The U.S. Food and Drug Administration supports the claim that 25 grams or more of soy protein daily reduces the risk of heart disease by lowering total blood cholesterol levels, including LDL, the "bad" cholesterol. Soy protein digests quickly, whereas casein, a dairy protein, digests slowly, providing a steady supply of amino acids to your muscles, which may lead to greater muscle development than fast-digesting proteins, according to the National Dairy Council. Although both soy and casein have beneficial properties, they are derived from very different sources.

Soybeans are legumes related to peas. Soy milk is made by heating ground soybeans that were first soaked in water. Casein, on the other hand, is a protein found in mammalian milk. Soy milk does not contain casein. To obtain slow-digesting casein, drink milk or consume other dairy products, such as yogurt and cheese.