A few days before his former teams would meet, Matt Birk sat in a Whole Foods parking lot in his adoptive home of Naples, Fla., speaking passionately about his dramatic weight loss, his new job with the NFL, his crusade against obesity and his belief that Adrian Peterson will finish his career as the greatest running back in NFL history.

Birk won't travel to Baltimore for the Vikings game Sunday, a year after he won a Super Bowl with the Ravens before retiring. "I wanted so badly to be there, but it might be best that I couldn't make it," he said. "If I had shown up, I'd probably be forced to pick a side. This way, I can be Switzerland.

"Now I know how Archie Manning feels when the Giants play the Broncos."

It wasn't a coincidence that Birk was sitting in a Whole Foods parking lot. He's lost about 75 pounds from his playing weight of 310 pounds, and posed for a picture that looks like his head has been photoshopped to the body of an Olympic gymnast.

"Who'd have thought you'd ever see that?" Birk said, laughing. "I know I didn't."

Birk thought he would lose weight quickly after he retired. The first 30 pounds melted off quickly. "Then, I got stuck," he said. He wound up trying a nutritional system called ViSalus. He became so ripped that he wound up posing for promotional photos for the company.

"Not to get too deep here, but I was a fat kid growing up," Birk said. "All my life, I struggled with weight. I say 'struggled' in quotation marks because being large plays well if you want to play in the NFL. Being big becomes part of your identity and part of who you are, good or bad."

The photo helped Birk become a nutritional consultant for current and former NFL players, and an advocate for children's nutrition.

"Because of what I went through, and that photo, I've been thrust into this role of helping people, and it's cool," Birk said. "I've embraced it. There really is a problem with obesity in our country, with our lifestyles, and how busy we are and how little emphasis we put on how we eat. That's troubling to me.

"So much of what we eat now is about convenience. Everything comes in a bag. It wasn't like that before. I'm trying to formulate some sort of plan or strategy to combat that problem."

The St. Paul native has moved his family to Naples. He works as an appeal judge for on-the-field fines, sometimes working out of the NFL offices in New York, sometimes in shorts and sandals at home.

"It's a great job, and I think it's important work right now," Birk said. "These fines mostly stem from player safety issues. The reason you're getting fined is you're endangering another player."

Birk's career was remarkable. He attended Harvard and was drafted in the sixth round by the Vikings. He became an All-Pro with the Vikings and won a Super Bowl with the Ravens. Sunday's game will spark all kinds of memories.

"When you're playing, it's all just kind of fun, like an embarrassment of riches," he said. "Everything is so great about it. Then you get to the other side and you can digest it and think about it and compare it to everyday life, and you're like, 'Man, that was even better than I thought it was, even greater than I thought it was when I was playing.' You get a heightened appreciation for it and I do feel humbled and grateful that I can fulfill this role right now and try to give back to the game, try to make the game a little better."

Birk played with a rookie named Adrian Peterson and a veteran star named Ray Lewis. What do they have in common?

"People take them for granted," he said. "They became victims of their own success. You'd have thought Adrian was having a horrible year this year if you listened to what was said about him, and he's leading the league in rushing and averaging 100 yards a game. You know how hard it is to average 100 yards a game?

"And he's doing it in a season in which he's been injured, he's dealt with personal tragedy, for a team where completing a forward pass seems like an impossible task."

Birk was reminded that Peterson, who passed 10,000 yards in career rushing last week, is only 28. "That's crazy," he said. "I'm on record as saying he'll be the best ever. If you're a kid who's 18, who's been watching Adrian for seven years, and Adrian Peterson has been the only great running back in your lifetime, you don't know how special he is. When I grew up, that guy was Barry Sanders. And Barry is great. But when all is said and done, Adrian will be the guy.

"I've never seen a guy approach every day at practice and every game with the mind-set he does. He does not take a day off, a play off, even in walk-throughs. The quarterbacks used to be taken aback by how hard he'd come for the handoff. He'd take your arm off. You've got to be ready for his handshake, and you've got to be ready for the handoff, or you'll get hurt. Adrian Peterson is great for a reason.''

Jim Souhan can be heard weekdays at noon and Sundays from 10 to noon on 1500 ESPN. His Twitter name is @SouhanStrib. • jsouhan@startribune.com