Rg5 & Rk1 : the secret tandem behind black ginseng
1. WHITE, RED OR BLACK GINSENG: A MATTER OF GINSENOSIDES
Ginsenosides are considered to be the main active substances from Panax ginseng roots. However, not all ginsenosides are equal in terms of health benefits. Indeed, the most common ginsenosides like Rb1 or Rg1 have poor bioavailability and therefore limited physiological impact by themselves. That said, it has been discovered that these ginsenosides can be partially converted by gut microbiota to more bioavailable forms of ginsenosides [1-2]. For instance, Rb1 is transformed in Rd which can then be transformed in Rg3, a ginsenoside considered to be bioactive (more info about Rg3 here soon). However, ginsenoside bioconversion by gut microbiota is both variable (depending of the microbiota status of the individual) and partial .
Alternatively, root processing by single steam cooking (= red ginseng), multiple steam cooking (=black ginseng), fermentation (fermented ginseng) or even enzymatic treatment can transform the ginsenoside profile and produce more bioavailable forms directly in the root, leading to better bioavailability of the active cosntituents [4-7]. For instance, red ginseng contains more Rg3 than white ginseng, black ginseng contains more Rg3 than red and white ginseng. Black ginseng also contains Rg5 & Rk1 (products of the dehydration of Rg3) which are considered to be bioactive forms too. It should be noted that black ginseng, due to the fact that it contains bioactive forms of ginsenosides, has been identified as more performant in a series of conditions and more specifically in the metabolic regulation area [8-12].
2. HEALTH POTENTIAL OF THE TANDEM Rg5:Rk1
2.1. Metabolic regulation
As previously mentionned, black ginseng has been identified as particularly interesting in the context of metabolic regulation, both for its activity on glycemia and on lipid metabolism. These properties has been correlated to bioactive ginsenosides such as Rg3, Rg5 and Rk1. The last two are exclusive to specific cooking procedures (black ginseng, sun ginseng, etc.) and rarely found in classical ginsengs. Therefore, they recently have been the subject of multiple studies to validate their impact on metabolic regulation. The experimental data confirmed the efficacy of the ginsenoside tandem in multiple models [13-17]. It should be noted that Rg3 has also been identified as potentially beneficial in these fields (see here). The increasing number of scientific data about these three bioactive ginsenosides and their positive impact on sugar and fats metabolism clearly invites to consider them as an option for early stages of “metabolic imbalance”.
2.2. Neurological benefits
As for Rg3, Rg5 & Rk1 have recently been identified as beneficial in the neurological area, including depression  and sleeping issues . However, most of the literature on the neurological impact of these ginsenosides, is referring to neuroprotective properties  including preservation of cognitive capacities [21-22] and positive impact in neurodegenerative models .
2.3. Cardiovascular effects
Compared to Rg3, scientific data on the cardiovascular benefits of Rg5 & Rk1 are relatively limited and mainly limited to confirmation of cardioprotective , vasculoprotective  and anti-aggregation activity  . However, that does not mean that they are less effective in this field. Indeed, Rg5 have been identified as a strong vasorelaxant ginsenoside  with Sildenafil-like activity in the erectile function context .
2.4. Potential in oncology
For now, Rg5 & Rk1 have essentially been tested in different breast cancer models, showing good potential [29-32]. However, several studies also suggest an interest in other cancer types [33-39]. It is however too early to say if this ginsenoside tandem is going to present an equivalent potential to Rg3 regarding oncology.
Like Rg3, Rg5 & Rk1 are considered as strongly bioactive forms of ginsenosides. The increasing number of investigations about their health potential seems to indicate they share “classical properties” associated to ginseng, but also suggests a particularly interesting activity in two health fields: Metabolic balance & Erectile functions. Rg5 & Rk1 are rare in classical ginseng. However, they can be found in premium ginseng preparations which were specifically designed to obtain optimized ginsenoside profile with maximal health benefits. Like for Rg3, all this data clearly prove that not every ginsenosides are equal in terms of health benefits and that you should dig deeper into ginsenoside profile if you want an ingredient of “clinical performance”.
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