, frozen desserts, canned soups, caramel candies and egg replacement products.