From the age of eight, his life has been plagued by difficult decisions.
The sprinter-turned-rugby player was hoping to join an elite band of athletes this year by competing in multiple disciplines at the Summer Olympics in Rio -- but he faces the dilemma of only being able to choose one.
Isles' childhood in foster care was a far cry from the lifestyle he leads now, and while it hasn't always been the glitz and glamor of the global sevens circuit and training sessions with NFL teams, he is accustomed to making sacrifices.
'It was basically survival of the fittest,' the Ohio native tells CNN.
'I had to fight. I went from home to poor schooling -- I had to eat dog food. We didn't celebrate birthdays, Christmases or anything like that.'
There was a time when Isles couldn't see the light at the end of the tunnel -- and it wasn't just himself he had to worry about.
His twin sister Cambra -- two minutes older -- was always by his side during their youth, after they witnessed their birth mother being driven away in the back of a police car, something Isles rarely speaks of.
He subsequently learned the hard way that he was very much in a race for his life.
'You just had to get by,' Isles reflects philosophically. 'It was about surviving. It was very difficult and, you know, I couldn't see past that darkness.
'So it was just a struggle to wake up every morning and just go through the same stuff over and over again.'
Things did eventually get better, however, when the twins were adopted by Starlett and Charles Isles just shy of their eighth birthdays.
Now 26, Isles' admiration for his adoptive mother is evident in every interview he gives, and she is often referenced as the leading influence in his life and one of the key reasons behind his success.
'She taught me values, wanted the best for us,' Isles explains proudly. 'In that situation she taught us how to get the best out of each other and out of ourselves.
'I always had some heart or some drive to be somebody, so she put me in positions where I could make the most of my dreams and my gift -- I'm always grateful for that.'
With stability at home, things started to fall into place and Isles quickly discovered that one thing set him apart from the rest of his contemporaries: Speed.
'I was pretty fast at eight years old,' Isles claims modestly, though it took some time to figure out what his true calling was with this newfound talent.
Initially starting out playing American Football, Isles soon discovered a penchant for track sprinting, running 60 meters in 6.68 seconds while enrolled at Ashland University, Ohio -- a record that still stands today.
After missing out on a Team USA athletics place at the London Olympics four years ago, Isles' first foray into rugby came in 2012 -- and he scored on his debut for the USA Sevens team against New Zealand.
Isles' 100 meters personal best, incidentally, would have been enough to see him line up alongside double Olympic champion Usain Bolt in the London 2012 semifinals.
But for a man that just loves running fast, you get a sense the sport he ended up playing was inconsequential, as long as he was leaving opponents trailing in his wake.
'I just feel alive,' Isles enthuses, as he opens up on the joy that running gives him. 'It's just this incredible feeling I get inside me -- it's just like I'm on top of the world, on cloud nine.
'It's a rush, you get your body just so in the moment. It's indescribable what I really feel.'
Quickly dubbed 'the fastest man in rugby,' Isles' super speed even earned him a call-up to the Detroit Lions practice squad in 2013, an NFL experience he admits he 'would like to try again.'
He then had a short stint with Scottish rugby union team Glasgow Warriors, but left without making a first-team appearance as he decided to return home to focus on being part of sevens' Olympic debut in Rio.
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His decision was quickly vindicated as he soon became the stand-out star in an explosive USA team.
A YouTube video compilation of his finest moments in a rugby shirt has racked up nearly seven million views, but with the emergence of South African speedster Seabelo Senatla and World Cup winner Bryan Habana transferring over to sevens, is Isles still the fastest man in rugby?
'Ohhh yeah, without a doubt,' he quips. 'But those guys are pretty quick...'
After the U.S. finished sixth in the past two Sevens World Series standings, Isles is confident he and his compatriots can secure a medal in Rio.
'I think a medal is a reasonable goal, I know we can do it,' he says.
'If we start putting everything together, continue to work hard and get better day by day, then without a doubt we can make that happen.'
Isles missed March's Vancouver Sevens in an attempt to resurrect his sprinting career and earn a place on Team USA's athletics roster.
He competed at the USA Indoor Track & Field Championships in Oregon, coming fifth in the 60-meter final with a time of 6.67 seconds.
But this is where things begin to get tricky for Isles.
USA Sevens' charismatic coach, Mike Friday, doesn't believe Isles is capable of breaking 10 seconds and propelling himself into the upper echelons of the country's fastest runners.
'There is no juggling (two sports). I'll quash this right here and now,' the Englishman told CNN at sevens' London finale, which Isles missed as he was recovering from injury. 'There's not a cat in hell's chance Carlin will be on the track team for Team USA.
'More importantly, Carlin's not quick enough to make the USA track team. I think seven to 10 U.S. track athletes went sub-10 seconds last year.
'So, while he is the fastest man in rugby union, and in a lot of other countries he would probably be fast enough to make their track team, he's just over 10 seconds,' Friday adds.
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'He could run 10 seconds flat with a lot of training, but there's seven of them ahead of him who have run sub 10.'
And, even if Isles' grueling training regime and competitive athletics meets power him to sub 10, he will have to face the most difficult decision of his career: Rugby sevens or athletics?
'It's not a question of me saying he can or can't do it -- he's not going to be able to do both,' the 44-year-old Friday explains.
'He can have a go, of course he can, but the reality is he's in danger of falling between two stalls, and he knows that.
'We let him go to the indoor nationals more as a release for him and an opportunity for him to see where he's at on the track, but he's fully committed to trying to make this Rio 12 (sevens squad), first and foremost. Let's lay that one to rest here and now.'
Friday says Isles' fighting attitude and determination, as well as the personal adversity he has overcome, played a part in allowing him to chase his dream.
'He's a big personality, he's a fierce competitor, he's a born fighter. He's had to fight for a lot of things before rugby and in his personal life, and you can see that the way he plays the game,' he says.
'There's no diva about Carlin, he's a born fighter. His commitment to being the best he can be, what he's had to endure in the public eye, coming across from athletics to rugby and having to do his education in the public eye in front of the screens.
'He got a lot of criticism in the first couple of years, which you'd expect because he's learning the game, but he persevered. He's got great resilience and he stays committed to the task. That's what I love about him -- he's an insane competitor and he's always trying to get better.'
Isles is yet to break the 10-second barrier over 100m, but he believes the muscle bulk he has developed in training for rugby can only serve to aid his double Olympic attempt.
A time of 10.16 seconds in a U.S. Olympic Trials event will secure Isles automatic qualification to the final July 1-10 trials.
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Though Friday has his doubts, Isles has total belief he can go below 10 seconds and put himself in contention for a sprinting spot in Rio.
'Yeah, I think I can go below that,' Isles replies quickly and confidently. 'I always thought I could, but it's about putting everything together.
'Over 60m I know I can run 6.5 seconds and over 100m I know I can go sub 10 seconds -- but it's about putting everything together to make sure that I'm good enough to be able to.'
While the odds appear stacked against Isles yet again, for a man who has already overcome so much adversity and once confidently claimed he could beat Bolt, you wouldn't bet against him.
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