Know yourself; love yourself; be true to yourself. These old adages have been batted around throughout the years by a whole cross section of artists, entertainers and philosophical types, from John Paul Sartre ("We only become what we are by the radical and deep-seated refusal of that which others have made of us") to Bob Dylan ("If you try to be anyone but yourself, you will fail") to Katharine Hepburn ("If you always do what interests you at least one person is pleased").
But how do you get to know yourself when you are constantly changing? Whether it's shedding skin, renewing the lungs or growing new hairs, the human body is in constant flux.
According to researchers, the body replaces itself with a largely new set of cells every seven years to 10 years, and some of our most important parts are revamped even more rapidly [sources: Stanford University, Northrup].
Some of you may be thinking, "Well, that explains why my spouse/sibling/parent/co-worker acts like a little kid." Others might be expecting those new cells to be the key to a longer life. Unfortunately, it's a little more complicated than that.
Body Rejuvenation 101
In the early '50s, researchers discovered the body's rejuvenating power by – yes, really – feeding and injecting subjects with radioactive atoms and observing their movement. They found that, on average, 98 percent of the Wade].]atoms inside the body – the smallest units of matter, which form the molecules that help comprise bodily cells – are replaced each year. Most new atoms are taken in through the air we breathe, the food we eat and the liquids we drink [source: NPR].
More than five decades later, Swedish molecular biologist Dr. Jonas Frisen studied body tissue renewal by measuring levels of a radioactive material called carbon-14. This material was released in the air before testing nuclear weapons aboveground was banned in 1963. Carbon-14 is breathed in by plants, which humans and animals eat every day, and is part of our DNA. But unlike other atoms and molecules that are constantly changing, a person's DNA remains the same from the day of a cell's birth – which occurs when a parent cell divides – throughout its life span. When a cell divides, in other words, the DNA incorporated in the new cell includes a certain level of carbon-14 that corresponds to the level of the material in the air around us at the time. This serves as a time stamp of sorts, by which researchers can determine when the cell was created based on the level of carbon-14 in its DNA [sources: Wade, Science Update].
What Frisen found is that the body's cells largely replace themselves every 7 to 10 years. In other words, old cells mostly die and are replaced by new ones during this time span. The cell renewal process happens more quickly in certain parts of the body, but head-to-toe rejuvenation can take up to a decade or so.
That explains why our skin flakes off, our nails grow and our hair falls out. But if we are constantly being filled with brand-spanking-new cells, why is it that the body grows old? Shouldn't this influx of new cells be like a shot of Botox? When it comes to aging, it appears that the secret lies not in our cells but, more specifically, in the cellular DNA [source: Wade].
The Life Span of a Cell
The body renews itself at varying paces. Just how long the cells in certain areas last depends on how much work they're asked to do. Red blood cells, for example, enjoy a quick life span of only about four months as a result of their arduous journey through the circulatory system, carting oxygen to tissues throughout the body [source: Wade].
Skin: The epidermis sees a fair amount of wear and tear, thanks to its role as the body's outermost layer of protection. These skin cells rejuvenate every two to four weeks.
Hair: The body's natural fuzz has a life span of about six years for women and three years for men.
Liver: The liver is the human body's detoxifier, purifying a wide variety of contaminants from our systems. It's aided in the process by a constant blood supply and remains largely immune to damage from these toxins by renewing itself with new cells every 150 to 500 days.
Stomach and Intestines: Cells that line the surface of the stomach and intestines have a difficult, short life. Constantly battered by corrosives like stomach acids, they typically last only up to five days.
Bones: Cells in the skeletal system regenerate almost constantly, but the complete process takes a full 10 years. The renewal process slows down as we age, so our bones get thinner.
Despite all this regeneration all the time, people who want to live forever shouldn't give up on that search for the fountain of youth. The truth is that we still get old and we still die. Frisen and others believe that this may be because of DNA mutations, which worsen as they're passed along to new cells over time [sources: Wade, Epstein].
There are also some cells that never leave us and may aid the aging process, or at least the body's breakdown over time. While the eye's cornea can regenerate itself in as little as one day, the lens and other areas don't change. Similarly, neurons in the cerebral cortex – the brain's outside layer that governs memory, thought, language, attention and consciousness – stay with us from birth to death. Because they aren't replaced, the loss of these cells over time can cause maladies like dementia. The good news is that other areas of the brain, like the olfactory bulb that helps us smell and the hippocampus that helps us learn, can and do rejuvenate [sources: Wade, Epstein].
So get out there and show off that big ol' brain like a clever version of a "Baywatch" lifeguard. It's one asset that won't last forever.
Author's Note: Does your body really replace itself every 7 years?
Goodbye cerebral cortex, I'm pretty sure I've lost the capacity to think. Like Elvis, my brain cells appear to have left the building. I spent a solid half an hour scouring my apartment for my wallet this morning. I checked all the usual targets – various tables and night stands – until I recalled that the last time I remembered having it was at the grocery store. So I opened the fridge and there was my cash, right between the milk and the eggs.
- Epstein, Angela. "Believe it or not, your lungs are six weeks old - and your taste buds just ten days! So how old is the rest of your body?" The Daily Mail. Oct. 13, 2009. (June 3, 2014) http://www.dailymail.co.uk/health/article-1219995/Believe-lungs-weeks-old--taste-buds-just-days-So-old-rest-body.html#ixzz33crMYBc7
- Northrup, Christiane. "Flourishing: Why a Healthier Mind Means Healthier Cells." The Dr. Oz Show. April 3, 2012. (June 3, 2014) http://www.doctoroz.com/blog/christiane-northrup-md/flourishing-why-healthier-mind-means-healthier-cells
- NPR. "Atomic Tune-Up: How the Body Rejuvenates Itself." July 14, 2007. (June 3, 2014) http://www.npr.org/templates/transcript/transcript.php?storyId=11893583
- Science Update. "Bombs and Brain Cells." June 24, 2013. (June 3, 2014) http://www.scienceupdate.com/2013/06/bomb/
- Stanford University School of Medicine. "Advancing Cancer Treatment." (June 3, 2014) http://stemcell.stanford.edu/research/cancer_stem_cells.html
- 3, 2014) http://stemcell.stanford.edu/research/
- Wade, Nicholas. "Your Body is Younger Than You Think." The New York Times. Aug. 2, 2005. (June 3, 2014) http://www.nytimes.com/2005/08/02/science/02cell.html?pagewanted=all