The heart may be only as old as its carrier.

In a new study, researchers found that transplanted hearts can shift toward the biological age of their recipients. Older hearts placed in younger bodies showed signs of getting younger, while young hearts in older recipients moved in the opposite direction.

The pattern was consistent, but not exactly complete. Some signs of age changed, while others stuck around.

Does this apply only to the heart? We don’t know yet, but there may be good reasons to believe that the age shown by an organ may be more flexible than scientists once thought.

To test whether a heart keeps the age of its donor or starts to resemble its new owner, Jesse Poganik of Harvard Medical School and his colleagues designed a controlled transplant experiment in mice.

Here, “biological age” does not simply mean how many years the heart has existed. It refers to how old the tissue appears at a molecular level — based on patterns of cellular wear and chemical changes that tend to accumulate with age. One of the most commonly used measures is DNA methylation, chemical tags attached to DNA that shift in fairly predictable ways over time. Scientists can use those patterns as an “epigenetic clock” to estimate whether a tissue looks biologically younger or older than its chronological age.

The researchers wanted to know whether that molecular age stays fixed after transplantation, or whether the new body can push it forward or backward.

They used three age groups: young mice, middle-aged mice, and old mice. Instead of replacing the recipient’s heart, the researchers added a second one. They surgically connected the donor heart to blood vessels in the recipient’s neck — the carotid artery and jugular vein — so blood circulated through the graft and kept it beating, while the mouse’s original heart continued doing the work of pumping blood around the body.

Some mice received a heart from a donor of the same age, while others received one from a much younger or older donor. Because the mice came from the same inbred strain, the team could largely avoid immune rejection and focus on the effects of age itself.

Four to six months later, the researchers compared the transplanted heart with the recipient’s original heart and other tissues. They focused first on DNA methylation. Using three different “epigenetic clocks,” they found a clear trend: the transplanted hearts began to resemble the age of the recipient rather than the age of the donor.

Young hearts placed in old mice acquired older molecular signatures. Old hearts placed in young mice shifted in the opposite direction and looked biologically younger. Gene activity pointed the same way. In particular, pathways involved in mitochondria and cellular energy production became more active in old hearts living in young mice and less active in young hearts living in old ones.

The transplantation procedure itself also produced some pro-aging signals, including increased inflammation. And not every feature of aging changed.

The finding builds on earlier work using heterochronic parabiosis, in which scientists surgically join young and old animals so that they share a bloodstream. Those studies have shown that exposure to young circulation can make some tissues in old mice appear biologically younger. The transplant experiment goes a step further by isolating the effect on a single organ.

The researchers then examined archived biopsies from 11 human heart transplants involving large age gaps between donor and recipient. Again, the transplanted hearts’ methylation ages tracked the recipients more closely than the donors.

They also looked at how well the transplanted hearts were working one year after surgery in data from hundreds of patients. In several measures, the heart behaved more like the age of the recipient than the age of the donor. Older recipients, for instance, tended to have lower exercise capacity and lower maximum oxygen use, regardless of how old the donated heart was.

But the effect was, again, not universal. Some electrical and structural features of the heart tracked with recipient age, while others did not. That suggests the transplanted heart may adopt some age-related traits of its new body without becoming completely “younger” or “older” in every respect.

“Some things are probably simply irreversible,” Poganik told Nature.

“Existing models such as the ones used in the study are still experimental and not necessarily an expression of real biological age yet,” Michael Sagner, clinical adviser in longevity and preventive medicine at King’s College London, who was not involved in the study, told Scientific American.

The clinical stakes are substantial. U.S. surgeons performed 4,636 heart transplants in 2024, up 81.5% from 2013, while donor organs remain scarce. Older hearts could enlarge that pool. Yet a 2024 analysis of U.S. transplant records found that hearts from donors aged 45 and older were associated with higher one- and three-year mortality and graft failure.

Clearly, older donor hearts are not interchangeable with younger ones. And aging, at least in a transplanted heart, appears neither completely fixed nor completely erasable. A younger body can turn part of the clock backward. It cannot yet wipe the slate clean.

The study is available in preprint form on bioRxiv.