The mechanism of the beneficial effects of Panax quinquefolius (Xiyangshen, American ginseng) on diabetes is yet to be elucidated. Recent studies show that Panax quinquefolius increases insulin production and reduces the death of pancreatic beta cells. Mechanism studies indicate that Panax quinquefolius improves cell's immuno-reactivity and mitochondrial function through various factors. Clinical studies show that Panax quinquefolius improves postprandial glycemia in type 2 diabetic patients. Further studies to identify the component(s) of Panax quinquefolius linked with pancreatic islets/beta cells in vitro and in vivo are warranted for better understanding of the full effects of Panax quinquefolius.

The main active components in ginseng responsible for ginseng's medical value have been identified as glycosides. Glycoside is a naturally occurring substance consisting of a sugar and non-sugar moiety. Some glycosides belong to a family of compounds named saponins which produce froth under agitation by reducing water surface tension. A group of saponins in ginseng have been named ginsenosides and classified into subclasses as Ro, Ra, Rb, Rc, Rd, Re, Rf, Rg and Rh. These ginsenosides can be differentiated in thin layer chromatography (TLC) based on their retention factor value, which is the distance that ginsenosides travel up the TLC column [12]. Panax ginseng can be distinguished from Panax quinquefolius by using ginsenoside profiles because Panax ginseng contains ginsenoside Rf which is absent in Panax quinquefolius [13]. The anti-hyperglycemic effects of the total ginsenosides extracted from Panax ginseng were studied in diabetic C57BL/6J ob/ob mice. The results indicate that the total ginsenoside extract has significant anti-hyperglycemic and anti-obesity properties [3]. In the same animal model, it was revealed that the anti-hyperglycemic effect of ginsenoside Re was due to improved muscle metabolism and that ginsenoside improved diabetes by alleviating inflammation through increasing C-reactive protein levels [14]. Rh2, another ginseng component, lowered plasma glucose level in Streptozotocin (STZ)-diabetic rats in a dosage dependent manner [15]. Recent studies support the notion that ginsenosides target steroid hormone receptors, which suggests that these ginseng components act as natural steroids, offering a diverse range of pharmacological activities [16].

Panax quinquefolius lowers blood glucose in diabetic patients [9], increases pancreatic beta cell insulin production and secretion, and prevents beta cell apoptosis [17]. The results also show that the effects of Panax quinquefolius in stimulating insulin production/secretion and anti-apoptosis are dosage dependent, suggesting a biological specificity of ginseng water extracts on beta cells. A high dosage of Panax quinquefolius is probably required to reverse IL-1 beta induced apoptosis. According to a clinical study by Vuksan et al., Panax quinquefolius attenuated postprandial glycemia in both type 2 diabetic and non-diabetic patients. While no difference was found in postprandial glycemia between placebo and ginseng groups in non-diabetic subjects when administered together with glucose challenge, in subjects with type 2 diabetes significant reductions were observed when ginseng was taken 40 minutes before the glucose challenge [8].

The mechanism for the beneficial effects of Panax quinquefolius has not been fully elucidated. According to our findings, Panax quinquefolius water extracts reversed an IL-1 beta induced increase of uncoupling protein-2 (UCP-2, a mitochondrial protein which uncouples electron transport from ATP production), decreased pro-apoptotic protein caspase-9 and increased the level of anti-apoptotic protein Bcl-2. This indicates that the anti-apoptotic effects of Panax quinquefolius may be achieved through regulating mitochondrial UCP-2 and ATP levels, thereby influencing insulin synthesis and apoptotic cascades in beta cells [17] (Figure (Figure1).1). While cell metabolism seems to be a critical factor targeted by Panax quinquefolius, there are other reports on additional factors targeted by ginseng such as immuno-reactivity [18]. Some other reports indicate that Rh2 lowers plasma glucose level induced by an increase of beta-endorphin secretion in STZ-diabetic rats, which activates opioid mu-receptors. The activation of opioid mu-receptors increases the expression of GLUT 4, which raises muscle and adipose tissue sensitivity to glucose stimulation, thereby promoting tissue absorption of glucose, which leads to a reduction in blood glucose level [15]. Another study on STZ-diabetic rats demonstrated that ginsenoside Re relieved oxidative stress in the kidney and eyes, and reduced blood glucose, cholesterol and triglyceride levels. These findings suggest that different components of ginseng may act together to achieve its overall therapeutic effects [19]. Protopanaxatriol (PPT), one of the ginsenoside metabolites, increases GLUT4 and improves insulin resistance [20]. Wang et al. reported that Panax quinquefolius exerted an anti-lipolytic effect on type 2 diabetic conditions through a signaling pathway different from that of insulin, suggesting that Panax quinquefolius not only affects pancreatic cells through increasing insulin production and cell viability, but also through creating an anti-lipolytic effect and targeting glucose receptors to counter hyperglycemia [21]. Not only has ginseng root been found to have positive effects, but also ginseng leaf and berry extracts have been found to have potent anti-oxidant activities against free radicals that are excessively produced in diabetic complications [22]. Indeed, these findings are merely the tip of the iceberg in terms of the information required for elucidating the mechanism for Panax quinquefolius to improve hyperglycemia and treat diabetes.

Due to a lack of standardization in the herbal medicine industry, it is not certain whether the beneficial effects on hyperglycemia hold true for all Panax quinquefolius products. It is also not certain whether different ginseng species would have the same effects on hyperglycemia. Sievenpiper et al. discovered that Panax ginseng showed both null and opposing effects on acute postprandial plasma glucose and insulin, which did not coincide with the findings with Panax quinquefolius [31]. Reproducible efficacy using an acute postprandial clinical screening model is necessary to standardize herbal products and to link herbal composition with efficacy in treating diabetes [32]. Further data are also required to confirm that Panax ginseng has the same effects on humans as it does in animal models.

Panax quinquefolius increases cell insulin production and cell viability through mediating mitochondrial proteins and apoptosis factors. These findings confirm that Panax quinquefolius improves pancreatic islet functions. The components of Panax quinquefolius that improve pancreatic beta cell function have not been identified. The precise actions of Panax quinquefolius components on other human cells such as muscle cells and lipocytes under diabetes therapy are yet to be investigated.

The author(s) declare that they have no competing interests.

ZW proposed to cooperate in this project and provided information for the manuscript. JL conceived this project, collected information and wrote the first draft of the manuscript. LL worked with JL to conceive this project, collected references andinformation, wrote and approved the manuscript. All authors approved the final manuscript.