Proteins, despite being grouped within a single classification, are some of the most versatile molecules on Earth. The building blocks of proteins are called amino acids. Their arrangement determines the composition and attributes of proteins. Twenty amino acids exist in nature, and most proteins contain tens or hundreds of amino acids, allowing for a staggering number of possible combinations. This versatility means that dairy and red meat proteins can differ significantly in their quality.
Comparison of Proteins
Both red meat and dairy contain all 20 amino acids necessary for the proper health of the human body. In this important respect, they are similar to other animal products such as eggs and poultry. However, they differ in more subtle ways. The problem with judging proteins separately is that many factors can determine protein quality, including the quantity of amino acids and the digestibility of the protein, which makes a direct comparison difficult. Dairy appears to be slightly inferior in digestibility but superior in overall amino acid value.
Amino Acid Score
According to the Food and Agricultural Organization of the United Nations and the World Health Organization, the protein digestibility-corrected amino acid score, or PDCAAS, is the best method for evaluating protein quality based on the amino acid requirements of humans and the ability to digest protein. The PDCAAS is 121 for cow's milk but only 92 for beef, a common red meat. However, some measurements truncate the PDCAAS at 100 percent, arguing that any protein beyond this threshold also exceeds the typical amino acid requirements for humans. Up until around 1993, the protein efficiency ratio, or PER, which measures the ability of protein to support growth, was a popular method of determining protein quality. Cow's milk has a PER of 3.1 vs. 2.9 for beef. However, PER has two large deficiencies: first, it's a measurement of growth in rats and not humans, and second, it is less applicable to adults.
Yet another measurement of protein quality is the biological value, which reflects the proportion of amino acids that the human body incorporates into proteins within the cells. Scientists can measure the biological value by looking at the amount of nitrogen — a distinguishing element within amino acids — that the human body retains. Milk has a biological value of 91 vs. only 80 in beef.
Milk contains two separate proteins, casein and whey. Whey alone tends to be superior to casein in most measurements of protein quality. In fact, whey is one of the best individual sources of protein. Casein, by itself, is better than red meat in some aspects, such as protein digestibility, but worse in others, such as PER. When judging dairy products, it's important to remember that cheese only contains casein. It is also possible to buy whey protein separately as a dietary supplement.