"In the last couple years, and especially in the last few months leading up to my May 5th fight, while sparring I would notice that when I got hit, it would affect me more and more," the 29-year-old Denis wrote in his blog.
"When I first started sparring I would run through punches unaffected. Not only that, but now training at Tristar (Gym in Montreal), I am literally training with the world's best. We are all training at the highest level, all for the same reasons.
"Could I fight in the UFC, against the best fighters in our solar system, literally trained killers, without sparring in training? Not really, so what was I to do? I have made the decision to retire."
Denis (11-3) is no ordinary fighter. He has bachelor's and master's degrees in biochemistry, and put his PhD studies on hold at the start of 2011 to focus on fighting.
He won his UFC debut in January, stopping Joseph Sandoval just 22 seconds into the fight with a string of nasty elbows.
It's believed to be the fastest knockout by a Canadian in the UFC since Gary Goodridge stopped Paul Herrera in 13 seconds at UFC 8 in 1996.
The five-foot-seven Denis collected US$45,000 for knockout of the night. He used some of money to take his girlfriend to Cuba for a week and to buy a two-horsepower, top-of-the-line Vitamix blender — the Rolls-Royce of blenders.
He lost by first-round submission to fellow Canadian Roland Delorme in May.
"I accomplished one of my goals," Denis said in his blog. "I made it to the UFC, signed a five- fight contract, made an amazing debut, some said one of the best debuts in the UFC ever, and had a great second fight that I am very proud of, but ended up breaking my orbital bone and ultimately losing by submission with a second left in the round.
"I am going to hold those moments close to me, because they will be as far as my dream goes. I have decided to bow out of MMA."
Denis said he started researching concussions after suffering his first loss in May 2009 by KO to Marlon Sandro in the Japanese promotion Sengoku.
Denis writes that Sandro "dribbled my head on the canvas like a basketball."
"As a graduate student at the university of Ottawa, I had access to all peer-reviewed scientific journals. No surprise to find that concussions = bad.
"However, I found something that had never occurred to me. Sub-concussive trauma. Basically, a blow to the head that doesn't lead to a concussion. When it happens, you feel fine, and continue on. Maybe you feel like you just had a little brain scramble, nothing big. Those who spar, know what I am talking about. However these add up. They accumulate, from training session to training session, year after year."
Denis says he told himself he would quit fighting if he had one more concussion, be it in the cage or just slipping on a patch of ice outside his house.
"But what if I never do get knocked out again? What if for the next decade I keep training hard and competing. I get in 'wars' and receive tons and tons of sub-concussive blows. Wouldn't that be orders of magnitude worse than one concussion?"
Denis says he started noticed that hits in sparring were taking a toll.
"I love MMA, and I have loved my experience with the UFC, Sengoku, and every other promotion along the way, but I am a human being first," he writes. "I don't define myself by my work, and nor should you. I am a human being, and I was born with only one brain, and I want to take care of it so that I will recognize the ones I love when I get older."
Denis got into MMA at 20, just looking for something different than lifting weights. He joined a gym and got to meet some fighters.
He tried out for "The Ultimate Fighter" reality TV show but did not make the cut. He caught the UFC's eye in October 2011 when he finished Nick Mamalis with a devastating suplex slam that knocked his opponent unconscious at a Wreck MMA card.
Prior to that bout, he had not fought since a March 2010 loss to Yuji Hoshino at Sengoku.
In the aftermath of that fight, he suffered a pinched nerve in training and his arm went numb.
The next morning, trying to shower or putting on a shirt were impossible. Months of physio followed as he waited for the injured nerve to grow back.
Unable to train immediately after the injury, he and a friend drove across Canada and back — camping or sleeping in their car. He made new friends along the way, taught a few martial arts seminars and tried his hand at surfing in Tofino, B.C.
"Lots of good memories,'' he said of the monthlong trip.
A native of Ottawa, he now lives in Montreal.
His fight nickname came from one of his coaches in Ottawa.
"Fight time I'm serious, like a ninja I guess you could say,'' Denis explained. "But in general I'm not a rough guy, I'm not a mean guy, I'm not going to talk trash. I'm just looking to have a good time — so I guess I'm the ninja of love.''