Dr Caroline Leaf – Still Contradicted by the Latest Evidence, Scripture and Herself

Leaf Cognitive Neuroscientist

Dr Caroline Leaf is a communication pathologist, world renowned author, public speaker, and self-titled cognitive neuroscientist. Her influence continues to grow. She is regularly invited to speak at some of the world’s largest churches. She spoke at her first TEDx conference in February, and she’s about to host her own conference for the second time. She has more than 120,000 Facebook followers, with many more on Twitter and other social media platforms. And she continues to top the sales charts of Christian best sellers.

She is a self-marketing machine.

But there are cracks appearing. More and more, people are realizing that beneath the facade of her numerous Instagram posts, happy snaps, and the allure of popular success, Dr Leafs teachings on science and the Bible don’t match up with actual science and good theology. While many in the church adorn themselves with her teaching, a growing minority are starting to realise that the Emperor has no clothes.

Almost two years ago to the day, I sat in the congregation of Kings Christian Church on the Gold Coast, and heard Dr Leaf speak live for the first time. What I heard troubled me, and I blogged about my concerns to open a dialogue on Dr Leaf and her teaching. Her husband, Mr Mac Leaf, dismissed my concerns out of hand, which only steeled me to take further action. Now, two years of intense research, dozens of posts and a book later, people are starting to take notice.

Not that Dr Leaf has changed her tune. Her fundamental teaching still relies on the idea that our thoughts control our physical and mental health, and toxic thinking causes disease because our thoughts change our DNA and the expression of our genes through epigenetics. And, if we ‘detox’ our thoughts, we will be restored to the health that God intended. Dr Leaf is also expanding her ministry to the subject of mental health and she plans to release a book on food in early 2016.

Dr Leaf can spruik whatever she likes, but her claims of expertise and her scientific and scriptural legitimacy are crumbling.

This post is a little longer than usual, but I’ve divided it up for easier reading:

  1. Dr Leaf is contradicted by her own qualifications
  2. Dr Leaf is contradicted by science
  3. Dr Leaf is contradicted by scripture
  4. Dr Leaf is contradicted by Dr Leaf

1. Dr Leaf is contradicted by her own qualifications

In her books, on TV, at churches, and in promotional material, Dr Leaf describes herself as a ‘cognitive neuroscientist’.

However, Dr Leaf does not have formal qualifications in neuroscience, has not worked at a university as a neuroscientist, has not worked in any neuroscience research labs, nor has she published any papers in neuroscience journals.

Actually, Dr Leaf is trained as a communication pathologist. A communication pathologist is an allied health professional which seems to be unique to South Africa where Dr Leaf trained. It’s a synthesis of audiology and speech pathology. It qualified her to work as a therapist, which Dr Leaf did for children with traumatic brain injuries. Dr Leaf also researched a narrow band of educational psychology as part of her PhD, and she also worked in a number of schools and for educational boards in South Africa. Dr Leaf hasn’t performed any university based research since her PhD was published in 1997.

In contrast, true cognitive neuroscientists actively carry out research into the biological basis of thoughts and behaviours – either mapping behaviours to certain brain regions using electrical currents from the brain, or with functional brain imaging like fMRI, or stimulating or suppressing the activity of a region of the brain and seeing how a person responds.

Simply having some training in neuroanatomy and psychology doesn’t make you a cognitive neuroscientist. Completing a PhD that involved a model for learning doesn’t make you a cognitive neuroscientist. Reading a lot of books on neuroscience doesn’t make you a neuroscientist either, just like reading the Bible doesn’t automatically make you a Pastor.

So no matter how much Dr Leaf may try to convince us that she’s an expert cognitive neuroscientist, truth be told, she is not.

Of more concern is that Dr Leaf is also trying to position herself as an expert in the fields of mental health and nutrition. But if she can’t get her facts right in an area in which she’s had some training, then it’s unlikely Dr Leaf’s teaching will be reliable in areas that she’s had no formal training or experience whatsoever.

