I don't think it is the low-carb diet per say, but rather the type of exercise. "Long and Slow" cardio and extreme endurace events to a greater degree push the wrong hormonal profile for one; high cortisone and low growth hormone and testosteron, while heavy weighlifting and HIIT type exercise pushes the opposite hormonal profile, low cortisone and high growth hormone and testosterone. The result is for athletes that may be consuming the same calories, marathoners will be dissapated, scrawny and have significant body fat percentages compared to a sprinter that will be heavily muscled and low body fat percentage. Also, there are very significant differences on how 'fat' is used as fuel, the end result being that "long and slow" tend to retain fat mass and HIIT tend to burn fat mass. (A good detailed explanation of this phenomenon is in "The Doctor's Heart Cure") Curiously, many "thin" people like CRonbies (brains! brains! brains!), super models and anorexics have much larger percentages of their body mass as fat than first appears. They have small amounts of lean tissue (muscle) and appear "thin". Physiologists where I work use the term "fat-skinny" to describe the condition, and with recent diet trends of starvation and little exercise is more commonly seen outside the cult extremists. I point being, the thin person you see on the street is most likely not very lean. That is another one of the reasons BMI is suspect and largely worthless as an individual guide to health status.

Kind regards