Differential absorption and metabolism of hydroxytyrosol and its precursors oleuropein and secoiridoids ; This study investigated and compared the absorption, metabolism, and subsequently, the tissue distribution and excretion of hydroxytyrosol (HT) administered either in its free form or through its naturally occurring esterified precursors, namely oleuropein (OLE) and its aglycone forms known as secoiridoids (SEC). Here, rats were fed a diet supplemented with the equivalent of 5 mg phenol/kg/day for 21 days and the HT metabolites in the gastrointestinal digesta (stomach, small intestine and caecum), plasma, urine and metabolic tissues (liver and kidney) were analysed. Compared to HT and SEC, OLE showed greater stability during digestion, and, consequently, the bioavailability based on the urine excretion of HT metabolites was higher. OLE, as a glycoside molecule, reached the colon unaltered generating more diverse microbial metabolites. In terms of bioavailability, findings suggest that OLE might be the most suitable precursor of HT for incorporation into foods or nutraceutical formulations.