Metabolite directly involved in the normal life processes of each cell.
Examples: Alcohols, amino acids, monosaccharides, nucleotides, organic acids.
Source:
PAC, 2018, 90, 1121. 'Terminology of bioanalytical methods (IUPAC Recommendations 2018)' on page 1168 (https://doi.org/10.1515/pac-2016-1120)