Introduction to Medicinal Plants
Medicinal plants from ancient times to the present
Plants have been used in treating human diseases for thousands of years. Some 60,000 years ago, it appears that Neanderthal man valued herbs as medicinal agents; this conclusion is based on a grave in Iran in which pollen grains of eight medicinal plants were found (Solecki and Shanidar 1975). One of these allegedly ancient medicinal herbs, yarrow, is discussed in this work as a modern medicinal plant.
Since prehistoric times, shamans or medicine men and women of Eurasia and the Americas acquired a tremendous knowledge of medicinal plants. All of the native plant species discussed in detail in this work was used by native people in traditional medicine. The fact that hundreds of additional species were also used by First Nations Canadians (Arnason et al. 1981) suggests that many of these also have important pharmacological constituents that could be valuable in modern medicine.
Up until the 18th century, the professions of doctor and botanist were closely linked. Indeed, the first modern botanic gardens, which were founded in 16th century Italy, in Pisa, Padova and Florence, were medicinal plant gardens attached to medical faculties or schools.
The use of medicinal plants is not just a custom of the distant past. Perhaps 90% of the world's population still relies completely on raw herbs and unrefined extracts as medicines (Duke 1985). A 1997 survey showed that 23% of Canadians have used herbal medicines. In addition, as much as 25% of modern pharmaceutical drugs contain plant ingredients (Duke 1993).
The number of medicinal plants
There are a huge number of medicinal plants. In the US, almost 1800 medicinal plant species are commercially available (Muller and Clauson 1998). It has been estimated that about 13,000 species of plants have been employed for at least a century as traditional medicines by various cultures around the world (Tyler 1993a). A list of over 20,000 medicinal plants has been published (see details in Deans and Svoboda 1990), and very likely a much larger number of the world's flowering plant species have been used medicinally. Sometimes the figure of 70,000 medicinal plant species is cited, but this includes many algae, fungi, and micro-organisms that are not really plants as the word is understood by botanists. In any event, there is no other category of plants useful to man (with the possible exception of ornamental plants) that includes so many species, and the question naturally arises why such a staggering number of plants have useful medicinal properties.
The medicinal qualities of plants are of course due to chemicals. Plants synthesize many compounds called primary metabolites that are critical to their existence. These include proteins, fats, and carbohydrates that serve a variety of purposes indispensable for sustenance and reproduction, not only for the plants themselves, but also for animals that feed on them.
Plants also synthesize a dazzling array of additional components, called secondary metabolites, whose function has been debated. Many secondary metabolites are
"antibiotic" in a broad sense, protecting the plants against fungi, bacteria, animals, and even other plants.
Every plant species contains chemicals that can affect some animals or micro-organisms negatively, strongly supporting the interpretation that secondary metabolites play a vital role in combating diseases and herbivores.
"Plants have been a rich source of medicines because they produce a host of bioactive molecules, most of which probably evolved as chemical defences against predation or infection" (Cox and Balick 1994).
Many animals too have evolved chemical defences, but on the whole the plant kingdom appears to far surpass the animal kingdom in this respect. It may be that sedentary animals, i.e., those that spend most of their lives attached to a given substrate just like plants (for example, reef-building coral species, barnacles), have also evolved extensive chemical defences to protect themselves against predators, just like plants. However, most sedentary animals (the majority of which are found in the seas of the world) are very difficult to collect or grow. For all these reasons, there are far more plants with medicinal uses than animals.
There are major questions regarding just how beneficial medicinal herbs can be, and just how harmful they can be. As pointed out above, plants contain a very wide variety of secondary compounds. It is clear that some of these compounds, at least in a pure state and at some dosage, are medicinal or toxic. However, it does not necessarily follow that the same compounds present in the herb are as toxic or medicinal as when extracted from the herb, since synergistic (interactive) effects of the chemical components of the herb are possible. An intriguing question about the blend of useful and toxic components of medicinal herbs was posed by Duke (1985, p. 101; repeated on pages 366 and 414):
"Can the homeostatic human body selectively take the one it needs?"
Most animals, including humans, have adapted over millions of years to a regular diet of plants. Consequently, the human system is adapted to a regular intake of plant constituents. Essential dietary constituents of plants are reasonably well understood, but the possible therapeutic role of most components of plants is not.
Almost certainly humans have been unconsciously ingesting and benefiting from medicinal plant components for hundreds of thousands of years. By contrast, humans are generally not naturally adapted to consuming the powerful modern purified drugs that have become the mainstay of Western Medicine, and deadly adverse reactions (as noted later) are far more common than with herbal medicines.
