Nov 10, 2026
You have probably heard that your body replaces itself every seven years. It makes a good dinner-party fact, but the research tells a messier and more interesting story. Some of your cells live for less than a week. Others are as old as you are, and a few structures you carry today formed before you were born.
Scientists got real answers only after they learned to date human cells using carbon left over from Cold War nuclear tests. Below, we walk through how long each part of the body takes to renew, which parts never do, and how those timelines shift as you age.
How long does it take your body to replace itself?
Your body never fully replaces itself. It makes about 330 billion new cells a day, matching its roughly 30 trillion cells every 80 days, but blood cells account for 86% of that turnover. Meanwhile, most brain neurons, the core of the eye lens, tooth enamel, and joint cartilage last a lifetime.
Those figures come from a 2021 analysis in Nature Medicine by Ron Sender and Ron Milo of the Weizmann Institute of Science. They estimated that a typical adult replaces about 80 grams of cells a day, or roughly 4 million cells every second. Red blood cells and the cells lining the gut make up the bulk of it. As a result, the “seven years” idea is wrong in both directions: some tissues turn over in days, and others not at all.
The tool that settled much of this came from an unlikely place. Above-ground nuclear tests in the 1950s and early 1960s sharply raised the amount of carbon-14 in the atmosphere, and every cell built since then carries a matching stamp in its deoxyribonucleic acid (DNA). By comparing that stamp to the atmospheric record, a team led by Kirsty Spalding first dated human cells in 2005, and many of the numbers below rely on the same method.
Cell replacement times by body part and age
The table runs from the fastest-renewing tissues to the ones that never renew. Each age column reflects the ages the researchers actually studied; where a study found no age effect, both columns match.
| Body part | Younger adults | Older adults | Study |
|---|---|---|---|
| Colon lining | About 3 days | Not measured | Sender and Milo, 2021 |
| Small intestine lining | About 5 days | Not measured | Sender and Milo, 2021 |
| Stomach lining | About 13 days | Not measured | Sender and Milo, 2021 |
| Skin surface layer | About 20 days (under 35) | About 28 days (over 60) | The Aging Skin Barrier, 2025 |
| Red blood cells | About 116 days | Not measured | Sender and Milo, 2021 |
| Liver cells | Average cell under 3 years old | Same | Heinke and colleagues, 2022 |
| Fat cells | About 10% a year; half replaced every 8.3 years | Same (ages 21 to 72) | Spalding and colleagues, 2008 |
| Whole skeleton | Renewed about every 10 years | Faster breakdown than rebuilding; remodeling rises after menopause | Langdahl and colleagues, 2016 |
| Skeletal muscle cells | Average cell about 15 years old | Not measured | Spalding and colleagues, 2005 |
| Heart muscle cells | 1% a year at age 25 | 0.45% a year at age 75 | Bergmann and colleagues, 2009 |
| Insulin-making beta cells | Formed in the first 30 years of life | No new cells detected after 30 | Perl and colleagues, 2010 |
| Memory-center (hippocampal) neurons | About 700 new neurons a day | Fairly steady into old age (debated) | Spalding and colleagues, 2013 |
| Cerebral cortex neurons | Never replaced | Never replaced | Spalding and colleagues, 2005 |
| Achilles tendon core | Never replaced after height growth ends | Never replaced | Heinemeier and colleagues, 2013 |
| Knee cartilage collagen | Never replaced after the skeleton matures | Never replaced | Heinemeier and colleagues, 2016 |
| Eye lens core | Formed around birth, never replaced | Never replaced | Lynnerup and colleagues, 2008 |
The split between “younger” and “older” here is by birth date, not biological age. No study we found has measured cell turnover against an epigenetic clock or another biological age test, a gap we return to below.
Fast renewers: blood, gut lining, and skin
Blood
Blood does most of the body’s rebuilding. Red blood cells last about 116 days in adults, and the body makes roughly 210 billion of them a day, according to Sender and Milo. White blood cells called neutrophils, the first responders to infection, circulate for less than a day. In children, red blood cells live only 60 to 80 days, so blood dominates turnover even more early in life. Our guide to blood health covers what routine blood work can tell you.
