Today on Health and Longevity, Dr. John Westerdahl's featured guest is National Geographic photographer, David McLain. In November 2005, David McLain's photojournalism  appeared in National Geographic's featured cover issue and story, "The Secrets of Living Longer," which for the first time, introduced three Blue Zone regions of the world; Okinawa, Japan; Sardina, Italy; and Loma Linda, California. Blue Zones are considered areas of the world where among the world's healthiest and oldest living people live, including many people over a hundred years of age. David McLain discusses his experiences in exploring and photographing these cultures of longevity. He shares the lessons he learned from observing the diet and lifestyle of the people he met, including several centenarians, that contributed to their health and longevity.