I might add, Dr Leaf’s insistence that she’s a cognitive neuroscientist and an expert on mental health and nutrition is also quite insulting for real psychologists, neuroscientists and nutritionists whose opinions are ignored in favour of a self-titled expert whose only ‘authority’ comes by popular demand, not training or experience.

2. Dr Leaf is contradicted by science

There are so many examples of Dr Leaf being directly contradicted by the science that she claims expertise in that I don’t have room in this blog to outline them all. What I can do in this limited space is to outline Dr Leaf’s most egregious and ironic fallacies as a taster.

The 98 percent

One of Dr Leaf’s most fundamental assertions is that “75 to 98 percent of mental and physical illness comes from ones thought life” [1]. She uses this little factoid all the time to justify her belief in the power of thoughts.

However, her statement is completely wrong. When considered in the historical and global context [2], most of human illness is related to preventable diseases that are so rare in the modern western world because of generations of high quality public health and medical care.

For example, Hunter et al state that, “diarrhoeal disease is the second most common contributor to the disease burden in developing countries (as measured by disability-adjusted life years (DALYs)), and poor-quality drinking water is an important risk factor for diarrhoea.” [3]

De Cock et al write, “Recent estimates of the global incidence of disease suggest that communicable diseases account for approximately 19% of global deaths” and that “2.5 million deaths of children annually (are) from vaccine-preventable diseases.” [4]

Routine screening with the much-maligned pap smear has decreased the death rate from cervical cancer in women by as much as 83% [5]. And having a competent midwife and obstetric support during childbirth can decrease the odds of dying in childbirth from 1 in 6 to less than 1 in 30,000 [6].

Midwives, vaccinations, pap smears, clean drinking water and internal plumbing have nothing to do with our individual thought life. We take all of this for granted in the first-world, so the impact of our thought life becomes artificially inflated. In reality, modern medicine and civil engineering, not our thought life, have everything to do with our good health..

Though what makes this meme such a good example of the weakness of Dr Leaf’s teaching is not just because it’s contradicted by actual science, but in trying to justify her conjecture, Dr Leaf has resorted to twisting, misquoting, and generally fudging information from her ‘sources’ in order to make them support her false conclusions.

For example, Dr Leaf quoted a source on genetics that was over thirty years old, from a time when genetic studies were still in the dark ages. She also misquotes her sources, significantly changing the meaning of the quotes in the process. One source didn’t even mention the figure she attributed to it. As if that’s not bad enough, Dr Leaf also cites biased sources, pseudoscientists, and other sources that directly contradict her assertion [7; Ch 10].

This pattern of relying on mistruths and factoids to paper over the gaping cracks in her irrational assertions is repeated throughout her teaching.

The heart is a mini-brain

Dr Leaf believes that the human heart acts as a mini-brain. She states that the heart has its own thought functions, is an electrophysiological regulator of every cell in the body, and is the source of the human conscience.

Such an assertion is ludicrous, and science proves it to be so – the “still small voice” comes from our brains [8-10], and everyday office-based medical tests prove that the electromagnetic signal from the heart is too small to have any meaningful influence on our body’s cells, let alone our thinking [7: Ch 11].

You control your DNA with your thoughts

Dr Leaf believes that our thinking can influence our DNA. She said this in her 2013 book [1: p35], and several times on her social media streams. The problem for Dr Leaf is that there is no credible scientific evidence that DNA is controlled by thoughts.

Her main evidence comes from a poster presentation at a 1993 psychotronics conference titled, “Local and nonlocal effects of coherent heart frequencies on conformational changes of DNA” [11]. She describes this paper as, “An ingenuous experiment set up by the HeartMath Foundation (which) determined that genuine positive emotion, as reflected by a measure called ‘heart rate variability’, directed with intentionality towards someone actually changed the way the double helix DNA strand coils and uncoils. And this goes for both positive and negative emotions and intentions.” [1: p111]

Actually, the experiment was based on faulty assumptions, and so full of flaws in the methodology and analysis, that it could show nothing at all [7: Ch 13]. All it could prove was that Dr Leaf was so desperate to grasp hold of anything that seemed to support her theory that she was willing to use a twenty-year-old study from a group of pseudoscientists that also believe in occult practices like ESP and telekinesis (http://psychotronics.org).

