Beneficial Effects of Plant Polyphenols on Obesity

Plant polyphenols are found in foods such as tea, apples, onions, berries, citrus fruits, plums, broccoli, cocoa, and coffee. Epigallocatechin-3-gallate, chlorogenic acid, curcumin, quercetin, and resveratrol are representative polyphenols and have been extensively studied for their health benefits. This review article provides evidence derived from epidemiological, clinical, and laboratory studies on the anti-obesity effects of plant polyphenols, and discusses their mechanisms of action. Most cell-based and animal studies have described their anti-obesity effects, although human studies have shown conflicting results, indicating that further studies are required to determine whether polyphenols indeed have preventive and therapeutic significance in obesity and related diseases. Basic research has suggested that the primary mechanisms of action of plant polyphenols include involvement of reactive oxygen species (ROS), 5’-AMP-activated protein kinase (AMPK), and the transcription factor NF-κB. The involvement of ROS in AMPK activation has been well documented, and the pro-oxidative properties of plant polyphenols can lead to the activation of AMPK, resulting in the modulation of various signaling pathways involved in adipogenesis, lipogenesis, and lipolysis. NF-κB is another key mediator of the mechanisms of action of plant polyphenols, and can modulate the protein and gene expression of proand anti-inflammatory cytokines that play important roles in obesity. The polyphenols described in this review are established antioxidants, and their ROS-scavenging properties may mitigate ROSdependent NF-κB activation. At present, the factors that direct these polyphenols to act as either pro-oxidants or antioxidants remain unclear, although polyphenol concentration and the presence of metal ions have been suggested. Furthermore, the observation that activated AMPK can downregulate NF-κB may provide an alternative explanation for the activities of plant polyphenols. Future studies are required to confirm the mechanisms of action of these polyphenols, and may subsequently contribute to their evidence-based application in human health.