Scientists used radioactive carbon-14 released during Cold War nuclear tests to determine whether the adult human heart can produce new muscle cells.
Above ground nuclear tests conducted between 1955 and 1963 nearly doubled the amount of carbon-14 in the atmosphere. The isotope entered the food chain and eventually became incorporated into human DNA, creating a unique timestamp that researchers could use to estimate when cells were formed.
For decades, scientists believed that heart muscle cells, known as cardiomyocytes, were created early in life and remained largely unchanged throughout adulthood.
However, researchers later analysed carbon-14 levels in the DNA of heart cells and found evidence that adult hearts can generate new cardiomyocytes.
A 2009 study estimated that heart muscle cells renew at a rate of about 1% per year at the age of 25. The rate gradually declines to around 0.45% per year by the age of 75.
The research suggested that fewer than half of a person’s heart muscle cells are replaced during a normal lifetime.
A follow up study in 2015 found that the total number of heart muscle cells is largely established within the first month after birth. The heart continues to grow during childhood mainly because existing cells increase in size rather than through the large-scale production of new cells.
Scientists say the findings changed the long standing belief that the adult human heart is completely incapable of regeneration. However, the exact rate at which heart muscle cells are replaced remains under scientific debate.
The discovery could have important implications for future research into heart disease and heart attack recovery, particularly if scientists can find ways to stimulate the natural production of new heart muscle cells.
The carbon 14 dating method, however, may become less useful for future generations. The continued burning of fossil fuels is diluting atmospheric carbon-14 levels, gradually weakening the unique radioactive signal created by nuclear testing.