My birth certificate might claim that I’m 30 years old, but how old is my body, really?
Many would argue that it’s not a day over seven years old. This notion goes back to a common bit of health lore claiming that our bodies go through cell regeneration every seven years. While a fresh set of cells and a brand-new body sound pretty great, a person has to wonder … is that even true?
Simply put, it’s not. Some cells take longer than that, and some don’t regenerate at all.
The cells in our bodies are constantly dividing, regenerating, and dying, but each cell’s life cycle is different. The cells lining the stomach, because they’re exposed to acid, replace themselves about every five days. Cells in the epidermis last about a week. Red blood cells live for approximately four months in the body, while hepatocytes (liver cells) live about five. These hardworking but disposable cells take a lot of punishment; they’re easily manufactured and easily replaced.
On the other end of the spectrum, some cells take much longer than seven years to regenerate. A bone completely remodels itself and replaces all of its cells every ten years or so. Cells in the intestinal tract (other than the lining of the stomach) last for about fifteen years, the same as certain muscles, such as the intercostals between the ribs.
Then there are the cells that rarely—if ever—turn over. For example, females do not regenerate oocytes; all the eggs they will ever have are present at birth. Teeth don’t regenerate. Cardiac tissues and neurons, although once thought to be irreplaceable, have been shown to be capable of regeneration, albeit at an extremely slow rate. According to a study published in the April 2009 issue of Science, researchers at the Karolinska Institute in Sweden ascertained that for cardiomyocytes, the yearly rate of cellular turnover is about 1 percent starting at birth, and that the rate declines steadily with age. They also estimated that a person who lived until age seventy-five would not yet have replaced even half of his or her original heart cells. In addition, scientists have discovered that although certain areas of the brain are indeed capable of neurogenesis, other areas, such as the cerebral cortex and the visual cortex, are not, and people’s neurons in those areas are with them from birth.
The human body contains about ten trillion individual cells. Taking into account all of the specialized tissues—those that regenerate quickly and those that don’t—an adult’s bodily cells are likely to be, on average, between eleven and fifteen years old.
Of course, one other big determining factor is chronological age. Cellular turnover slows as part of the normal aging process; stem cells divide less quickly and less efficiently, and eventually the rate of dying cells outpaces the rate of new cells being born. An elderly person is likely to have a body whose tissues are far older than eleven to fifteen years, because the body can’t regenerate those cells as efficiently as it used to.
Most of your body’s cells are significantly younger than your chronological age, but the process of turnover becomes much less efficient and effective as you get older. Some of the most important cells in your body—those of your bones, your brain, and your heart—don’t regenerate much at all, so treat them with care.