Nucleotides are the building blocks for DNA and RNA, and carriers of metabolic energy (i.e. ATP). Although cells can synthesize nucleotides themselves, nucleotides are considered semi-essential nutrients as under certain conditions the own synthesis capacity of animals is too low. For instance, immune cells and cells of the gastrointestinal tract have a relatively low nucleotide synthesis capacity. Also in rapidly dividing cells, like in young animals, the synthesis capacity is too low. Moreover, exogenously supplied nucleotides reduce the energy requirements of cells and consequently animals can better withstand stress and show an enhanced resistance against pathogens.