On and on, the same pattern continues. She claims that our thoughts are powerful enough to control our DNA and our brain, except that the opposite is true – it’s our DNA code, with some influence from our environment, that creates our pattern of neurons responsible for our stream of thoughts. She teaches that thoughts cause stress, when again, the evidence is the opposite – psychological stress starts as a subconscious process which changes our stream of thoughts. Dr Leaf teaches that in order to improve our mental and physical health, we need to fight any ‘negative’ or ‘toxic’ thoughts, when studies show that cognitive therapy isn’t effective when compared to behavioural activation. (This is explained in more detail, and with the appropriate references, in my book [7]).

Dr Leaf even goes so far as to say that our thoughts can control physical matter! [1: p33,38]

Over and over again, Dr Leaf’s teaching conflicts with modern science. That Dr Leaf also regularly misquotes her sources and relies on unpublished opinion from pseudoscientists and new-age practitioners also brings her reputation as an expert scientist into disrepute.

3. Dr Leaf is contradicted by scripture

In her books and on social media, Dr Leaf often quotes scripture in an attempt to reinforce her reputation as some form of Biblical expert. Everything’s fine when she simply quotes scripture, but problems arise when she tries to interpret it. Like her use of science, Dr Leaf often misquotes or paraphrases scripture, or uses it out of context, in order to try and Biblically justify her tenuous hypotheses.

2 Timothy 1:7

One of Dr Leaf’s favourites is 2 Timothy 1:7: “For God hath not given us the spirit of fear; but of power, and of love, and of a sound mind.” Dr Leaf interprets the phrases of “spirit of fear” and “a sound mind” as “anxiety” and “mental wholeness” respectively. For example, on the 12th of May 2014, she posted to her social media feeds, “Your mind is all-powerful. Your brain simply captures what your mind dictates. 2 Timothy 1:7” And in her book “Switch on your brain” [1], she said on page 33, “For now, rest in the assurance that what God has empowered you to do with your mind is more powerful and effective than any medication, any threat, any sickness, or any neurological challenge. The scripture is clear on this: You do not have a spirit of fear but of love, power and a sound mind (2 Tim 1:7).”

Simply checking the verse in its full context, and in a different translation, shows it in a completely different light to the way Dr Leaf promotes it. From the NIV, “I am reminded of your sincere faith, which first lived in your grandmother Lois and in your mother Eunice and, I am persuaded, now lives in you also. For this reason I remind you to fan into flame the gift of God, which is in you through the laying on of my hands. For the Spirit God gave us does not make us timid, but gives us power, love and self-discipline. So do not be ashamed of the testimony about our Lord or of me his prisoner. Rather, join with me in suffering for the gospel, by the power of God.” (2 Timothy 1:5-8)

The Greek word for “fear” in this scripture refers to “timidity, fearfulness, cowardice”, not to anxiety or terror. The Greek word that was translated “of a sound mind” refers to “self-control, moderation”, not to mental wholeness. So Paul is teaching Timothy that God doesn’t make him timid, but full of power, love and self-control. Paul is simply saying that through the Holy Spirit, we have all the tools: power, love and the control to use them, so we don’t have to be afraid.

This scripture has nothing to do with our mental health. It certainly doesn’t say that our minds are “more powerful and effective than any medication, any threat, any sickness, or any neurological challenge”. Dr Leaf’s use of this scripture is misleading.

Proverbs 23:7

Another favourite of Dr Leaf’s is Proverbs 23:7, “For as he thinketh in his heart, so is he”.

She used this scripture a number of times on her social media feeds, including on the 4/2/2015, “‘The more you believe in your own ability to succeed, the more likely it is that you will. Shawn Achor’ – ‘For as he thinketh in his heart, so is he …’ Proverbs 23:7”, and the 29/5/2015, “Mind In Action: ‘Genes cannot turn themselves on or off. In more scientific terms, genes are not ‘self-emergent’. Something in the environment has to trigger gene activity.’ Dr Bruce Lipton’ – That ‘something’ is your thoughts! Read Proverbs 23:7”. Dr Leaf also used the same scripture to try and explain how the woman with the issue of blood managed to obtain her healing [1: p111].

