I don't post often on forums but I saw this thread and dipped in as I had also checked up optimal race weight and saw the Stillman formula. From taking a brief look at it I realised that the weights expected for optimal performance were way too low for me. I am 40 and female, and although I know I could do with losing a stone to run faster and feel better I also know that I am a size 10 and look ok doing as I am. My non running friends would say I would look too thin if I even dropped a stone.
I work in psychology and worked in Eating Disorders for 3 years. From that experience and knowledge I do know the recommended BMI's. It's 3 years since I worked in the field and things may have changed but we took any BMI between 18-25 as normal (also accepting that a BMI between 25-30 could be ok if the person was an athlete e.g rugby player as more muscle mass, so can be misleading as otherwise a BMI of that would show to be overweight).
A BMI below 17.8 as being anorexic. So, BMI of 15-17.8 was classed as anorexic and BMI of 15 -13.5 as severe anorexia. A BMI of 14-13.5 would require hospital admission for re feeding and often forced sectioning. What is also worrying is the fact that muscle weighs more. So, if people who have a low BMI and also exercise alot it gives a false weight as we all know muscle weighs more than fat, so often actually lower than someone who does no sport or exercise.
These results were lower for children and teens as they do tend to weight less, but as an adult were seen as the guidlines. So If the gentleman who posted the thread does have a real BMI of 14.5 and is doing alot of running it is likely his true BMI is lower as very little fat and mainly muscle. If that is the case, and this is not a wind up I would suggest there maybe a problem and should seek some professional help!
Other complicating factors include whether the person uses other compensatory means for weight loss such as laxatives/diuretics or purging. Interestingly exercise is also seen as a compensatory method, and of course you have the problem of electrolyte problems with rhythm of heart (particularly sodium and potassium which are very important in cardiac conductivity).