A donor heart may not keep the biological age it arrived with. Early research suggests that after transplantation, a heart's biological age shifts toward that of its new body, so young hearts placed in older recipients appear to age faster, while older hearts placed in younger recipients appear biologically younger.

The findings come from a preprint posted to bioRxiv, meaning the study has not yet gone through peer review. They were reported by Scientific American on Sept. 28.

If the results hold up, they could matter for one of medicine's scarcest resources. Doctors generally prefer younger donor hearts because older organs have been linked to worse outcomes, which narrows an already limited supply.

The need for organs is large. Nearly 110,000 people are on the U.S. national transplant waiting list, and 12 people die each day waiting for an organ, according to the federal organ donation program run by HRSA. More than 49,000 transplants of all organ types were performed in 2025.

Mouse Hearts Took On Their New Body's Age

Jesse Poganik, an instructor in medicine at Harvard Medical School and a co-author of the study, and his colleagues transplanted hearts between young, middle-aged, and old mice. Each recipient kept its own native heart, so the transplanted organ was an additional graft.

The team measured DNA methylation, chemical marks on DNA that change with age, and used three epigenetic clocks, tools that convert those patterns into an estimate of biological age.

Young hearts placed in older mice appeared biologically older than expected, while old hearts placed in younger mice appeared to have de-aged. Other measures of biological aging showed the same pattern, according to Scientific American.

"The young systemic environment can reverse certain aspects of biological age," Poganik said, adding that others may be irreversible.

Genes involved in mitochondria, the structures that produce energy in cells, also became more active in old hearts placed in young mice and less active in young hearts placed in old mice. The transplant did not appear to change the biological age of the recipient's own heart, liver, or blood. Transplantation itself also caused some pro-aging changes, including increased inflammation.

Human Biopsies Pointed the Same Way

To test whether the pattern extends to people, the researchers analyzed archived heart biopsies from 11 human transplant recipients with large donor-recipient age gaps. Again, the transplanted hearts' biological ages were more closely associated with the recipient's age than the donor's.

The team also examined a database of hundreds of human transplant recipients. Some functional measures of heart health, including exercise capacity and VO2 max, a measure of how well the body uses oxygen during exercise, were lower in older recipients regardless of donor age. That suggests the transplanted heart was aging alongside the rest of the body.

"Older organs that would currently be passed over for donation could be considered," Poganik said of younger recipients, while noting that much more work is needed before any changes to the donor system could be implemented.

Limits of an Unreviewed Study

Several limitations stand out. The study is a preprint, so outside experts have not formally reviewed its methods or conclusions. The human biopsy analysis involved only 11 patients. And the database findings show associations, not proof that the recipient's body caused the heart to age or rejuvenate.

The tools themselves are debated. Michael Sagner, a clinical adviser in longevity and preventive medicine at King's College London who was not involved in the study, told Scientific American that such models "are still experimental and not necessarily an expression of real biological age yet."

Earlier peer-reviewed work also points to complications. A 2024 mouse study in the American Journal of Transplantation found that transplanting older hearts induced senescence in younger recipients, a form of cellular aging, along with reduced physical and cognitive performance. How those findings fit with the new preprint remains an open question.

Another outside commentator quoted by Scientific American, Peter Scriven, co-founder and clinical and scientific director of LONGEVITY, a private aging clinic in England, suggested that signals such as mitochondrial activity might pass from recipient cells to a transplanted heart. That idea is speculative and was not tested in the study.

Exactly what drives the shift is unknown. Poganik suspects the body's overall environment, rather than a single factor, may help clear accumulated damage or expose the organ to younger immune signals. That remains a hypothesis.

No Change Yet for Patients and Donors

For now, nothing changes for transplant patients or registered donors. Organ allocation in the United States is governed by national policy through the Organ Procurement and Transplantation Network, and this study does not alter those rules. People waiting for a transplant should follow the guidance of their transplant team.

The next steps are peer review, larger human studies, and independent replication. Until then, the findings are best read as an intriguing early signal, not a guide to transplant decisions.

What Readers Want to Know

What did the study find?

In mice, transplanted hearts took on the biological age of their recipients. Young hearts in older mice looked older, and old hearts in younger mice looked younger.

Was this tested in humans?

Partly. Researchers analyzed biopsies from 11 human transplant recipients and a larger database, and found heart age tracked the recipient more than the donor.

Is the study peer reviewed?

No. It is a preprint posted to bioRxiv and has not been formally reviewed by independent experts.

Could this change who receives donor hearts?

Not now. Co-author Jesse Poganik said much more work is needed before any changes to the donor system could be considered.

What is an epigenetic clock?

It is a method that estimates biological age from chemical marks on DNA called methylation. Some experts say these tools are still experimental.

Should transplant patients do anything differently?

No. Patients should follow the guidance of their transplant team. This early research does not change current medical practice.