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Fighting Free Radicals: The Role of Bark Extractives

by Kelvin Duncan, Ph.D.(more info)

listed in antioxidants, originally published in issue 44 - September 1999

A stand of young Pinus radiata

A stand of young Pinus radiata

The rapidly growing volume of evidence in the scientific literature about the nature and role of free radicals has lead to an increasing awareness of their importance in health and disease. Free radicals have been implicated in a great number of human conditions and the literature on the subject is vast (Ames et al., 1993, Bland, 1995, Diplock, 1994, Halliwell, 1996 and the references in Table 1).


Table 1. Recent references to health conditions in which flavonoids are thought to be beneficial.

Degenerative disease/ helps prevent or slow down
General diseases/ prevents or ameliorates

Cancer and cardiovascular/ protects
Inflammatory bowel disease/ helps
Heart attacks/ reduced risk
Strokes/ reduced risk
Clots/ prevent abnormal clots
Endothelial cells/ protect and strengthens
Plaque/ reduced rate of formation

Hair/ increases hair growth and follicle cell density
Anti-inflammatory, esp. arthritis/ reduces

Oedema/ reduces
Tumours/ inhibits early tumours
Tumours/ reduces incidence of spontaneous tumours in elderly mice
Allergies/ anti-allergenic
Carcinogens and mutagens/ chemoprotective
Skin cancer/ reduces or prevents skin cancers induced by UV
Prostate cancer/ anticarcinogenic against prostate cancer
Insulitis/ protect pancreas and help lower or prevent insulitis

HIV, may help HIV
Ames et al., 1993

Cross et al., 1987
Gey, 1993
Grishanan, 1994
Hertog et al., 1993
Keli et al., 1996
Rice-Evans, 1996
Kaneko et al., 1999
Xu et al., 1998
Takashashi et al., 1999
You et al., 1999
Pelzer et al., 1998,
Lairir-Chatterjee et al., 1999
Duncan, 1998
Cheong et al., 1998
Sohn et al., 1998

Ahmad et al., 1998

Zi et al., 1998
Soto, et al., 1998
Scrhramm & German, 1998
Kitamura, et al., 1998


However, not all free radical reactions in the body are harmful; some are entirely natural and are necessary for the correct functioning of many metabolic processes. (Cheesman and Slatter, 1993). These benign, natural free radicals do not concern us here since they are well controlled by the body's metabolism. Rather it is the damaging free radicals, which are largely caused by non-natural events, that are the focus of this article.

How do free radicals form?

Free radicals can form naturally since some of the body's molecules have weak bonds that spontaneously break and cause the molecule to become a free radical. Also, for unknown reasons a small proportion of normal oxidative reactions result in the formation of free radicals. Other causes of damaging free radicals are ionising radiation, such as light, or ultra violet, or other forms of radiation. Mariner's skin, that scourge of skin which makes people look far older than they really are, is a result of excessive exposure to light and to UV radiation: the skin changes in thickness, it becomes dehydrated, its collagen becomes thickened and hardened, and wrinkles and dryness result. Radioactivity, whether from natural or man made sources, also causes free radicals to form. Finally, a great number of chemicals, especially human-produced synthetic compounds can cause free radical formation.


The damaging effects of free radicals can be countered by antioxidants. These work by either stopping free radical damage by donating an electron without becoming a damaging free radical themselves, or by preventing oxidation commencing.

There are three main types of antioxidants that are important in human metabolism. These are: antioxidant enzymes produced by the body; essential nutritional dietary compounds such as vitamin C, and small plant-derived substances which intercept free radicals and prevent them from causing damage. The full range of antioxidants found in the body include vitamins C and E, carotenes, glutathionine, uric acid, taurine, and plant flavonoids and flavonoid derivatives, and some other compounds. This article considers only the flavonoids, which are small compounds synthesised by plants, but not animals, so must be taken in our diet. They are water soluble compounds based on a unit involving carbon ring structures containing phenols (-OH) groups. There are more than four thousand known, not all of which have antioxidant activity, and a great number remain to be discovered (Colgan 1994). They scavenge free radicals without becoming themselves becoming damaging free radicals or causing other chemical species to become free radicals.

In living plants, flavonoids are produced as pigments, defences against fungi and bacteria, anti-parasitics, and as antioxidants protecting against cellular oxidative stress.