What’s interesting is how Dr Leaf only ever uses the first half of this verse. The whole verse (in the King James Version) reads, “For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.”

So what’s with the second half of the verse? What’s the eating and drinking half of the verse got to do with our thought life?

The explanation is that this verse has nothing to do with our thought life at all. Dr Leaf has simply been misquoting it for years, and no one checked to see if she’s right. According to the Pulpit commentary found on the Bible Hub website, “The verb here used is שָׁעַר (shaar), ‘to estimate … to calculate’, and the clause is best rendered, ‘For as one that calculates with himself, so is he’. The meaning is that this niggardly host watches every morsel which his guest eats, and grudges what he appears to offer so liberally … He professes to make you welcome, and with seeming cordiality invites you to partake of the food upon his table. But his heart is not with thee. He is not glad to see you enjoy yourself, and his pressing invitation is empty verbiage with no heart in it.” (http://goo.gl/nvSYUh)

Thus, the scripture does not prove that our thoughts define us as Dr Leaf would suggest. Dr Leaf’s use of this scripture is misleading.

James 1:21

Another example, on the 26 May 2014 on her social media feeds, Dr Leaf said, “James 1:21 How you react to events and circumstances of your life is based upon your perceptions” and then a week later, “James 1:21 Our thoughts and perceptions have a direct and overwhelmingly significant effect of the cells of our body” (4/6/2014).

Except that James 1:21 actually says, “Wherefore lay apart all filthiness and superfluity of naughtiness, and receive with meekness the engrafted word, which is able to save your souls”, and has absolutely nothing to do with our perceptions and our cellular biology.

The same pattern is repeated on social media and in her books. Dr Leaf finds scriptures where one version mentions words like “thinking” or “choice”, isolates them from their context and reinterprets them to suit her meaning, rather the actual meaning of the verse in the original language and the original context.

4. Dr Leaf is contradicted by Dr Leaf

Not only is Dr Leaf’s teaching contrary to science and scripture, but even her own teaching contradicts itself. Dr Leaf also makes claims about her research and achievements that aren’t backed up by her published papers.

To gift or not to gift …

In her 2009 book, “The gift in you” [12], Dr Leaf teaches about the gifts that we have, specifically, our gifts are something uniquely hardwired into our brain, something that we cannot change even if we wanted to, and that it’s our brain structure that gives rise to the way in which we think, the actions that we take, and the gifts we are given from God.

On page 47, Dr Leaf said,

The mind is what the brain does, and we see the uniqueness of each mind through our gifts. This, in itself is delightful and, intriguing because, as you work out your gift and find out who you are, you will be developing your soul and spirit.” (Emphasis added)

This quote in and of itself isn’t actually that significant until we compare it to a quote from the first chapter of Dr Leaf’s 2013 book, “Switch on your brain.” [1]

“The first argument proposes that thoughts come from your brain as though your brain is generating all aspects of your mental experience. People who hold this view are called materialists. They believe that it is the chemicals and neurons that create the mind and that relationships between your thoughts and what you do can just be ignored.
So essentially, their perspective is that the brain creates what you are doing and what you are thinking. The mind is what the brain does, they believe, and the ramifications are significant. Take for example, the treatment of depression. In this reductionist view, depression is a chemical imbalance problem of a machinelike brain; therefore, the treatment is to add in the missing chemicals.
This view is biblically and scientifically incorrect.” [1: p31-32] (Emphasis added)

So … our gifts are hardwired into our brain and can’t be changed because our mind is what our brain does OR our brain is what our mind does, so our gifts aren’t uniquely hardwired into our brain, and we should be able to change our gifting if we want to, based on our choices. Which is it? It can’t be both. Dr Leaf’s fundamental philosophies are mutually exclusive.

Now, we all make innocent mistakes. No one is perfectly congruent in everything they say. But this isn’t just getting some minor facts wrong. These statements form the foundation for Dr Leaf’s major works, and are in print in two best selling books, from which she has used to present countless sermons and seminars around the globe.

