Foods that are rich in nitrogen include meat, fish, potato, milk, eggs, cereals and legumes. Most of the nitrogen in foods are found in proteins as amino acids, although nitrogen can also be present in various forms such as free amino acids, creatine, choline, creatinine, purines, pyrimidines and amino sugars. These trace compounds are called non-protein nitrogen, or NPN.
The nitrogen in meat comprises free amino acids and peptides, while the nitrogen found in fish is in the form of methyl-amino substances. Urea nitrogen makes up half of the nitrogen composition in human milk, and 50 percent of the nitrogen content in the potato is NPN.