The Effects of Allium Cepa and their Derivatives on Respiratory Diseases and the Possible Mechanisms of these Effects

Allium cepa (A. cepa) or onion is an important condiment plant consumed worldwide. The plant and its constituents, such as quercetin (Qt), thiosulphinates, and phenolic acids, show various pharmacological and therapeutic effects, including the effects on respiratory disorders. In this chapter, various pharmacological and therapeutic effects of A. cepa and its constituents on respiratory disorders are reviewed. Using keywords such as onion, A. cepa, A. cepa constituents, therapeutic effects, pharmacological effects, and respiratory disorders in different online databases were searched until the end of July 2020. A. cepa showed a significant relaxant effect in tracheal smooth muscle (TSM) comparable to the effect of theophylline, indicating the bronchodilatory or reliving effect on obstructive pulmonary diseases. The possible mechanisms of this effect were reported as β2-adrenergic stimulatory and/or calcium channel blockade. The plant and its constituents also demonstrated a preventive effect on inflammatory respiratory diseases by improving lung pathological changes and tracheal responsiveness in various lung diseases. The possible effect of preventive effects of onion and its constituents on lung disorders is their inhibitory effects on inflammatory cells, inflammatory mediators, cytokines, cyclooxygenase and lipoxygenase activity, and oxidative stress markers. Therefore, A. cepa, and its constituents could be of therapeutic value in treating respiratory disorders both through reliving and prevention of lung diseases.