Does Your Body Really Replace Itself Every Seven Years?

In the early 2000s, Swedish researchers from Karolinska Institute studied body tissue renewal by measuring levels of a radioactive material called carbon-14. There was a sudden increase of carbon-14 into the air during the 1950s and '60s when nuclear weapons were tested aboveground until the Limited Test Ban Treaty was signed 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 timestamp of sorts, by which researchers can determine when the cell was created based on the level of carbon-14 in its DNA.

What the researchers found is that the average age of all cells in the human body is seven to 10 years. That doesn't mean the cells replace themselves every seven to 10 years. At the individual level, cell renewal happens at different rates in the body. Cells in our colon, for instance, are replaced every three to five days, but our muscle and fat cells can take up to 70 years to renew.

If our bodies are constantly being renewed with brand-spanking-new cells, why do we grow 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 inside the body.

As we've mentioned, cells in the body renew at varying rates. Just how long certain cells last depends on how much work they're tasked with doing. Red blood cells, for example, have an average life span of about 120 days because of their arduous journey through the circulatory system, carting oxygen to tissues throughout the body.

Here are the life expectancies for other cells:

Despite all this regeneration and the fact that the average age of all our cells is about seven years, the truth is that we still get old and die because of mutations that appear in our DNA as our cells replicate over time. So if you think you're going to live forever, you'll will have to find the Fountain of Youth.

Still, there are 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.

So get out there and show off that big ol' brain. It's one asset that won't last forever.