Net protein utilisation (NPU) is a classic measure of protein quality that combines two factors: how completely a protein is digested and how well its absorbed amino acids match the body’s needs. It is calculated as the product of biological value and digestibility, and expressed as the percentage of ingested nitrogen that the body retains. A high NPU, as seen with eggs and milk, means little of the protein is wasted as urea or excreted. The measure helped establish the hierarchy of protein sources used in mid-twentieth-century nutrition science and in formulating feeds and therapeutic foods. Although superseded for regulatory purposes by PDCAAS and DIAAS, NPU still appears in textbooks and older research as a compact summary of protein quality.