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Olive leaf is the leaf of the olive tree (Olea europaea). While olive oil is well known for its flavor and health benefits, the leaf has been used medicinally in various times and places. Olive leaf and olive leaf extracts (OLE), are now marketed as anti-aging, immunostimulator, and antibiotic agents. Though there is some laboratory evidence for these effects in biological standardization experiments (i.e., bioassays), clinical evidence in humans is inconclusive.
Clinical evidence has been conflicting regarding any blood pressure lowering effect of carefully extracted olive leaf extracts. Bioassays support its antibacterial, antifungal, and anti-inflammatory effects at a laboratory level. A liquid extract made directly from fresh olive leaves gained international attention when it was shown to have an antioxidant capacity almost double green tea extract and 400% higher than vitamin C.
The silvery green leaves are oblong, measuring 4–10 centimetres (1.6–3.9 in) long and 1–3 centimetres (0.39–1.2 in) wide.
The primary active compounds in unprocessed olive leaf are believed to be the antioxidants oleuropein and hydroxytyrosol, as well as several other polyphenols and flavonoids, including oleocanthal. Elenolic acid is a component of olive oil and olive leaf extract. It can be considered as a marker for maturation of olives. Oleuropein, together with other closely related compounds such as 10-hydroxyoleuropein, ligstroside and 10-hydroxyligstroside, are tyrosol esters of elenolic acid.
|This section needs more medical references for verification or relies too heavily on primary sources. (July 2012)|
Researchers have found that olive leaf dilates isolated rat aorta. A study conducted on monozygotic borderline hypertensive twins showed significant reduction of blood pressure in a dose related manor. 
Recent research in rodents has shown that olive leaf extracts may reduce infarct volume, brain edema, as well as improve blood–brain barrier permeability and neurological deficit scores after transient middle cerebral artery occlusion (stroke).
Olive leaf harbours antioxidant properties that help protect the body from the continuous activity of free radicals. Free radicals are highly reactive chemical substances that can cause cellular damage if left unchecked. Some recent research on the olive leaf has shown its antioxidants to be effective in treating some tumors and cancers such as liver, prostate, colon, skin and breast cancer, clinical studies lacking; Olive leaf is especially potent when used in combination with other antioxidants.
A randomized controlled double-blind crossover trial in New Zealand found that olive leaf extract capsules significantly improved insulin sensitivity and pancreatic β-cell responsiveness in middle-aged overweight men.
Olive leaf can be taken as a liquid concentrate, dried leaf tea, powder, or capsule. The leaf extracts can be taken in powder, liquid concentrate, or capsule form though the fresh-picked leaf liquid extracts are quickly gaining popularity due to the broader range of healing compounds they are thought to contain.
Olive leaf extracts are combined with olive oil in soaps and skin creams for application to the skin or other body surfaces.