To summarise, Dr Leaf has directly called her own beliefs and teaching “biblically and scientifically incorrect”, and not noticed. The confusion and embarrassment are palpable.

But wait, there’s more.

(Not) Making a Difference

From the pulpit, in her books, and in her promotional material, Dr Leaf refers to her ground-breaking research – how her “Switch On Your Brain 5 Step Learning Process” and the Geodesic Information Processing model (which underpins her program), have helped thousands of children to increase their learning and improve their academic results.

For example, Dr Leaf claims that, “The Switch On Your Brain with the 5-Step Learning Process® was assessed in a group of charter schools in the Dallas [sic]. The results showed that the students’ thinking, understanding and knowledge improved across the board. It was concluded that The Switch On Your Brain with the 5-Step Learning Process® positively changed the way the students and teachers thought and approached learning.” (http://drleaf.com/about/dr-leafs-research/ – Original emphasis)

In her TEDx talk, Dr Leaf stated, “I wasn’t sure if this was going to have the same impact cause until this point I’d been working one on one. Well I’m happy to tell you that we had the same kind of results … The minute that the teachers actually started applying the techniques, we altered the trend significantly.” and,
“I stand up here saying this with conviction because I have seen this over and over and over in so many different circumstances … in this country I worked in Dallas for three years in charter schools, and we found the same thing happening.” [13]

Though there is the minor problem of her research results not demonstrating any actual change.

In Dr Leaf’s first case, Dr Leaf herself admitted that the demonstrated improvement of her single patient was just as likely to be related to spontaneous improvement, and not Dr Leaf’s intervention. In Dr Leaf’s PhD thesis, the students improved almost as much in the year without Dr Leafs intervention as they did with her program. In the Dallas charter schools study, Dr Leaf’s intervention either disadvantaged the students or showed no significant difference. In academic circles, Dr Leaf’s research hasn’t so much as generated a stale whimper [14].

So while Dr Leaf may claim that her research has changed the learning and lives of thousands of students all over the world, but her own published research disputes her claims.

The Emperor has no clothes, but no one wants to say anything

In Hans Christian Andersen’s legendary tale, the Emperor was conned by two swindlers into believing that “they were weavers, and they said they could weave the most magnificent fabrics imaginable. Not only were their colors and patterns uncommonly fine, but clothes made of this cloth had a wonderful way of becoming invisible to anyone who was unfit for his office, or who was unusually stupid.”

If you don’t know the story, you can read it here. In the end, the Emperor was duped so badly that he paraded in front of all his subjects au naturel, but “Nobody would confess that he couldn’t see anything, for that would prove him either unfit for his position, or a fool. No costume the Emperor had worn before was ever such a complete success.”

My analogy here is not to suggest that Dr Leaf is deliberately conning the church. Rather, our natural instinct is to suppress our own judgement, even when it’s right, in favour of everyone else’s. We assume information to be true because others in authority tell us it is. We assume that the Emperor must be wearing something because the trusted ministers and noblemen are holding his imaginary train high in the air.

Likewise, it’s very natural for Christians to believe that Dr Leaf’s teaching must be ok because our pastors and leaders vouch for it. Our pastors and leaders vouch for Dr Leaf’s teaching because it’s been endorsed by world-renowned Christian leaders like Kenneth Copeland and Joyce Meyer. And no one wants to say anything, because they don’t want to look sheepish (or be ostracised). Dr Leaf’s ministry may look like a complete success, but only until someone finally says, “But, the Emperor has no clothes …”

It’s time to call Dr Leaf’s ministry for what it is. In my humble opinion, I suggest that Dr Leaf’s ministry is not based on scientific acumen, but on popularity and reputation. And her reputation, in turn, is based on slick self-promotion and an availability cascade (a self-reinforcing process by which an idea gains plausibility through repetition).

Dr Leaf’s teachings are not supported by science, nor by scripture. Her own fundamental philosophies contradict each other. Her assertions about her title and the results of her work are in conflict with her own official data.

