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Our body is in a constant process of replacing old cells with new ones. But does that mean this dad’s body has the same age as his baby’s? Not exactly. Marvy Andreina Jaimes/Getty ImagesKnow yourself; love yourself; be true to yourself. These timeless sayings have been passed down over time by artists, entertainers, and philosophers alike, from Jean-Paul Sartre ("We only become what we are by the radical and deep-seated refusal of that which others have made of us") and 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 come to know yourself when your body is constantly changing? Whether it's shedding skin or growing new hair, the human body is always in a state of flux.
Research suggests that our bodies continually replace many of their nearly 30 trillion cells. Every day, about 330 billion cells are replaced, equaling roughly 1 percent of all the cells in our body. Some cells, like those in our gut, renew every week.
You may have hoped that your new cells would lead to a longer life, but it's more complex than that. For example, certain cells in our body, such as those in our brain, heart, and eyes, remain with us throughout our lives.
Body Rejuvenation 101
In the early 2000s, researchers from the Karolinska Institute in Sweden studied body tissue renewal by measuring levels of radioactive carbon-14. This material's levels spiked during the 1950s and 1960s due to above-ground nuclear weapons tests, which stopped after the Limited Test Ban Treaty was signed in 1963. Carbon-14 is absorbed by plants, which humans and animals consume, and it becomes a part of our DNA.
Unlike other molecules that are constantly changing, a person's DNA remains unchanged from the moment a cell is born — which happens when a parent cell divides — to the end of its life. When a cell divides, the DNA in the new cell carries a specific level of carbon-14 that reflects the atmospheric levels at the time, essentially acting as a timestamp that allows researchers to determine when the cell was created based on the carbon-14 concentration in its DNA.
Researchers discovered that the average age of all the cells in the human body is between seven to 10 years. However, this doesn't mean that cells replace themselves on this exact schedule. The rate of cell renewal varies across the body. For instance, colon cells are renewed every three to five days, while muscle and fat cells may take as long as 70 years to regenerate.
If our bodies are continuously renewing with fresh cells, why do we age? Shouldn't this influx of new cells act like a rejuvenating boost, like Botox? It seems that the key to aging doesn't lie in the cells themselves, but rather in the cellular DNA within the body.
Doctors and scientists believe that cancer can develop in the body when cancerous cells replenish themselves by dividing. Chemotherapy, a common treatment, works by targeting a wide array of cells, rather than focusing solely on the cancerous ones. By understanding how and when cells self-renew, researchers aim to identify the sources of cancer and stop those cells from replicating without affecting other healthy cells.
The Life Span of a Cell
This diagram illustrates the life cycle of red blood cells, which regenerate roughly every 120 days in our bodies. Source: VectorMine/ShutterstockAs we've previously mentioned, cells in the body renew at different rates. The longevity of certain cells is determined by the tasks they perform. For example, red blood cells have an average lifespan of about 120 days due to the strenuous journey they undergo in the circulatory system, transporting oxygen to tissues across the body.
Here are the average life spans for various other cells:
- Skin: The outer layer of the body undergoes a lot of wear and tear, and its cells regenerate every two to four weeks.
- Hair: Hair cells live for about six years in women and three years in men.
- Liver: The liver, responsible for detoxifying the body, stays healthy by renewing itself with new cells every 150 to 500 days, thanks to a constant blood supply.
- Stomach and intestines: The cells lining the stomach and intestines have a challenging life, often battered by corrosive stomach acids, and only last about five days.
- Bones: Bone cells regenerate constantly, though the process takes about ten years. As we grow older, this regeneration slows, leading to thinner bones over time.
Despite all the regeneration and the fact that the average age of all our cells is about seven years, the reality is that we still age and eventually die due to mutations in our DNA as our cells replicate over time. So, if you believe you're going to live forever, you might want to search for the Fountain of Youth.
There are still some cells that remain with us throughout life and may contribute to aging or the gradual breakdown of the body over time. While the cornea in the eye can regenerate within a day, the lens and other regions do not. Similarly, neurons in the cerebral cortex — the brain's outer layer responsible for memory, thought, language, attention, and consciousness — stay with us from birth until death. Since these cells aren't replaced, their loss can lead to conditions like dementia.
The good news is that other parts of the brain, such as the olfactory bulb, which helps us smell, and the hippocampus, which plays a role in learning, do regenerate and rejuvenate.
So get out there and show off that big ol' brain. It's one asset that won't last forever.
