Describes protein that the body cannot use efficiently because its amino acid pattern poorly matches human needs or because it is poorly digested. Proteins of low biological value, such as those in most cereals eaten alone, are limited by shortfalls in essential amino acids like lysine, or by anti-nutritional factors and tough structures that resist digestion. The concept is measured by how much absorbed nitrogen the body retains for growth and maintenance. Combining complementary proteins, as in beans with rice, raises the overall biological value of a meal. Food formulators and dietitians use the measure to assess protein quality in diets and products.