Answer:
Pigs need protein to grow and most importantly, to develop muscle tissue (muscles contain chiefly protein and water). Protein is made up of amino acids, the 'building blocks' of protein, linked together in chains. Amino acids contain nitrogen and this is what distinguishes them from other food groups such as fat and carbohydrates.
Source: https://www.daf.qld.gov.au/business-priorities/agriculture/animals/pigs/feed-nutrition/nutrients-diets/basics