Gut lining
The single layer of cells lining your intestines renews faster than almost any other tissue: about every 3 days in the colon and 5 days in the small intestine. The stomach lining lasts closer to 13 days.
However, the lining is only part of the gut. When Spalding’s team dated whole samples of small intestine from three adults, the cells averaged 10.7 years old. Once they set aside the fast-renewing lining, the remaining cells of the gut wall averaged 15.9 years. In other words, your gut lining is days old, while the rest of the gut wall is closer to 16 years old. For more on keeping that system working, see Healthy Digestive System.
Skin
The outer layer of skin, the epidermis, turns over in roughly 25 to 38 days in adults, the range Sender and Milo used. Its very top layer of dead cells renews faster in the young. In data by Grove and colleagues summarized in a 2025 review in the Journal of Cosmetic Science, that layer took 19.8 days to replace on the forearms of people under 35, compared with 28.1 days in people over 60. For daily care, see Feel Comfortable In Your Own Skin.
Slow renewers: liver, fat, bone, muscle, and heart
Liver
The liver is famous for regrowing after injury, and it also renews steadily without one. Using the carbon-14 method, a team led by Olaf Bergmann found that liver cells average under 3 years old. Cells with the usual two copies of each chromosome renewed more than 7 times faster each year than cells carrying extra copies.
Fat
Fat cells come and go more than most people assume. In 2008, Spalding and colleagues reported that about 10% of fat cells are replaced each year, so half turn over roughly every 8.3 years. The rate held steady across ages 21 to 72 and across body weights. However, the total number of fat cells stays constant through adulthood, so new fat cells mostly replace old ones instead of adding to the count.
Bone
Bone looks permanent, but it constantly breaks down and rebuilds in small patches. According to a 2016 review by Langdahl, Ferrari, and Dempster, “the adult skeleton is renewed by remodeling every 10 years.” About 80% of that work happens in spongy inner bone, even though it makes up only 20% of the skeleton.
Age tips the balance. After peak bone mass, breakdown and rebuilding stay matched for a decade or two. After that, breakdown speeds up and rebuilding slows, and the drop in estrogen after menopause increases remodeling further. Our guide to bone health covers how to slow that loss.
Skeletal muscle
Muscle fibers are long-lived. In the 2005 carbon-dating study, rib muscle cells from two adults aged 37 and 38 averaged 15.1 years old. Two people is a very small sample, and no study we found has measured how this figure changes with age. Muscle repairs itself with dedicated stem cells, a process we cover in Injury Recovery Time by Age.
Heart
Scientists long debated whether heart muscle cells renew at all. Bergmann’s team showed otherwise in a 2009 study in Science: about 1% of heart muscle cells renew each year at age 25, falling to 0.45% a year by 75. Even so, fewer than half of your heart muscle cells are replaced over a normal lifespan. That limited capacity is one reason heart health rests on prevention.
Pancreas
The insulin-making beta cells of the pancreas are mostly made by early adulthood. In a 2010 study of 10 people aged 17 to 74, Perl and colleagues found that beta cells were born within each person’s first 30 years. In the eight people over 30, they found no beta cells that had divided during the study’s labeling period.
Parts of the body that never replace themselves
Several parts of your body are as old as you are, or nearly so. Damage to them adds up over a lifetime, which makes protecting them more useful than hoping they will renew.
Brain
Most neurons in the cerebral cortex, the brain’s outer layer, are never replaced. When Spalding’s team sorted neurons from the visual cortex in 2005, the neurons matched the age of the person within the method’s two-year margin. Neurons in the cerebellum, which coordinates movement, formed by about age 3. In contrast, the brain’s support cells were much younger, so the brain as a whole does renew some of its cells.
The hippocampus, a memory center, is the main exception. In a 2013 carbon-dating study, Spalding and colleagues estimated that adults add about 700 new neurons a day there, with the renewing share turning over at 1.75% a year and only a modest decline into old age. That finding remains debated: a 2018 study in Nature reported that new hippocampal neurons drop to undetectable levels by adulthood. Our guide to brain health covers what helps protect the neurons you have.
Eye lens
The proteins at the center of the eye lens form around the time of birth and never turn over. Lynnerup and colleagues dated lens proteins in 2008 and found that, once formed, the lens crystallins are “never subject to turnover or remodelling.” The lens has no blood supply, and its outer capsule keeps old cells from shedding. For protecting your eyes, see our guide to eye care.
