The former sixth overall pick in the 2010 draft has completely transformed his life in retirement ... embarking on a mental and physical journey through long-term fasting.

Okung -- who weighed more than 300 pounds during his playing days with the Seahawks, Broncos, Chargers and Panthers -- went on his first fast earlier this year, in which he lost more than 100 pounds.

The former offensive lineman is now 192 pounds after completing his second round of fasting over the weekend ... but he makes it clear the challenge is not all about losing weight.

The 34-year-old explained his decision to go through with the process for a second time ... saying, "Ultimately, I was still transitioning out of the NFL, I had a lot of questions about the world."

"My entire worldview was completely shifting. There's still a lot of questions that I have. I don't want to say that they went away, but I do want to acknowledge that I have much more clarity, peace and acceptance that there are things that I cannot change," he added.

"There are things that I can change and I am really [interested] in bringing my entire self to every experience that I encounter. I'm very happy 'cause I feel rooted. I feel grounded. I know why I'm here."

Russell shared a video of his first meal on Sunday ... and he was overcome with emotion as he sipped juice.