Mark Sisson (born July 1953) is an American fitness author and blogger, and a former distance runner, triathlete and Ironman competitor. Sisson finished 4th in the February 1982 Ironman World Championship. He has written several books, including The Primal Blueprint, which incorporates aspects of the popular Paleolithic diet. The Primal Blueprint is his fifth book. Sisson also blogs at his website Mark's Daily Apple.