A novel method has been developed to determine protein requirements, which is called Indicator amino acid oxidation (IAAO). This technique has been validated by comparison with the “gold standard” nitrogen balance”. Using IAAO we have shown that minimum protein requirements have been under estimated by 30-50%. The National Academy of Sciences have for macro-nutrients proposed “Acceptable Macronutrient Distribution Ranges”, which for protein is 10 to 35% of total energy. In practice, we suggest 1.5-2.2 g/kg/d of a variety of high-quality proteins.