Does the Human Body Really Replace Itself Every 7 Years? by Benjamin Radford, Live Science Contributor | April 04, 2011 11:33am ET 438 141 18 Submit 151 Reddit Credit: Dreamstime View full size image It's a neat idea, and one that has caught the popular imagination. Here's how the story goes: Every seven years (or 10, depending on which story you hear) we become essentially new people, because in that time, every cell in your body has been replaced by a new cell. Don't you feel younger than you were seven years ago? It is true that individual cells have a finite life span, and when they die off they are replaced with new cells. As The New York Public Library's Science Desk Reference (Stonesong Press, 1995) notes, "There are between 50 and 75 trillion cells in the body.... Each type of cell has its own life span, and when a human dies it may take hours or day before all the cells in the body die." (Forensic investigators take advantage of this vaguely morbid fact when determining the cause and time of death of homicide victims.) Red blood cells live for about four months, while white blood cells live on average more than a year. Skin cells live about two or three weeks. Colon cells have it rough: They die off after about four days. Sperm cellshave a life span of only about three days, while brain cells typically last an entire lifetime (neurons in the cerebral cortex, for example, are not replaced when they die). There's nothing special or significant about a seven­year cycle, since cells are dying and being replaced all the time. It's not clear where this myth began; perhaps some well­meaning but innumerate person simply added up the all the lifespans of the body's various types of cells and (mistakenly) assumed that all the cells are renewed after seven years. Does Hypnosis Really Work? What Are the Ingredients of Life? Why Does Acid Make People Trip? More from LiveScience Aww! Primordial Reptile Parents Should Limit Wow! The Most Amazing Most Interesting Science Fossils Show Mother Kids' Pizza Consumption, Images in Science This News Articles of the Caring for Babies Researchers Say Week Week Couples Who Work Mummy Mask May Together to Get Healthy Reveal Oldest Known Have More Success Gospel Author Bio Benjamin Radford Benjamin Radford is a the Bad Science columnist for Live Science. He covers pseudoscience, psychology, urban legends and the science behind "unexplained" or mysterious phenomenon. Ben has a master's degree in education and a bachelor's degree in psychology. He has written, edited or contributed to more than 20 books, including "Scientific Paranormal Investigation: How to Solve Unexplained Mysteries" and "Tracking the Chupacabra: The Vampire Beast in Fact, Fiction, and Folklore." He sometimes appears on television but doesn't like to watch himself. He has also written and directed two short films and created a board game..