Joey Chestnut's longtime opponent revealed the news in the Netflix documentary, "Hack Your Health: The Secrets of Your Gut" ... saying he's ready to focus on his well-being and repair the damage he's done to his body over the past two decades.

Kobayashi rose to fame after winning his first Fourth of July contest in 2001 ... and went on to earn the mustard belt five more times.

Chestnut dethroned the Japanese standout in 2007 ... and the two would go on to have epic battles for years to come.

But everything came to a screeching halt in 2010 ... when Kobi reportedly found himself in a contract dispute with Major League Eating. He famously attended that year's hot dog contest as a spectator ... and was arrested after hopping onstage at the conclusion of the event.

He continued to show off his wild eating accomplishments over the next several years ... but never returned for a rematch against Jaws.

Kobi explained his decision ... saying he lost his appetite and ability to even smell food -- which was a wake-up call for him to make drastic lifestyle changes.