Tree bark in history

Tree bark is a particularly rich source of a wide variety of kinds of medicinal compounds, and this has been known to many cultures for many centuries. Two thousand years ago Hippocrates, the Greek physician known as the father of medicine, recommended chewing on white willow bark to relieve pain and fever. In England, this was a common practice in the Renaissance. The active ingredient was isolated in 1828 and was given its current name, salicylic acid in 1938. It was stabilised in 1897 as acetylsalicylic acid. Even though the new product enjoyed great market success, its mode of action was not known until John Vane, a British Pharmacologist, found that they inhibited the body's production of prostaglandins that promote inflammation and thereby cause pain. Vane received a Nobel Prize for this work. Salicylic acid, a natural product, has been found to have widespread and beneficial effects on many human conditions, including heart attacks and strokes (Graeda and Ferguson 1993).

Decoctions or infusions of barks and leaves were commonly used as poultices and therapies throughout Europe and Asia. The great navigator, Captain James Cook, was well aware of the value of bark and leaves as antiscorbutics (antiscurvy agents). After an enormously long and difficult voyage to New Zealand he would set out to collect materials for "spruce beer", an optimistic name designed, no doubt, to make the bitter concoction more palatable to his reluctant crew. Spruce, or white, or Norwegian beer was well known then, as it continues to be today, and even if not loved, it was at least accepted for its effects! But Cook's spruce beer was neither spruce, nor was it beer. What it was is revealed in a letter he wrote to the errant Furneaux, the captain of the accompanying ship on the second of Cook's voyages. Unlike Cook, Furneaux had a serious scurvy problem amongst his crew because he had not previously followed Cook's detailed instructions. So, in his letter Cook gave instructions to "brew" beer of the inspissated (thickened) juice of wort, essence of spruce and tea plants. By spruce he meant anything that vaguely resembled spruce, since there was then no spruce in New Zealand, and by tea plants he meant manuka-like plants. Manuka is a ubiquitous and cheerfully scruffy charmer of a scrub or small tree in New Zealand. Like many plants, Manuka contains powerful anti-microbial compounds and other useful biologically active ingredients. Recent work has shown that those evergreen southern equivalents to conifers, the Podocarps, to which Cook's "spruce" belongs, have an abundance of flavonoids . As much as they hated his brews, Cook's crews did consume them and their health was remarkably good (Hough 1994). Cook delivered a paper to the Royal Society describing his work in conquering scurvy and other health problems on long sea voyages. He did not claim to have discovered the health benefits of vegetables and plant extracts, as others had experimented with such diets and treatments earlier, but his fame was such that he was greatly instrumental in popularising them to the enormous benefit of seafarers' health.

Asian cultures used bark extensively to treat and heal. Indian Ayurvedic medicine has a 5000 year history. Punarvasu in about 800 BC, wrote the Susrita Samhita which describes 1500 plants and 300 medicines of therapeutic value. Barks are important components of Ayurvedic practice. The bark of the arjuna tree has been used for at least 3000 years for the treatment of heart failure and for reducing the swelling due to fluid accumulation in ankles and legs when the heart is not pumping properly. This traditional use has been confirmed by western science and it is used by many healers today including Western-trained healers.

The dried bark of the varuna tree has provided relief from kidney and bladder stones. It is now being used in Western medicine to prevent stone formation and related urinary tract infections (Chevellier 1996).

The Chinese, too, have used bark products in their traditional medicine for very many centuries. A herbal and medicinal source book, written over 2500 years ago, the Shen Nung Pen Tsao Ching of China lists over 360 medicinal drugs made from plants. Barks are used extensively. Every Chinese herbalist uses a wide variety of barks, each with its own characteristics and specific uses. Cinnamon, the dried bark of Cinnamomum cassia, is used to control fever and diarrhoea, to aid menstrual problems, and to soothe indigestion. Recently, scientific medicine has confirmed its potency as an antiseptic agent and has shown its potency in reducing the insulin dependency in diabetics. Magnolia barks are prescribed in Chinese herbal medicine as a skeletal muscle relaxant, analgesic and anti-hypertensive. Phelledendron spp., are commonly prescribed to treat diarrhoea and inflammation (Griggs, 1993).

Extensive research in China and Japan is elucidating the active ingredients in these remedies, but only a small fraction of the myriad complex compounds in bark have been identified.

