No sympathy expected, but my body-fat percentage (according to my fat-calliper and bizcalcs.com 4-fold) is 14% - and according to bodyfatchart that makes me within a kilo or two of being underfat. Another source or two puts me somewhere near the fit/athletic borderline. According to the nonsense they call WHO/NHS BMI, I am overweight at BMI 26.2!
My father was 24 stone when he was diagnosed with diabetes, and three other blood-relatives are chronically morbidly obese - so it seems difficult to imagine that anyone in my family could get underweight!
I was thinking of doing another 68-hour fats this weekend, but I like to get a pound or two above my target weight before fasting.