Note that all of the authors of this paper work for a company that sells this snake oil.

This entire idea of charging or discharging your body for some physiological effect is fundamentally flawed. It is indeed true that touching the ground will change your electrical potential to that of the ground, however that's as far as it goes, the rest is just nonsensical.

Let's start with the very first part of that, specifically changing your electrical potential to match the ground. The problem with this is that the ground is 0V because we define it as 0V, not because of any sort of fundamental property. Between two points on the Earth you can see hundreds of kilovolts. So the obvious question is why should the electrical potential of a particular piece of soil where your grounding rod is installed be physiologically preferable to whatever potential your body happened to be at before?

Now lets go on to the next part of what these scam artists usually claim. That is that the Earth's electrons enter your body and do good things of some description. When you touch the ground some small charge does enter or leave your body, however that stops nearly instantly when the potentials equalise[1]. How is the Earth meant to to keep putting electrons into your body if there's no voltage? That's just simply not how electricity works.

As an aside, note that I said enter or leave. A positive static charge and a negative static charge can both easily build up in your body. Therefore any claims that specify that electrons enter your body (or leave you body) is immediately nonsensical because it could go either way.

I'll leave it to someone else to explain all the nonsensical biological claims that follow on from the already fundamentally wrong electrical side since that's not my area. I will however note that none of these sponsored papers actually show and measurable biological effect, they all just hand-wave about redox reactions.

[1]: Yes, you're coupled to mains wiring and other changing fields that are around, however that's getting too far into the weeds for a short comment. If the claim made by the snake oil salesmen was that a current going through your body is good then they'd also suggest wrapping yourself in an electrical cord to maximise the effect, or just taping batteries to your skin.