Olive leaf has a wide number of constituents, including oleuropein and several types of flavonoids (e.g., rutin, apigenin, luteolin).11 While olive leaf is traditionally associated with a wide number of medicinal claims, few of these have been verified by experimental study. In an animal study oleuropein (when given by injection or in intravenous form) was found to decrease blood pressure (e.g., systolic and diastolic) and dilate the coronary arteries surrounding the heart.12 This ability to lower blood pressure may justify the traditional use of olive leaf in the treatment of mild to moderate hypertension.13 However, human studies are needed to clearly establish olive leaf as a potential treatment for high blood pressure.
In addition, a test tube study has revealed that oleuropein inhibits the oxidation of LDL ("bad") cholesterol. LDL oxidation is one part in a series of damaging events that, if left unchecked, can lead to the development of atherosclerosis.14 This action may provide one clue as to why those consuming a Mediterranean-based diet may lower their risk of developing atherosclerosis.
Oleuropein from olives may also have antibacterial properties. When unheated olives are brined to preserve them, oleuropein is converted into another chemical called elenolic acid. Elenolic acid has shown antibacterial actions against several species of Lactobacilli and Staphylococcus aureus and Bacillus subtilus in a test tube study.15 Whether or not the oleuropein in the leaf undergoes such a transformation is open to question at this point, raising some question as to its antibacterial effects and potential use for this purpose in humans.
Olive leaf extracts have been employed experimentally to lower elevated blood-sugar levels in animals with diabetes.16 These results have not been reproduced in human clinical trials and as such, no clear conclusions can be made from this animal study in the treatment of diabetes.