With bulimia nervosa, a person feels the need to purge following food consumption.
Exercise bulimia, however, means engaging in excessive exercise to eliminate calories from the body - which causes the person to be at the gym for hours on end. It is, as Mr O'Neil describes, "compulsive exercise as a form of purging".
"Exercise bulimia manifests itself in different ways - from excessive exercise to compensate for calories consumed, to starving oneself but continuing to exercise, to an all-consuming obsession with exercise to the point of serious self-harm, as in my case at the moment," he writes.
"In all cases, the results can be debilitating, both mentally - emotional distress when you're unable to exercise sufficiently - and physically - bone density loss from lack of nutrition, joint pain, constant muscle soreness, recurring injuries, and persistent fatigue."
Mr O'Neil admits he goes to the gym at least 360 days in the year.
"You wouldn't know it by looking at me," O'Neil says.
"I am by no means the picture of health or even particularly muscular-looking - not for someone who exercises this much, and definitely not compared to most of the men I see at my gym."
Or maybe he is, but the disorder has given him doesn't "correspond with reality".
"I see a fat piece of sh*t, and then I think to myself that it's time to punish my body for letting me down."
But Mr O'Neil's motivation is to "earn the next day's meal". His relationship with exercise is a form of punishment, and only concludes when he feels like he's earnt it. The disorder is "as destructive as any other type of addiction".
"Once I feel I have earned it, I will eat a large meal, thereby resetting the cycle of guilt, and begin the process all over again tomorrow," he writes.
Once upon a time, Mr O'Neil said he thought his exercise regimen was "admirable, a point of pride".
But now, as he explains, his exercise obsession is far more than just an active lifestyle.
"I will run until my knees ache and my back stiffens," he writes.
"I will manage the ensuing pain with too much Advil. Being skinny, even with back pain, feels a lot better than being chubby."
According to Australia's National Eating Disorders Collaboration, the rates of body dissatisfaction in males are rapidly approaching that of females.
"For males, body dissatisfaction is more commonly manifested as the pursuit of a muscular, lean physique rather than a lower body weight," they write.
"For some males, heightened concerns about muscularity may become part of an eating disorder, characterised by distorted perceptions about muscle bulk, and /or distorted eating and exercise patterns."
Mr O'Neil hopes his account and recognition of his disorder will help other acknowledge symptoms of "exercise bulimia".
"You're not alone, and it's OK to talk about," he writes.
"You're not going to die from the embarrassment. Your eating disorder, on the other hand, might do the trick if you let it."
To read Luke O'Neil's full story, visit www.esquire.com.