Johnny Manziel says he withered from 210 pounds all the way down to 170 just months after his NFL career ended ... explaining his body morphed due to "a strict diet of blow."

The former Heisman Trophy winner opened up about his struggles following his Cleveland playing days during a recent, two-hour interview with Shannon Sharpe.

In the chat on the "Club Shay Shay" podcast ... Manziel admitted he was at nearly rock bottom once he had failed the Browns in 2015 at 23 years old -- so he turned to the Las Vegas party scene to pick him up.

Apparently, his trip to Sin City included a lot of cocaine ... something he says changed his figure into an unrecognizable one.

"At that point in time, man," Manziel said, "I would look in a mirror and I didn't see myself any different than when I was in Cleveland. Until I stepped on a scale at the Cosmopolitan Hotel in Las Vegas, I didn't realize I had lost 40 pounds at all."

"I remember these pictures came out," he continued. "I was like, 'Damn. What am I doing? Ah, whatever, we'll figure that out later. Let's go again.'"

Manziel said in addition to coke, he was also using OxyContin and Percocets.

Johnny Football went on to lament other parts of his life in his talk with Sharpe -- including his strained relationship with LeBron James.

After Cleveland had drafted the Texas A&M QB, LeBron signed him to his agency ... and throughout his time with the Browns, Manziel said the NBA superstar tried to take him under his wing. But, while battling poor play and subsequent depression, he said he pushed the hooper away.

"LeBron would text me every week to come over to the house and watch a game or play poker with the boys and just tried to be there," Manziel said. "And I was so depressed for the first time in my life that even my biggest role model and inspiration in my life couldn't get me out of bed to come and hang out with him."

Manziel added that he still feels to this day he let the future basketball Hall of Famer down.