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.
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.