## 2 Material and Methods
### HPLC Analysis of Phenolic Compounds Extracted from Wild Olive Leaves  
The phenolic compounds from wild olive leaf extract were separated by high-performance liquid chromatography on an HPLC system (Perkin Elmer, Waltham, MA, USA) using COI/T method 20/Doc. No. 29 [35]. The system is labelled Series 200 and consists of units for the automatic sample application, a binary pump, a system for removing gases from the mobile phase using a vacuum, a column heater, and a UV/VIS detector. The separation was performed on an Ultra Aqueous C18 column (250 \(\times\) 4.60 mm) with a stationary phase diameter of 5 \(\upmu\)m (Restek, Bellefonte, PA, USA). The mobile phase consisted of 0.2% phosphoric acid (solvent A) and a 1:1 mixture of methanol and acetonitrile (solvent B). Gradient elution was achieved by a pump programme that initially provided 4% solvent B, and increased the proportion of solvent B to 50% over a period of 40 min. The increase in solvent B to 60% continued over the next 5 min, with the proportion of solvent B increasing to 100% from 45 to 60 min. In the next 8 min, solvent B flowed through the column. A steep linear decrease of solvent B to 4% occurred from 68 to 70 min, and the achieved solvent ratio was maintained for the last 10 min to ensure stabilisation of the column to the initial conditions. The column was thermostatted at 25 \({}^{\circ}\)C, the flow rate was 0.8 mL/min, and the injection volume was 20 \(\upmu\)L.  
A UV detector with a wavelength of 280 nm was used for the detection of phenol. The quantification of oleuropein aglycone is expressed in \(\upmu\)g/mL oleuropein, while separate calibration lines were established for the other 14 standards (Supplementary Materials Figure S1a-m). Oleuropein was identified by the retention time of the standard and quantified using the calibration curve of the standard in the range of 5.16 to 201.60 \(\upmu\)g/mL (R\({}^{2}\) = 0.9997). All results represent the average of two measurements.  
Standards and solvents were of analytical grade and were purchased from Sigma-Aldrich (Steinheim, Germany). Deionised water (Milli Q) was used to prepare the solvents.  
Collection of Human Digestive Enzymes and Determination of the Enzymatic Activity of Digestive Juices  
The collection of human digestive juices from the stomach and duodenum was done according to the method described by [PERSON] et al. (2016), while the determination of the enzymatic activity of collected juices was performed according to the method described by [PERSON] et al. (2016). Briefly, human digestive juices from the stomach and duodenum were collected from four healthy volunteers using an endoscope. Digestive juices were collected separately from the stomach and duodenum, collected in a sterile tube and centrifuge at 10,000 rpm at room temperature (my Spin Minicentrifuges, Thermo Fisher Scientific, Waltham, MA, USA) to remove mucosa and cell debris. Juices were divided into aliquots, treated with inert gas (argon), and stored at \(-20\)\({}^{\circ}\)C until further analysis. The enzymatic activity of collected human digestive juices was determined using bovine haemoglobin solution for gastric juices and casein solution for duodenal juices. One unit of enzyme activity (U) represents the amount of enzyme that causes the absorbance change of 1 between the blank and the sample at the wavelength of 280 nm (Microplate Reader, Tecan, Mannedorf/Zurich, Switzerland).