Fitness writer Hugo Rivera explains at About.com that the biological value of a protein refers to the value that measures how well the body can absorb and utilize a protein, and so a high biological value protein absorbs, uses and retains more nitrogen. This means that proteins with a higher biological value promote the most lean muscle gains.
The European Food Information Council also explains that a high biological value in a protein means that it contains the essential amino acids in a proportion similar to that required by humans. The council states that some proteins have scarce essential amino acids and those that come in the lowest supply become limiting amino acids. However, when two foods providing vegetable protein combine in one meal, the amino acids of one protein may compensate for the limitations of the other, which results in a combination of a higher biological value.
The council also explains that animal sources of protein such as meat, poultry, fish, eggs, milk, cheese and yogurt provide the highest biological values. Plants, legumes, grains, nuts, seeds and vegetables provide the lowest values. However, Rivera states that whey protein has the highest biologoical value. According to him, egg and milk proteins have the second and third highest values, respectively.