Bioavailability of phenols in extra virgin olive oil
The classical definition for nutrient bioavailability is the degree to which the amount of an ingested nutrient is absorbed and available to the body. Thus, absorption and release into the circulation are points at which bioavailability can be obstructed. Interference with absorption could be when a nutrient is found in a food source, but there is also an inhibitor present in the food which would render the compound not able to be absorbed. For example, a food table would list spinach as a source of the mineral calcium. However, the calcium in spinach is bound by oxalic acid, which means the calcium found in spinach is not available for absorption. If a nutrient can be absorbed into the intestinal absorptive cell, there is often a need for an available carrier on the circulation side for the compound to enter the blood. For example, dietary iron can be absorbed into the intestinal cell but if the person’s iron status is sufficient, the transport protein for iron (transferrin) is not made so the absorbed iron will not enter the circulation but stays in the intestinal absorptive cell and will be released back into the intestinal tract when the cell dies and is then sloughed off into the gut lumen to become part of the fecal matter.
Phenols in Extra Virgin Olive Oil
There has been some debate over the years as to the bioavailability of the phenols in extra virgin olive oil. It is the phenols in extra virgin olive oil that provide the health benefits as evidenced by studies that compare refined olive oil to extra virgin olive oil with a known phenol content 1-11. Studies show benefit starting with a phenol content of approximately 150 mg/kg 7,11 and studies comparing extra virgin olive oils with more than one phenol content suggest that the health benefits may be positively related to the phenol content of the extra virgin olive oil 3,7,11.
Studies looking at the absorption of the phenols in olive oil indicate that the absorption ranges from 50-95%.12 However, the fate of the phenol post absorption has been questioned when the analysis was for phenols in the free form in plasma. 13 Studies comparing in vitro with in vivo activity of phenols and LDL oxidation show that the in vivo lag time of LDL oxidation does not change with phenol consumption while it does improve with in vitro testing with the same phenol.14,15 This discrepancy suggests that in additional to the classical components of bioavailability, one could add the variable of “laboratory analysis” of the nutrient in question in the living tissue to the definition of bioavailability. Earlier studies that report an in vitro improvement in LDL oxidation with phenols but not with in vivo test have used the conventional method of ultracentrifugation to assess the phenol presence in the blood.14,15 In an elegant paper by Tung, et al. 16 the authors provide evidence that phenols will attach to both the low-density lipoprotein cholesterol (LDL-c) and high density lipoprotein cholesterol (HDL-c) particles. The authors hypothesize that use of ultracentrifugation in the analysis for phenols in plasma changes the lipoprotein conformation which would lead to the release of phenols, thus rending them undetectable. In their experiment, they used the affinity column procedure which is quicker than and not as harsh as ultracentrifugation and would not lead to phenol release from the lipoprotein. Using this milder method, they did show improvement in LDL oxidation lag time in vivo post phenol ingestion.
The attachment of ingested phenols to the lipoproteins would mean that the phenol could decrease oxidation on the lipoprotein it is attached to. For LDL, this would decrease oxidized LDL-c, which is the form of LDL that increases risk of atherosclerosis. Oxidized HDL-c is not as commonly studied as oxidized LDL, but studies show that EVoo compared to refined olive oil will improve HDL function even if the total amount of HDL-c does not change indicating biological activity for the phenols.5 The ability of the olive oil phenols to bind to lipoproteins would also provide transport throughout the body and could bring the phenol to various sites in the body where they would be available for biological activity.
As extra virgin olive oil has been shown to provide health benefits not found in refined olive oil, it is not surprising that the olive oil phenols are quite bioavailable post absorption. It just took the appropriate laboratory technique to establish the presence and thus activity of the phenols once they were absorbed into the body.