Polynesian and American indigenous people also made use of bark (Whistler, 1991; MacDonald, 1993, Garrett and Garrett, 1996; Wyatt, 1994).


Pinus radiata bark

Pinus radiata bark

Industrial preparation of flavonoids

Today, various barks are used extensively for health purposes. Following the great success of Taxol, there has been a massive search by ethnobotanists and biochemists for useful bark extractives and a number of useful substances have been discovered. Bark flavonoids are amongst the most useful. They can be extracted industrially using solvent extraction or hot water extraction. This process isolates and concentrates the flavonoids by partitioning them between two different phases of mutually insoluble solvents – like oil and water. More of the desired compounds dissolve in one phase and the unwanted material in the other so it is then easy to separate the two phases mechanically. Further purification can be undertaken by "salting out" – precipitation of the desired material by adding increasing amounts of common salts, such as magnesium sulphate. The salt remains in solution but throws the less soluble flavonoids out of solution as a precipitate so they can be collected. The trouble is that these two processes, solvent partitioning and salting out, do not yield completely pure products. Some of the undesirable compounds get through, necessitating repeated cycles of solvent extraction and salting down in order to obtain yields of sufficient purity. But three main problems still remain with this approach: solvent residues may contaminate the product, undesirable by-products may contaminate the product, and micro-organisms may survive the processing to be present in the product. Furthermore, the process is expensive.

In contrast, the process used to produce a recent new product – enzogenol – is based on water extraction in the absence of oxygen (to prevent the possibility of oxidation of the flavonoids) of clean and selected bark from the Monterey Pine – a coastal species from the Pacific Northwest of North America which is grown extensively in New Zealand. The desired mixture of flavonoids can be selected from the decoction or liquor by a purely physical process. This excludes all the undesirable by- products and micro-organisms to yield a very pure product of controlled composition. The by-products are used as a soil enhancer, so no wastes are produced from the process. Even the water can be recycled (Gilmour in Duncan, 1998).

Bark as a source of antioxidants

Why should bark contain so many antioxidants? The reason is that oxidative stress is a great problem faced by the stem of trees. Stems are intended to last many decades or centuries, so that they have to have powerful and long-lasting protection against attack, decay and disease. Bark is the structure that performs this protection, and since oxygen diffuses from the outside of the stem through the bark, antioxidants are present in the bark and the tissues immediately underneath it so as to provide protection in the event of free radical formation.

Table 2. Kinds of flavonoids and their origin
The six classes of flavonoids:
flavonones, found in citrus
flavones, found in herbs
flavonols, found in all fruits and vegetables
isoflavonoids, found in legumes
anthocyanidins and flavans, found in fruits.

None are synthesised by animals, and our sources are entirely from the plants we eat. We should eat a wide variety of types and not rely on only a few.

Human metabolism does produce some antioxidants naturally, but these are not sufficient to combat all the free radicals formed in our bodies. We rely on dietary sources to augment and complete our defences. The trouble is that the rate of formation of free radicals has probably increased over the last few centuries due to increas- ing sources of free radical forming agents as industrial processes extend further and further into our lives. Further- more, our diet has become more and more deficient in free radicals owing to changing dietary habits away from sources rich in antioxidants (raw leafy vegetables, onion, nuts, fruits) toward overly processed foods from which flavonoids, Vitamin E and many other nutrients have been removed. More people are living longer, so these two influences are affecting more and more people. It seems a great pity that modern food processors exploit our two Achilles heels of diet – our fondness for fat and sugar. Most of us are far too fond of these for our own good. This was not a problem when our only sources of food had limited amounts of fats and sugars, but today we can greatly modify our food and over-process it to an extent that it poses a risk to human well being. Nor do I believe that supplementation of these over-processed foods by adding back in synthetic forms of the extracted nutrients is at all wise. There is evidence that such practices are doubly inimical to human health – not only has the over processing removed essential nutrients, the addition of synthetic forms of these poses chronic risk through the loading of the body with antichiral synthetics (manufactured forms of nutritional or medical compounds which have the correct gross chemistry, but which have the wrong stereoisometry for appropriate biochemical action).

Determining the health value of flavonoids

There are six main ways by which the effects of dietary flavonoids on human health may be evaluated: direct measurement of oxidative stress levels, epidemiological studies, repeated measures tests, laboratory (in vitro or in glass) biochemical or cytological studies, randomised double blind trials, and animal testing.