Traditional medicinal usage of herbs by humans, however imperfect and
"unscientific" by modern standards, is the result of countless trial-and-error tests that people have conducted, and so traditional usage points the way to natural therapeutic usage. As we later stress, however,
"natural" does not necessarily mean
"safe." Some herbal products are extremely effective but so dangerous that they should only be used in the hands of skilled medical professionals. Others, however, are sufficiently safe that they can by used by laypeople to help prevent or alleviate minor health problems. Sometimes the herbal drugs are preferable, but as we stress throughout this work, qualified medical personnel should always be consulted.
Herbal medicines versus pharmaceuticals
Two classes of use of medicinal preparations are commonly recognized, often under the titles herbal and pharmaceutical. Pharmaceuticals, discussed below, are refined or synthesized drugs. The World Health Organization has defined medicinal herbals as follows (WHO 1996):
"Finished, labelled medicinal products that contain as active ingredients aerial or underground parts of plants, or other plant material, or combinations thereof, whether in the crude state or as plant preparations. Plant material includes juices, gums, fatty oils, essential oils, and any other substances of this nature. Herbal medicines may contain excipients [inert additives such as starch used to improve adhesive quality in order to prepare pills or tablets] in addition to the active ingredients. Medicines containing plant material combined with chemically defined active substances, including chemically defined, isolated constituents of plants, are not considered to be herbal medicines. Exceptionally, in some countries herbal medicines may also contain, by tradition, natural organic or inorganic active ingredients which are not of plant origin."
Increasing popularity of medicinal plants
The high costs of western pharmaceuticals put modern health care services out of reach of most of the world's population, which relies on traditional medicine and medicinal plants to meet their primary health care needs. Even where modern medical care is available and affordable, many people prefer more traditional practices. This is particularly true for First Nations and immigrant populations, who have tended to retain ethnic medical practices.
In the last decade, there has been considerable interest in resurrecting medicinal plants in western medicine, and integrating their use into modern medical systems. The reasons for this interest are varied, and include:
- low cost: herbals are relatively inexpensive and the cost of pharmaceuticals to governments and individuals is rising
- drug resistance: the need for alternative treatments for drug-resistant pathogens
- limitations of medicine: the existence of ailments without an effective pharmaceutical treatment
- medicinal value: laboratory and clinical corroboration of safety and efficacy for a growing number of medicinal plants
- cultural exchange: expanding contact and growing respect for foreign cultures, including alternative systems of medicine
- commercial value: growing appreciation of trade and other commercial economic opportunities represented by medicinal plants
However, the pace of re-adopting the use of traditional medicinal plants is by no means uniform in western medicine (Duke 1993, Cox and Balick 1994).
In parts of Europe, especially in Germany, herbal medicine (or phytomedicine) is much more popular than is the case in North America. Some 67,000 different herbal products are available in Germany (Foster 1995). The already well-established medicinal plant trade of Europe is increasing at an annual rate of about 10%.
In Canada, and the US, the regulatory climate has been much less receptive to herbal medicines (Tyler 1993b). This is because lack of proper scientific evaluation, limited regulation, absence of quality control, limited education of many herbal practitioners, and the presence of
"snake-oil salesmen" have all combined to give herbal medicine a bad reputation. However, in response to public demand for
"complementary" medicine, this situation is changing. At least 20% of Canadians have used some form of alternative therapy, such as herbalism, naturopathy, acupuncture, and homeopathy (Kozyrskyj 1997).
Herbs are the fastest-growing part of the pharmacy industry of North America, with an annual growth variously estimated as 15 to 20%, and thousands of herbal products are now available to Canadians (Carmen-Kasparek 1993). Herbal remedies have been estimated to have a current value of between two and ten billion dollars in North America, depending on how comprehensively the category of medicinal herbs is interpreted (Marles 1997). Foster (1995) predicted that with appropriate research and regulation,
"herbal medicine will regain its rightful status as an important and integral aspect of classical medicine."
Nutraceuticals and functional foods
Medicinal plants are finding a new, expanding market as herbal components of health foods and preventative medicines, especially under the marketing term
"nutraceuticals" (about as frequently spelled nutriceuticals) (Insight Press, 1996a, 1996b; also see Childs 1997). An economic analysis of nutraceuticals in Canada is found in Culhane (1995) and a good general discussion is in Spak (1998). Essentially synonymous phrases include
"functional foods Footnote [note 1]". All of these terms are applied to substances that may or may not be considered foods or parts of food, but provide health benefits when eaten.
