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Plants and the Human Brain

Plants and the Human Brain by David O. Kennedy. Oxford, UK: Oxford University Press; 2014. Hardcover; 400 pages. ISBN: 978-0199914012. $59.95.

David O. Kennedy, PhD, professor of biological psychology at Northumbria University and director of the Brain, Performance and Nutrition Research Centre has produced a landmark contribution to psychopharmacology and human health. In this sophisticated text, he presents an impressive multi-disciplinary treatise in which he systematically and convincingly expounds on the premise that the co-evolutionary battle between plants and their insect predators, and human neurophysiological similarity to the latter, are the underlying reasons that phytochemicals have provided the preponderance of our psychotropic drugs throughout the ages.

Dispensing with foreword and introduction, the book begins Part I Why Do Plant Secondary Metabolites Affect Human Brain Function? with a chapter titled From Shamans to Starbucks, which provides a whirlwind history of psychopharmacology and the role of plant drugs in religion, divination, recreation, and medicine. The remainder of this part systematically explains the role of plant secondary metabolites as attractants to pollinators, or defense compounds against grazing bugs and animals, and the evolutionary biochemical similarity of insect and human brains. In each instance, the text is accompanied by relevant biochemical structures and diagrams as well as extensive, up-to-date references, totaling 1,434 in all. The presentation is scholarly, and the author examines each concept from molecular to molar levels to support each concept.

As an author who has trod similar phytopharmacological territory in the past1, I expected the book to examine the usual alkaloidal and herbal suspects such as morphine, cocaine, the entheogens (dimethyltryptamine, mescaline, psilocybin), and St. Johns wort (Hypericum perforatum, Hypericaceae), ginkgo (Ginkgo biloba, Ginkgoaceae), kava (Piper methysticum, Piperaceae), valerian (Valeriana officinalis, Caprifoliaceae), ginseng (Panax spp., Araliaceae), et al., which it does quite admirably. However, I was delighted to note that Kennedy has chosen to go far beyond the usual constricted pale in incorporating book sections on phenolics and terpenes. These ubiquitous plant chemicals have heretofore received short shrift in scientific investigation, and certainly in the dietary advice we receive, but are deserving of much greater emphasis. Thus, the reader learns that these agents that provide much of the color, odor, and taste of our plant foods also yield essential antioxidants that may protect us from development of cancer, slow aging, and allay dementia. These agents also subtly or otherwise alter our brain chemistry, providing possible solutions to issues of anxiety, depression, and other prickly problems of modern life.

Evolutionarily, this makes perfect sense, as our hunter-gatherer forebears were habitual opportunists who availed themselves of whatever fruit, berry, or nut was available on the trail. Such dietary diversity may be lacking contemporaneously. Our current preoccupation with exotic cuisines may also betray evolutionary longing, not simply for tastes to cover spoiled food, but for rather trace dietary elements that tweak our neurochemistry and correct nutritional imbalances. After learning the underlying reasons to diversify our intake of plant-based nutrients, the reader may well be motivated to seek out outstanding chemovars in the store or grow them personally. Jo Robinsons recent book would be a worthy companion in this regard.2

In the course of the narrative, Kennedy expounds on some fascinating concepts. While readers may be familiar with the theory of hormesis that exposure to a low-level stressor may steel an organism against a more serious future threat perhaps fewer will be aware that ingesting products of stressed plants may be beneficial to humans because of xenohormesis, the theory that, for example, the antioxidant protection that flavonoids offer plants may extend to the humans that ingest them. A salient example would be the ability of polyphenols to interact with mammalian sirtuin genes that regulate cellular aging processes. This is an active area of research that offers promise that we may not only live longer, but lead more healthy lives along the way.

Other fun facts include that the jasmonates in plants, which modulate production of secondary metabolites, are analogous to prostaglandins in animals, that morphine and dimethyltryptamine are trace endogenous compounds in mammals; that ginsengs allelopathy to prevent germination of neighboring plants derives from the same compounds that serve as human adaptogens; and the observation that a fortuitous evolutionary mutation (SNP, single nucleotide polymorphism) in GABA receptor pore structures distinguishes humans from insects such that the ginkgolides and bilobalides that are overtly toxic to them have a 10,000-fold safety margin for us, allowing free use for their benefits of cognitive enhancement and as bulwarks against dementia. 

Finding deficiencies in this book is a fools errand. I noted a dearth of typos, only an incomplete reference and a couple of factual quibbles. It is extensively and rationally referenced, well-indexed, and impeccably presented. More than an impressive collection of facts, the author has succeeded brilliantly in integrating the evidence on the foundation of a clear premise and building each successive layer of evidence to support it with compelling facts and reasoned synergistic examples. Much of science today is effective in dissecting mechanisms and explaining how things work, but it is rarely so brilliant in its exposition or demonstrating why things are as they are. Students, teachers, and researchers of herbal medicine, biochemistry and phytochemistry, nutrition, psychopharmacology, ecology, and entomology should all avail themselves of the opportunity and pleasure to read this beautifully written book.

Ethan Russo, MDMedical Director, PHYTECSVashon, Washington


1.          Russo EB. Handbook of Psychotropic Herbs: A Scientific Analysis of Herbal Remedies for Psychiatric Conditions. Binghamton, NY: Haworth Press; 2001.

2.          Robinson J. Eating on the Wild Side: The Missing Link to Optimum Health. New York, NY: Little, Brown and Co.; 2013.