Olive oil is synonymous with the Mediterranean diet, and the health benefits of both are well documented.

Olive oil reduces the risk of heart disease, cancer, diabetes and premature death. Olives also contain numerous healthy nutrients.

Now evidence is mounting about the health benefits of olive leaves, including from studies in a recent review.

Here's what's in olive leaves and who might benefit from taking olive leaf extract.

What's in olive leaves?

Olive leaves have traditionally been brewed as a tea in the Mediterranean and drunk to treat fever and malaria.

The leaves contain high levels of a type of antioxidant called oleuropein. Olives and olive oil contain this too, but at lower levels.

Generally, the greener the leaf (the less yellowish) the more oleuropein it contains. Leaves picked in spring also have higher levels compared to ones picked in autumn, indicating levels of oleuropein reduce as the leaves get older.

Olive leaves also contain other antioxidants such as hydroxytyrosol, luteolin, apigenin and verbascoside.

Antioxidants work by reducing the oxidative stress in the body. Oxidative stress causes damage to our DNA, cell membranes and tissues, which can lead to chronic diseases such as cancer and heart disease.

Are olive leaves healthy?

One review and analysis combined data from 12 experimental studies with 819 participants in total. Overall, olive leaf extract improved risk factors for heart disease. This included healthier blood lipids (fats) and lowering blood pressure.

The effect was greater for people who already had high blood pressure.

Most studies in this review gave olive leaf extract as a capsule, with daily doses of 500 milligrams to 5 grams for six to 48 weeks.

Another review and analysis published late last year looked at data from 12 experimental studies, with a total of 703 people. Some of these studies involved people with high blood lipids, people with high blood pressure, people who were overweight or obese, and some involved healthy people.

Daily doses were 250-1,000mg taken as tablets or baked into bread.

Individual studies in the review showed significant benefits in improving blood glucose (sugar) control, blood lipid levels and reducing blood pressure. But when all the data was combined, there were no significant health effects. We'll explain why this may be the case shortly.

Another review looked at people who took oleuropein and hydroxytyrosol (the antioxidants in olive leaves). This found significant improvement in body weight, blood lipid profiles, glucose metabolism and improvements in bones, joints and cognitive function.

The individual studies included tested either the two antioxidants or olive leaf incorporated into foods such as bread and cooking oils (but not olive oil). The doses were 6-500mg per day of olive leaf extract.

So what can we make of these studies overall? They show olive leaf extract may help reduce blood pressure, improve blood lipids and help our bodies handle glucose.

But these studies show inconsistent results. This is likely due to differences in the way people took olive leaf extract, how much they took and how long for. This type of inconsistency normally tells us we need some more research to clarify the health effects of olive leaves.

Can you eat olive leaves?

Olive leaves can be brewed into a tea, or the leaves added to salads. Others report grinding olive leaves into smoothies.

However the leaves are bitter, because of the antioxidants, which can make them hard to eat, or the tea unpalatable.

Olive leaf extract has also been added to bread and other baked goods. Researchers find this improves the level of antioxidants in these products and people say the foods tasted better.

Is olive leaf extract toxic?

No, there seem to be no reported toxic effects of eating or drinking olive leaf extract.

It appears safe up to 1g a day, according to studies that have used olive leaf extract. However, there are no official guidelines about how much is safe to consume.

There have been reports of potential toxicity if taken over 85mg/kg of body weight per day. For an 80kg adult, this would mean 6.8g a day, well above the dose used in the studies mentioned in this article.

Pregnant and breastfeeding women are recommended not to consume it as we don’t know if it's safe for them.

What should I do?

If you have high blood pressure, diabetes or raised blood lipids you may see some benefit from taking olive leaf extract. But it is important you discuss this with your doctor first and not change any medications or start taking olive leaf extract until you have spoken to them.

But there are plenty of antioxidants in all plant foods, and you should try to eat a wide variety of different colored plant foods. This will allow you to get a range of nutrients and antioxidants.

Olive leaf and its extract is not going to be a panacea for your health if you're not eating a healthy diet and following other health advice.