Intrinsic and Extrinsic Factors Associated with Hair Graying (Canities) and Therapeutic Potential of Plant Extracts and Phytochemicals

This review aims to gain insight into the major causes of hair graying (canities) and how plant-derived extracts and phytochemicals could alleviate this symptom. Research articles on human hair graying were searched and selected using the PubMed, Web of Science, and Google Scholar databases. We first examined the intrinsic and extrinsic factors associated with hair graying, such as the reduced capacity of melanin synthesis and transfer, exhaustion of melanocyte stem cells (MSCs) and melanocytes, genetics and epigenetics, race, gender, family history, aging, oxidative stress, stress hormones, systematic disorders, nutrition, smoking, alcohol consumption, lifestyle, medications, and environmental factors. We also examined various plants and phytochemicals that have shown a potential to interfere with the onset or progression of human hair graying at different levels from in vitro studies to clinical studies: the extract of Polygonum multiflorum and its major components, 2,3,5,4′-tetrahydroxystilbene-2-O-β-D-glucoside and emodin; the extract of Eriodictyon angustifolium and its major flavonoid compounds, hydroxygenkwanin, sterubin, and luteolin; the extracts of Adzuki beans (Vigna angularis), Fuzhuan brick tea (Camellia sinensis), and Gynostemma pentaphyllum; bixin, a carotenoid compound found in Bixa orellana; and rhynchophylline, an alkaloid compound found in certain Uncaria species. Experimental evidence supports the notion that certain plant extracts and phytochemicals could alleviate hair graying by enhancing MSC maintenance or melanocyte function, reducing oxidative stress due to physiological and environmental influences, and managing the secretion and action of stress hormones to an appropriate level. It is suggested that hair graying may be reversible through the following tactical approaches: selective targeting of the p38 mitogen-activated protein kinase (MAPK)–microphthalmia-associated transcription factor (MITF) axis, nuclear factor erythroid 2-related factor 2 (NRF2), or the norepinephrine–β2 adrenergic receptor (β2AR)–protein kinase A (PKA) signaling pathway.