Tooth enamel
Enamel is the other well-known permanent tissue, as Lynnerup’s team noted. Unlike bone, it contains no living cells to rebuild it, so worn or decayed enamel does not grow back. See Listen to Your Mouth for dental care.
Joint cartilage and tendons
The collagen frame of your joints is permanent too. Heinemeier and colleagues measured carbon-14 in knee cartilage from 23 people and found virtually no collagen replacement after the skeleton matured, even in people with osteoarthritis. A 2013 study of the Achilles tendon found the same pattern: the tendon core matched carbon levels from the donors’ first 17 years of life, meaning it forms while you are still growing and is essentially not renewed after that. In practical terms, the main collagen structure you have at 20 is the one you have at 70.
Does cell renewal slow with age?
Cell renewal slows with age in some tissues and holds steady in others. Heart muscle renewal falls by more than half between 25 and 75, and the skin’s surface layer takes about 40% longer to replace after 60. By contrast, fat and liver cells renew at the same pace from young adulthood into old age.
Sender and Milo point to one likely reason for the slowdown: stem cells in older people divide less often, which lowers turnover. That fits a broader pattern researchers call stem cell exhaustion, one of the hallmarks of aging described in our Reversing Aging article.
Slower stem cells are only one piece of the story. For the bigger picture of why bodies age at all, from programmed biological timing to damage that builds up over decades, see What Causes Aging?
Where biological age fits in
No study we found has compared cell turnover with biological age as measured by an epigenetic clock or a blood-based aging test. Every age effect in this article comes from comparing people by birth date. Because of that, we cannot give separate replacement times for a biologically “young” 60-year-old and a biologically “old” one.
Even so, two findings suggest that a person’s biological state matters more than the calendar. First, menopause speeds up bone remodeling, and menopause arrives at different ages for different women. Second, people with obesity make more than twice as many new fat cells each year as lean people, even though the share replaced is the same. In both cases, what is happening inside the body changes the numbers more than age alone.
If you want a measure of how your own body is aging, our guide to biological age tests compares the options. Keep in mind that none of them measures cell turnover directly.
What this means for how you care for your body
The turnover data split your body into two groups, and each calls for a different approach.
Protect what lasts a lifetime. Cartilage, tendon cores, cortical neurons, the lens, and enamel do not renew, so the wear they take is cumulative. That makes prevention the main tool. Our guides to eye care, dental care, flexibility, and brain health are good starting points.
Support what renews. Bone, blood, skin, and the gut lining rebuild constantly, and they need raw materials and signals to do it well. Our guides to strength training, healthy eating habits, and sleep cover the basics.
Watch the research, skeptically. Scientists are testing ways to restore youthful renewal, from partial cellular reprogramming to stem cell therapies. Most of that work remains in animals or early human trials. Our articles on cellular reprogramming and rejuvenation and regeneration track where it stands.
For personal decisions about bone density, heart health, or joint problems, talk with your primary care provider.
Frequently asked questions
Does your body really replace itself every seven years?
No. Your body makes about 330 billion new cells a day, but most of them are blood and gut cells that last days to months. Fat cells take about 8 years to half renew, and the skeleton about 10 years. Most brain neurons, the eye lens core, tooth enamel, and joint cartilage collagen are never replaced at all.
What parts of the body never regenerate?
Most neurons in the brain’s outer layer, the proteins at the center of the eye lens, tooth enamel, the collagen in joint cartilage, and the core of the Achilles tendon are not replaced in adulthood. Carbon-dating studies show these tissues are about as old as the person, or date from childhood and adolescence.
How often does your skin replace itself?
The outer layer of skin renews in roughly 25 to 38 days in adults. Its top layer of dead cells replaces itself faster when you are young: about 20 days on the forearm in people under 35, compared with about 28 days in people over 60. That slower turnover is one of the ways skin changes with age.
Do cells regenerate more slowly as you get older?
In some tissues, yes. Heart muscle cells renew at 1% a year at age 25 but only 0.45% a year by 75, and skin turnover slows by about 40% after 60. Fat cells and liver cells, however, renew at a steady rate across adult ages. No study has yet compared renewal with biological age.
