Don't know of jeans or khakis made of different materials, or I would. :p
That's what I do. My diet is fairly balanced, for an American anyway. :P Plenty of fruits and vegetables.
I'm a little overweight, but I'm not "very" fat. I'm male, and I know it's not an allergy because I regularly wear shorts of the same materials, with no problems at all. That also rules out hormones. It literally JUST happens when I wear long pants and get sweaty.