I'm male, but for the longest time I was 5'10" (still am), but weighed right around 100 pounds. I tried everything to gain weight, but it just wouldn't come...until I hit my mid to late thirties, then boom, the weight came. (Up to 175-ish now, and feel much healthier!) So, I always had the same problem but in reverse: girls don't like rail-thin guys much. Probably a survival trait from ancient times - at 100 pounds (wet), how much of a protector could I be?