The Chemistry of Plants
Although people have been using herbs for centuries, we don't know a whole lot about the pharmacology, or chemical makeup, of many of them. Unlike pharmaceuticals — drugs created in a lab from a precise chemical recipe — herbs are often chemical mysteries.
Unraveling the Mysteries
In recent years, scientists have been deconstructing herbs to determine the chemical compounds — called phytochemicals — behind their actions.
For example, researchers have determined that garlic (Allium sativum) owes much of its antibacterial and cholesterol-lowering action to a phyto-chemical called allicin. Ginger (Zingiber officinale) apparently gets its stomach-settling powers from two chemicals, 6-gingerol and gingerdione. And cayenne peppers (Capsicum annuum, C. frutescens) contain capsaicin, which dulls pain and produces a warming sensation.
Primary and Secondary
In most cases, the compounds that are so helpful to humans are actually secondary to the plant's survival. The most important things, at least as far as the plant is concerned, are the primary constituents, which are used in the plant's primary metabolic processes (like photosynthesis) and include things like sugars and chlorophyll.
Secondary constituents are things that the plant developed over the course of its evolution to defend itself against animals, insects, disease, or environmental stresses such as changes in temperature or water levels. Quite often these compounds — which include vitamins, minerals, essential oils, and phytochemicals — are key to both the plant's viability and its bioactivity (the effect it has on another living organism or tissue).
According to the latest estimates, there are 122 scientifically identified plant compounds being used as drugs throughout the world, which are drawn from just 94 plant species. With several thousand known medicinal plants now in use, scientists still have quite a few more chemicals to name.
Luckily, the chemicals that make a plant unappealing to microbes can serve as antimicrobial agents in humans, too. And the neurotoxic chemicals that a plant uses to defend itself against foraging deer can work as sedatives, muscle relaxants, or anesthetics in people.
Most of the secondary constituents in plants can be lumped into three categories: terpenoids, alkaloids, and polyphenols.
Many terpenoids are either toxic or just unappetizing to grazing animals; others make a plant more appealing to pollinating insects. Feverfew (Tanacetum parthenium) and chamomile (Matricaria recutita) contain medicinal terpenoids.
Alkaloids can have potent medicinal activity. Examples are nitrogen, caffeine, quinine, morphine, and nicotine. The Chinese herb ma huang (Ephedra sinica) contains the alkaloid ephedrine. Cocaine is found in the leaves of the coca shrub (Erythroxylum coca).
Polyphenols include tannins and flavonoids. Tannins are astringent chemicals (once used to tan animal hides) found in the seeds and stems of grapes (Vitis vinifera), the leaves and bark of trees or shrubs like witch hazel (Hamamelis virginiana), and tea leaves (Camellia sinensis). Flavonoids are a broad class of compounds that act as antioxidants (agents that counteract the process of oxidation, which can damage cells and trigger disease). They are found in ginkgo (Ginkgo biloba), hawthorn (Crataegus monogyna, C. oxyacantha), and milk thistle (Silybum marianum), among others. Iso-flavones are one type of flavonoid that act as phytoestrogens (plant compounds that mimic the effects of estrogen in the body). Isoflavones are found in soybeans (Glycine max).
The Sum Versus the Whole
But while scientists have identified the active constituents in many herbs, many more remain a mystery, in part because the chemicals in the plant appear to work synergistically instead of individually.
That means that, in most cases, we don't know which ingredient in a plant is causing a therapeutic effect, or if that ingredient is acting alone or in combination with other ingredients. Moreover, because it's a natural thing, an herb's biochemical composition is inherently variable and can change from year to year or crop to crop.