The most widely used of these terms, nutraceutical, was coined by Dr. Stephen DeFelice of the Foundation for Innovative Medicine, a New Jersey based industry group. His definition was
"a food derived from naturally occurring substances which can and should be consumed as part of the daily diet, and which serves to regulate or otherwise affect a particular body process when ingested."
The term is now commonly applied to an extremely wide variety of preparations with perceived medicinal value but not necessarily with apparent food value (such as amino acids, essential fats, dietary fibres and fibre-enriched foods, plant and animal pigments, antioxidants, vitamins, minerals, sugar and fat substitutes, fatless meat, skim milk, genetically engineered designer foods, herbal products and processed foods such as cereals, soups and beverages). Some have contended that fruits and vegetables should be included in
"functional foods" because they are so nutrient-packed, while others would reserve the term for foods fortified in some fashion for health (in this sense, the first functional food seems to have been calcium-fortified orange juice).
"phytonutrient," which should be used for plant materials that by definition have nutritional value, has been applied to medicinal plant preparations without apparent food value.
"Phytomedicines" have been defined as therapeutic agents derived from plants or parts of plants, or preparations made from them, but not isolated chemically pure substances, such as menthol from peppermint (Foster 1995). Unlike pharmaceuticals, which are usually potentially toxic medications that can only be prescribed by a medical doctor, nutritional supplements for the most part can be purchased from a health food store, herbal practitioner or independent distributor. Because they are much less expensive than drugs, herbal preparations or extracts, as additions to diet, have been advanced as a new, cost-effective health care system.
Plant-based vitamins and a wide variety of chemical constituents in fruits and vegetables provide many of the benefits of medicinal plants (fruits and vegetables are medicinal plants, although rarely thought of as such), and concentrated extracts from them are commonly marketed today as nutriceuticals.
"an apple a day keeps the doctor away" reflects the essential medical wisdom of a sensible plant-based diet. This wisdom is quite ancient, as reflected by the saying of Hippocrates (460?-?377 B.C., Greek physician, considered to be the father of medicine)
"Let food be thy medicine."
Pharmaceutical compounds from plants
Above, we have discussed the use of medicinal plants in the form of raw herbs and crude extracts. Modern pharmacology, however, relies on refined chemicals - either obtained from plants, or synthesized. The first pure medicinal substance derived from plants was morphine, extracted from the opium poppy at the turn of the 19th century.
Often, chemicals extracted from plants are altered to produce drugs. For example, diosgenin is obtained from various yam (Dioscorea) species of South America, and is converted to progesterone, the basis of the oral contraceptive pill. Aspirin-like chemicals were once obtained from willows (Salix species) and European meadowsweet (Filipendula ulmaria), but aspirin is now synthesized in the laboratory.
Numerous medicines in use today are extracted from plants. About 50 to 60% of pharmaceutical drugs are either of natural origin or obtained through use of natural products as starting points in their synthesis (Verlet 1990, Balandrin et al. 1993).
The commercial value of biologically active compounds from plant sources has been estimated to approach $30 billion annually worldwide (Deans and Svoboda 1990). Higher plants have given rise to about 120 commercial drugs and 10-25% of all prescription drugs contain at least one active compound from a higher plant (Duke 1993, Cox and Balick 1994).
The tradition for developing plant-based drugs in modern Western medicine is largely based on a paradigm (model) that there is a single active ingredient in medicinal plants, or at least a primary chemical, that is responsible for the medical effectiveness. However, it may be that many preparations used in traditional herbal medicine are effective because of synergistic (interactive) therapeutic effects of several ingredients.
Certainly many traditional herbal drug preparations are compounded from several plants. Such drug mixtures are not of interest to pharmaceutical firms, because they generally cannot be patented (although under some conditions natural products can secure patent protection).
On the other hand, as a visit to a pharmacy or
"health-food" store quickly reveals, numerous companies are marketing plant mixtures as
"dietary supplements," which are in fact being utilized as non-prescription drugs, although there is generally limited or no modern research proof of effectiveness. Since the private sector has limited interest in this issue, there is a clear need for public supported (government) research.
Seeking new drugs from plants
For several decades, the pharmaceutical industry has debated the relative merits of seeking new drugs by synthesis in the laboratory or by screening and testing chemical constituents of plants. The majority of commercial refined plant-based drugs come from only about a hundred plant species.