We can measure the site and extent of oxidative stress in human subjects by urine analysis. The beneficial effects of antioxidants can be assessed by measuring the decrease in oxidative stress levels following administration of dietary supplements. This is being researched at the present time and results are still to be evaluated, but initial results appear extremely promising.

Epidemiological studies can be divided into two types: observational and experimental. Observational studies compare the incidence of disease and longevity in flavonoid-rich populations, such as in the Mediterranean countries, with flavonoid-poor countries, such as the United Kingdom and the United States. The results indicate that a flavonoid-rich diet does, indeed, lead to increased longevity and better health, but other factors, such as genetic factors, may be involved. It is essential to maintain a wide range of different flavonoids from a variety of sources if you rely on natural dietary sources alone.

Experimental epidemiological studies rely on repeated measures, which are a scientific kind of "before and after" studies. An example of such studies is the Spanish women smokers study completed last year. Little benefit was recorded in lowering the incidence or outcome of lung cancer from a diet rich in flavonoids, but this is what may be expected in such a rampant form of cancer as lung cancer. Other studies have indicated that flavonoid supplements have very great benefits in both preventing diseases and in mitigating their effects.

Laboratory studies have shown that plant-derived antioxidants have great antioxidant activity. Platelet aggregation, thrombosis formation and plaque deposition are all reduced, thus explaining the epidemiological observations of reduced incidence of strokes and heart disease. The effects of flavonoids on cancer formation and propagation are also becoming understood. Again, the evidence is accumulating that flavonoids can help prevent certain cancers forming, growing and undergoing metastasis.

However, randomised double blind clinical trials of chronic effects of flavonoids are not as common, mainly because of expense and experimental difficulties. If you are studying the rate and age of onset chronic degenerative diseases it is scarcely practical to undertake clinical trials of twenty or thirty year duration. For similar reasons, "longitudinal" (life long) animal trials are rare. They are very expensive and are liable to be affected or destroyed by factors beyond control. The longer the duration of the experiment, the more likely these destructive events are to occur. So chronic experimentation, whether it is by the double blind methodology or fully controlled animal trials, tends to be rare. There have been some, however. Trials on vinegar flies in the early 1970s showed greatly increased longevity, and recent trials on mice have shown greatly reduced incidence of spontaneous old-age cancers, healthier and thicker coats, better cognitive skills, and increased longevity (Duncan, 1998).

Short-term studies of flavonoids are more popular amongst researchers. Recent studies are given in the references to this article.

Other benefits of flavonoids

Besides combating free radicals, flavonoids have been shown to have other beneficial effects including: adhesion receptor expression, slowing down or preventing bacterial replication (one of their main functions in living plants), slowing down viral replication, inhibiting proteolytic enzyme action, oestrogenic effects and carbohydrate induced AGE, (advanced glycosylation end products, where glucose and its polymers bond on to protein and cause the proteinacous materials of the body to become mucoid or amyloid material that "gums up the works". An example of this is ageing of skin where collagen thickens and becomes less flexible because of glycosylation, and less flexible because of the cross linking due again to glycosylation. This, coupled with the loss of subdermal fat due to free radical damage and glycosylation, causes the appearance of old skin).


Free radicals are implicated as a major cause of many disease states in the human body, particularly chronic inflammatory and degenerative diseases such as arthritis, heart disease and cancer. There is a great deal of evidence suggesting that positive health benefits can be achieved by the adoption of a healthier lifestyle, a healthier diet richer in flavonoids and taking dietary supplements if the normal diet is deficient in flavonoids. The incidence of oxidative cell damage and general degenerative diseases is lowered by dietary flavonoids. There is also excellent evidence that the onset of these conditions can be delayed or even prevented by diets rich in flavonoids.


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About Kelvin Duncan, Ph.D.

Kelvin (Kelly) Duncan is the Dean of Science at the University of Canterbury, Christchurch, New Zealand. He has researched bioactive defence compounds in animals, plants and micro-organisms for many years. He also develops new technologies to extract the compounds he discovers. Many of his discoveries have become commercial – enzogenol is a recent example. He is currently working on photobioreactors to grow photosynthetic micro-organisms in a more controlled manner, a new cell disruption technology called "potentiation" that enables a greater range of better products to be extracted from plant and microbial cells, "potentiated" pollen and "potentiated"grasses, and a new class of non-toxic (to humans) natural antibiotics of great curative powers.

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