Our church leaders need to come clean about why they publicly endorse Dr Leaf’s ministry. I can justify why I think Dr Leaf should not be preaching from our pulpits – in this and many other blog posts, and in my 68,000 word rebuttal to Dr Leaf’s published works. Can Kenneth Copeland and Joyce Meyer, or churches such as Cottonwood Church or Hillsong Church, produce evidence where they performed due diligence on Dr Leaf’s scientific credibility before endorsing her ministry? I would be happy to publish any responses they may be willing to make, complete and unabridged.

If Dr Leaf is preaching at your church, politely ask your pastor to produce his or her evidence that Dr Leaf’s teaching is scientifically and scripturally sound. If your church leaders can’t show that Dr Leaf’s teachings are scientifically and scripturally accurate, then politely ask them why she’s been invited to preach from their pulpit or to sell her wares in your church? Feel free to share your experiences in the comments section.

Critics and sceptics love to use any opportunity they can to embarrass the church, but by parading our own naivety, we’re simply embarrassing ourselves.

It’s time we dressed ourselves in God’s glory, not our own ignorance and ignominy.


[1]        Leaf CM. Switch On Your Brain : The Key to Peak Happiness, Thinking, and Health. Grand Rapids, Michigan: Baker Books, 2013.
[2]        World Health Organization. GLOBAL HEALTH ESTIMATES SUMMARY TABLES: DALYs by cause, age and sex. In: GHE_DALY_Global_2000_2011.xls, editor. Geneva, Switzerland: World Health Organization,, 2013.
[3]        Hunter PR, MacDonald AM, Carter RC. Water supply and health. PLoS medicine 2010;7(11):e1000361.
[4]        De Cock KM, Simone PM, Davison V, Slutsker L. The new global health. Emerging infectious diseases 2013 Aug;19(8):1192-7.
[5]        Dickinson JA, Stankiewicz A, Popadiuk C, Pogany L, Onysko J, Miller AB. Reduced cervical cancer incidence and mortality in Canada: national data from 1932 to 2006. BMC public health 2012;12:992.
[6]        Ronsmans C, Graham WJ, Lancet Maternal Survival Series steering g. Maternal mortality: who, when, where, and why. Lancet 2006 Sep 30;368(9542):1189-200.
[7]        Pitt CE. Hold That Thought: Reappraising the work of Dr Caroline Leaf. 1st ed. Brisbane, Australia: Pitt Medical Trust, 2014.
[8]        Mendez MF. The neurobiology of moral behavior: review and neuropsychiatric implications. CNS spectrums 2009 Nov;14(11):608-20.
[9]        Zysset S, Huber O, Ferstl E, von Cramon DY. The anterior frontomedian cortex and evaluative judgment: an fMRI study. NeuroImage 2002 Apr;15(4):983-91.
[10]      Glascher J, Adolphs R, Damasio H, et al. Lesion mapping of cognitive control and value-based decision making in the prefrontal cortex. Proceedings of the National Academy of Sciences of the United States of America 2012 Sep 4;109(36):14681-6.
[11]      Rein G, McCraty R. Local and nonlocal effects of coherent heart frequencies on conformational changes of DNA. Proc Joint USPA/IAPR Psychotronics Conf, Milwaukee, WI; 1993; 1993.
[12]      Leaf CM. The gift in you – discover new life through gifts hidden in your mind. Texas, USA: Inprov, Inc, 2009.
[13]      Leaf CM. Ridiculous | TEDx Oaks Christian School | 4 Feb 2015. YouTube: TEDx, 2015;20:03.
[14]      Pitt CE, The TEDx Users Guide to Dr Caroline Leaf, cedwardpittcom; 2015   Mar 26, https://cedwardpitt.com/2015/03/26/the-tedx-users-guide-to-dr-caroline-leaf/

Olive Leaf Extract – A potential treatment, but not for what you think

Here in Australia, it’s winter. It’s currently warmer in the fridge than it is outside. We’ve just been blasted by a wall of frigid air straight from Antarctica, and much of the south-eastern corner of our continent has snow drifts over parts that not so long ago were baking under the hot Autumn sun. It’s not something we’re used to in Australia.