On the whole, laboratory-based chemistry has been supplanting the search for natural drugs, because testing plants is comparatively labour-intensive and random plant testing has been shown to have a relatively low rate of return. For example, many thousands of plants have been tested for drugs effective for treating cancer, but the success rate of finding an effective chemical or chemical derivative, like taxol from the Pacific yew (discussed in this work), was found to be only one in several hundred.
Worse, the chance that a pharmaceutical company's investment in plant-based drugs will produce a profitable drug has been estimated as perhaps no better than one in several thousand.
In the United States, bringing a new drug to market costs $125 million (Mendelsohn and Balick 1995), so it is no wonder that drug research is undertaken with great caution. [See Feinsilver and Chapela (1996) for the viewpoint that prospecting for new pharmaceuticals among wild plants
"has little probability of success."]
Nevertheless, a number of companies have invested in the last several years in the search for plant-based drugs. There are several reasons for this.
First, there is a need to study biodiversity, especially in third world countries, and the traditional (folklore) medicinal knowledge that native peoples have, before advancing civilization destroys both the plant species and knowledge of their use. Ethnobotany is the branch of biology specifically dedicated to researching the economic relationships between plants and so-called
"primitive" human societies.
Second, improvements in automation and robotics have facilitated laboratory evaluation of large samples in a short time.
Third, synthetic chemists have proven to need examples of effective natural drugs from plants as structure-function models in order to rationally design analogous drugs on the basis of molecular structure; having a natural example of how a novel plant-derived enzyme functions on human receptors may enable the engineering of analogous synthesized molecules with predicted biological activity.
Most of the world's supply of medicinal herbs is obtained by wild collection (often called
"wildcrafting"), not by cultivation. Harvesting renewable wild resources is perfectly legitimate so long as this is conducted in a sustainable fashion that does not eliminate populations or degrade the habitat where the plants grow.
There are still many minor medicinal plant species in Canada that are abundant in nature and can be collected in a sustainable fashion. However, because of shrinking wild resources and a strengthening sentiment that biodiversity should be preserved, cultivation is becoming increasingly important.
When a plant is (or becomes) popular medicinally, its commercial value is likely to lead to over collection. Many very important Canadian drug plants grow in the shade of trees (for examples, ginseng, goldenseal, Mayapple, and Pacific yew) and, because they grow very slowly, are especially susceptible to over collecting. Such non-timber forest resources are of importance to the forest industry, which is looking for alternative crops.
Ginseng has been over collected to the point that the wild Canadian reserve has been designated as
"threatened." Native supplies of Pacific yew (which furnishes the anti-cancer drug taxol) are decreasing, and can no longer meet market demand. Sometimes cultivation is preferable even when there is a wild supply, because of the advantages of growing certain cultivars (for example, uniform maturation or consistency of chemical concentrations), proximity of supply, or quality considerations (for example, being able to certify that a product has been grown organically).
Cultivation offers the possibility of not only preserving economically important wild plants in their natural habitats, but also of providing farmers with new crops.
Domestic and foreign markets for medicinal plants are growing rapidly and provide important opportunities for the development and diversification of Canadian agriculture. Currently, ginseng dominates the medicinal crops of Canada. Ginseng (including both the American and Asian species) is the world's most widely used medicinal plant, and Canada's most important medicinal crop, contributing about $100 million annually to the Canadian economy.
Canadian farmers, entrepreneurs and pharmaceutical companies have increasingly been searching to exploit additional medicinal plants that can be grown in Canada, but have been limited by the difficulty of acquiring information on the many promising possibilities that exist.
In the following chapters on selected native Canadian medicinal plants, we provide summary information intended to improve the utilization of economically important plants of Canada, by providing a guide to critical and relevant information. Except for ginseng, information resources are limited and often difficult to obtain. Although our primary focus is economic, we have also tried to include information of general interest, since the topic of medicinal plants is both crucial and fascinating.
- Footnote 1
In September 1996, Health Canada made available a discussion paper entitled "Recommendations for defining and dealing with functional foods." This contained the following working definitions, which are far more restrictive than found in common usage: "A functional food is similar in appearance to conventional foods, is consumed as part of a usual diet, and has demonstrated physiological benefits and (or) reduces the risk of chronic diseases beyond basic nutritional functions." "A nutraceutical is a product produced from food but sold in pills, powders (potions) and other medicinal forms not generally associated with food and demonstrated to have a physiological benefit or provide protection against chronic disease."