A man with a mysterious case of full-body hives in Abu Dhabi. A Californian rushed to the hospital with a life-threatening allergic reaction. A fit, gym-going man in Texas with soaring blood sugar — inexplicably headed for pre-diabetes. Doctors working on the bleeding edge of longevity care in high-end clinics around the globe — from concierge practices in wealthy enclaves of California to premier longevity centers in the Middle East — say they're seeing an uptick in both interest in, and