**8. Bioavailability of Oleuropein**

The beneficial e ffects of olive leaves or di fferent preparations (e.g., infusions, extracts) have been known since ancient times, and have been used as traditional herbal remedies for the treatment of many diseases (such as diabetes mellitus, artherial hypertension, and bronchial asthma), or to alleviate their symptoms. Olive leaves can be a good source for the development of new potentially functional foods that may contribute, through basic nutrition, to optimal health conditions reducing the risk of NCDs. Therefore, special attention is paid to the recovery, recycling, and upgrading of food waste and by-products [166–169]. Ole is the major constituent of the secoiridoid family in olive leaves. HT is the primary metabolite of Ole/OleA, and shares some of the above described biological properties, but possesses a greater antioxidant capacity. Although this feature increases the importance of HT, obtaining it in a synthetic form is expensive, so methods using Ole as a source of HT production, have been developed [170–172].

The possibility that these biophenols may exert their biological e ffects depends on the probability of reaching key molecular targets in human tissues at a su fficient dose, which is dependant to their metabolism and bioavailability. However, the data on the metabolism of oleuropein from EVOO or olive leaves in humans are poor, and often the results on the level and the form found in plasma, and/or excreted in urine, are conflicting [173–175]. This discrepancy may be explained by the fact that oleuropein bioavailability is influenced by several factors, such as the route of administration, genotype, age, sex, interaction with food, and by the di fferent extraction processes and analytical methods used [176]. A recent human trial showed that oral Ole ingestion is resistant to the acidic conditions of the stomach, and it is rapidly absorbed (55–60%) in the intestinal tract, reaching a maximum plasma concentration (23–30 min, depending on the preparation, liquid vs. capsule) earlier than conjugated

metabolites of HT, glucuronidated and sulfated (at 64–93 min), that made up 96–99% of the Ole phenolic metabolites detected in plasma and urine after intake [177]. These data sugges<sup>t</sup> a potential complete metabolization of Ole to HT, and other degradation products. The efficacy in vivo of these compounds regarding their absorption and metabolism kinetics once ingested, should be checked. In fact, the major criticism of the in vitro studies using these molecules is that the doses used are at greater concentrations (μmol/L–mmol/L) compared to the metabolite concentrations measured in plasma, which are only at the nmol/L concentration [178]. Therefore, delivery systems have been developed based on the esterification/lipophilisation and encapsulation of phenolic compounds, or using the creation of liposomes and/or nanoparticles of bioactive compounds, to increase their bioavailability and bioaccessibility [179–182].

Recently, much attention has been given to the gu<sup>t</sup> microbiota, considered as a metabolic "organ" which impacts host nutrition, and may influence the bioavailability and bioaccessibility of olive phenolic compounds *via* biotransformation into other active substances, which have interesting beneficial health properties in bowel diseases [166,183]. Mosele et al. [184], in an in vitro model experiment, observed that Ole was rapidly deglycosylated during 6 h of incubation with human fecal microbiota, becoming OleA; the latter was degradated into elenolic acid and HT by microbial esterase activity, until it disappeared after 48 h. On the contrary, HT constantly increased during the same fermentation period. This finding was confirmed by the same authors in an in vivo study, that showed that after intake of phenol-rich olive oil for three weeks, the concentration of free HT was significantly increased in the faeces of all the participants in the study. Other reserches have shown that the conversion of Ole into HT was performed by lactic acid bacteria, in particular by *Lactobacillus plantarum* [185], and recently some authors developed oral granules for the co-delivery of *L. plantarum* and a standardized olive leaf extract (Phenolea®Active F, PhenoFarm s.r.L, Rome, Italy), in order to foster Ole metabolism and provide high levels of HT [171].