Of course, now that winter is firmly entrenched, more people are coming to see me with their viral upper respiratory tract infections, better known as ‘colds’. Yes, ’tis the season to be sneezin’!  Over my years of practice, I’ve seen enough people with a cold to last me a thousand winters.

What always fascinates me are the things that people try to use to cure their cold. I think I’ve heard everything over the last decade: garlic, ginger, peppermint, chicken soup, honey, tea, honey mixed with lemon mixed with tea, or honey mixed with lemon mixed cayenne pepper mixed with tea.   Some people rub Vicks on their feet. Other people douse their pillows in eucalyptus oil.

Another common recommendation that gets around the grape vine and social media is olive leaf extract, used in traditional ‘medicine’ for thousands of years, and those witch-doctors and shamans can’t all be wrong.

One published review described the ‘science’ of olive leaf extract: “Constituents of the olive tree, Olea europaea, have been studied and utilized in folk medicine for centuries. Olive leaf extract, derived from the leaves of the olive tree, contains phenolic compounds, specifically oleuropein, that have demonstrated potent antimicrobial, antioxidant, and anti-inflammatory activity. Oleuropein and derivatives such as elenolic acid have been shown to be effective in in vitro and animal studies against numerous microorganisms, including retroviruses, coxsackie viruses, influenza, and parainfluenza as well as some bacteria. Research suggests that olive leaf constituents interact with the protein of virus particles and reduce the infectivity and inhibit replication of viruses known to cause colds, influenza, and lower respiratory infection. Olive leaf extract has also been shown to stimulate phagocytosis, thereby enhancing the immune response to viral infection. Anecdotal reports indicate olive leaf extract taken at the onset of cold or flu symptoms prevents or shortens the duration of the disease. For viral sore throats, gargling with olive leaf tea may alleviate symptoms, possibly by decreasing inflammation and viral infectivity.” [1]

It’s always a concern when a supposedly peer reviewed journal allows an article to get through which seriously discusses anecdotal evidence as something worthy of attention. Anecdotal evidence is the weakest level of evidence possible. Anecdotal evidence is essentially just stories and opinion [2]. There’s anecdotal evidence for the Tooth Fairy. The other ‘evidence’ that this review describes is from in vitro studies, which are trials in test tubes not in people. In vitro evidence is only helpful in a general sense. Just because a reaction happens in a test tube or petri dish doesn’t mean that it will happen in a real life human being.

So then, do the claims for olive leaf extract stand up to the rigors of modern scientific enquiry or is it like every other cold and flu ‘remedy’ – just another individualised mythology?

Being sceptical, I wanted to find out. So I searched through the published medical literature for quality clinical trials that studied olive leaf extract in humans, and I found only six trials. Interestingly, all of the trials studying olive leaf extract weren’t looking at its effect on immune function but on cholesterol and blood sugar control, blood pressure, and oxidative stress.

In 2009, Kendall et al published a single-centre, randomized, single-blinded, prospective pilot comparison of the effect of dietary supplementation with olive leaf extract on the markers of oxidative stress in 45 healthy young adult volunteers. They found that olive leaf extract had no effect on oxidative stress compared to the control group [3].

Susalit et al (2011) published a double-blind, randomized, parallel and active-controlled clinical study looking at the tolerability, cholesterol-lowering and anti-hypertensive effect of Olive leaf extract in comparison with Captopril (a common blood pressure medication) in patients with early hypertension. After 8 weeks of treatment, there were similar reductions in blood pressure in both the olive leaf extract and the blood pressure pill groups. There was a significant drop in triglyceride levels in the olive leaf extract group, but not in Captopril group [4].

