The human body does not replace itself completely every 7 years, contrary to popular belief. Its cells renew according to widely differing schedules determined by the nature and function of each cell. While some cells live for hours or days before being replaced, others remain for many years, and some may accompany a person from birth or early childhood until death.
Cell lifespans and functions vary throughout the body
Some white blood cells are among the shortest-lived cells in the body, including neutrophils, eosinophils, monocytes and granulocytes. These cells may live for only hours or days, although some can survive for a few months. Their continuous renewal helps keep the immune response effective against pathogens and limits the potential effects of chronic inflammation.
Red blood cells, meanwhile, are replaced roughly every 4 months in a continuous cycle that preserves their characteristics and ability to perform their functions. This reflects the body’s need to replace old cells regularly, but does not mean that other tissues and organs renew at the same pace. The epithelial cells lining the intestine live for only about 5 days.
Their short lifespan corresponds to the nature of their role: they are constantly exposed to food, acids and the absorption of nutrients. The intestinal lining therefore needs rapid renewal to replace cells worn down and damaged while carrying out these functions. Cells in the outer layer of the skin renew roughly every 20 days, although this cycle may lengthen by about 10 additional days with age.
By contrast, renewal is faster in areas of skin most exposed to the external environment, reflecting differences in rates even among cells in the same tissue, depending on their location and the stresses they face. Other cells, however, live for much longer periods.
Memory T cells live for years
Memory T cells in the blood, lymph nodes, lungs and intestines can survive for between 1 and 2 years, while those in the spleen live for 3 to 10 years. These estimates appeared in an analysis by researchers from the United States, published in September 2025 in the journal Immunity.
Liver cells are estimated to have an average lifespan of less than 3 years, consistent with the organ’s high metabolic activity. Replacing them helps preserve the liver’s vitality and functional efficiency, although studies examining the actual lifespan of human liver cells remain limited. Heart cells renew at a slower rate.
Researchers from Sweden used the radioactive carbon-14 isotope to determine the ages of these cells and concluded that around 1% renews each year in a person aged 20. This proportion gradually declines with age, reaching about 0.3% at age 75. This low rate means that the total proportion of heart cells replaced by the body over a normal human lifetime remains below half.
The heart therefore cannot be included in a model assuming that all the body’s cells disappear and are replaced by new cells during a fixed 7-year cycle. Neurons are also among the longest-lived cells. The lifespan of grey-matter cells in the cerebellum is close to that of the person, as they may be present from birth, or from the first 2 years afterwards, and remain until death.
Neurons make up around 92% of the cerebellum’s grey matter, meaning that a large part of this neural structure persists from childhood and does not renew after its formation is complete. Grey-matter cells in the occipital cortex, the visual-processing centre at the back of the head, however, appear to be younger than their counterparts in the cerebellum.
This suggests that the rate of cell renewal may be higher in the cerebral cortex than in the cerebellum, although large areas of neural tissue remain long-lived. These findings are based on an analysis published by researchers from Sweden and the United States in July 2005, using carbon-14 to determine the age of cells in the occipital cerebral cortex.
Research based on this isotope has helped estimate the ages of cells whose life cycles are difficult to track directly. The long lifespan of many neurons does not mean that all brain cells are incapable of renewal.
The scientific dispute over renewal of hippocampal cells
For around 3 decades, researchers have tried to settle the dispute over whether new neurons continue to form in the hippocampus, located deep in the temporal lobe within the limbic system. This region plays a central role in forming memories and converting them from short-term to long-term memories. The dispute continues because of conflicting results from immunohistochemical studies of tissue.
Some findings indicate that neuron renewal in the hippocampus continues during physiological ageing, while others support a different view: that the process stops early in childhood. In 2017, Professor Jonas Frisen of Sweden’s Karolinska Institute and several colleagues published a study on microglial cells in the brain.
These cells perform immune functions and renew slowly, but the researchers estimated their renewal rate at about 28% a year, meaning that more than 96% are replaced during a person’s lifetime. These differences show that the body’s needs and the cell’s function are among the factors determining the speed of replacement.
Some cells renew continuously, others go through extremely slow cycles, while additional types can survive throughout life without being replaced. These differing patterns do not produce a single point at which the body becomes an entirely new cellular version of itself. The renewal of large numbers of cells does not restore the body’s youth, because ageing is not linked solely to the age of each individual cell.
As people age, the body is exposed to internal and external factors that affect the stability of genetic material and the information it carries, even when new cells continue to be produced in some tissues. These factors include physical, biological and chemical influences, as well as oxidative processes and errors that can occur during chromosome division or the replication of genetic material.
Over time, these processes cause mutations and molecular changes to accumulate within cells and tissues. Some mutations may be passed on to new cells when cells divide, while others are not, in a phenomenon known as “genetic mosaicism”. A new cell therefore does not begin its work inside a body returned to square one; it exists within a bodily environment that continues to age and accumulate multiple changes.
The efficiency of the mechanisms responsible for repairing damage to genetic material also declines over time. Ageing therefore continues despite the replacement of large numbers of cells, and its effects appear in organ function and the body’s outward appearance, without being cancelled by the ongoing renewal cycles in the blood, skin, intestines and elsewhere.
The idea that the entire body renews itself every 7 years is thought to have spread after a news article in the reviews section of the journal Nature Reviews Molecular Cell Biology, based on a study Frisen published in 2005. The article stated that the average lifespan of cells in an adult human body ranges from 7 to 10 years.
Frisen and his colleagues’ findings, however, also showed wide variation between cell types, not a single cycle covering them all. Some brain cells can live for well over 10 years, while others survive for only days or months. The average therefore does not represent a point at which the body replaces all its cellular components.
Research continues to determine the precise lifespans of different cell types and understand the mechanisms behind their renewal, particularly in areas where study results remain conflicting, such as the hippocampus.
The firm conclusion is that the body does not rebuild itself all at once, and there is no biological clock that erases and replaces all its cells every 7 years.