Wainstein et al (2012) performed a randomized controlled trial on 79 adults with non-insulin dependent diabetes, comparing a single 500mg dose of olive leaf extract with placebo over 14 weeks. They measured the HbA1c (a surrogate measurement of the average blood sugar over a three month period) and plasma insulin levels. They also did studies in rats to study the mechanism of action of the olive leaf extract. In the human trials, the subjects treated with olive leaf extract exhibited significantly lower HbA1c and fasting plasma insulin levels. This effect was thought to be reflected in the rat study which suggested that olive leaf extract reduced the digestion and absorption of starch from the intestines [5].

de Bock et al (2013) did a randomized, double-blinded, placebo-controlled, crossover trial on 46 patients in New Zealand, over a 30 week period. The participants were middle aged and overweight. The researchers were primarily studying insulin sensitivity but they also looked at glucose and insulin profiles, cytokines, lipid profile, body composition, 24-hour ambulatory blood pressure, and carotid intima-media thickness. The olive leaf extract group had a statistically significant improvement in insulin sensitivity and responsiveness of the pancreatic β-cell. Interestingly, the olive leaf extract supplementation improved some inflammatory markers, but not others, and made no difference to the patients lipid profile, blood pressure, body composition (their body fatness), carotid intima-media thickness (a risk predictor of cardiovascular disease), or liver function [6].

For completeness, de Bock lead another trial, also published in 2013, although this trial was more a study of the absorption of the compounds in olive leaf extract than a study of their effects [7]. There was a 1996 Belgian study which was written in French. I’m not very good with French, but according to the English abstract, there was no difference between the olive leaf extract and placebo in terms of blood pressure and blood sugar levels [8].

Reconciling the research on olive leaf extract makes for an interesting narrative. There are a couple of really strong, methodologically robust trials on olive leaf extract, and with positive results in favour of it. However, I can count them on one hand, and while the results are encouraging for proponents of olive leaf extract, there needs to be a lot more research before those claims can be made with certainty. And in contrast to its usual selling points, those positive effects for olive leaf extract were for blood sugar control, not the prevention or treatment of viral illnesses.

The bottom line – olive leaf extract may one day prove to be a useful herbal supplement, but there’s not enough clinical evidence to support it at the present moment. And there’s certainly no evidence that olive leaf extract will do anything for your viral upper respiratory tract infections.

So next time you get a cold, don’t bother spending money on olive leaf extract. Have a couple of paracetamol, a long hot shower and a good rest.

And if symptoms persist, don’t forget to see your GP.


[1]        Roxas M, Jurenka J. Colds and influenza: a review of diagnosis and conventional, botanical, and nutritional considerations. Alternative medicine review : a journal of clinical therapeutic 2007 Mar;12(1):25-48.
[2]        Fowler G. Evidence-based practice: Tools and techniques. Systems, settings, people: Workforce development challenges for the alcohol and other drugs field 2001:93-107.
[3]        Kendall M, Batterham M, Obied H, Prenzler PD, Ryan D, Robards K. Zero effect of multiple dosage of olive leaf supplements on urinary biomarkers of oxidative stress in healthy humans. Nutrition 2009 Mar;25(3):270-80.
[4]        Susalit E, Agus N, Effendi I, et al. Olive (Olea europaea) leaf extract effective in patients with stage-1 hypertension: comparison with Captopril. Phytomedicine : international journal of phytotherapy and phytopharmacology 2011 Feb 15;18(4):251-8.
[5]        Wainstein J, Ganz T, Boaz M, et al. Olive leaf extract as a hypoglycemic agent in both human diabetic subjects and in rats. Journal of medicinal food 2012 Jul;15(7):605-10.
[6]        de Bock M, Derraik JG, Brennan CM, et al. Olive (Olea europaea L.) leaf polyphenols improve insulin sensitivity in middle-aged overweight men: a randomized, placebo-controlled, crossover trial. PloS one 2013;8(3):e57622.
[7]        de Bock M, Thorstensen EB, Derraik JG, Henderson HV, Hofman PL, Cutfield WS. Human absorption and metabolism of oleuropein and hydroxytyrosol ingested as olive (Olea europaea L.) leaf extract. Molecular nutrition & food research 2013 Nov;57(11):2079-85.
[8]        Cherif S, Rahal N, Haouala M, et al. [A clinical trial of a titrated Olea extract in the treatment of essential arterial hypertension]. Journal de pharmacie de Belgique 1996 Mar-Apr;51(2):69-71.