A recent study examining both human and animal models has found that transplanted hearts may adjust to match the biological age of their recipients. In experiments, young donor hearts transplanted into older individuals showed signs of rapid aging, while older donor hearts transplanted into younger individuals appeared to rejuvenate. This discovery could potentially allow for a broader range of donor ages in the future, which may help address the growing shortage of heart transplants. Heart transplants are becoming more common, especially among older individuals, due to the aging global population. However, the demand for donor hearts far exceeds the supply, leading to many patients dying while waiting for a transplant. One reason for this shortage is the current selection criteria for donor hearts, which often consider the donor's chronological age. While there is no universal age limit, hearts from donors under 45 are typically preferred. Older donor hearts can still be used, particularly if they do not have significant coronary disease. Some countries have already relaxed these age limits for older donors with healthy hearts to help meet the demand. Heterochronic transplants, where the donor and recipient have different ages, are becoming more common as a way to address the organ shortage. A pre-published study on the platform bioRxiv suggests that transplanted hearts may adapt to the biological age of the recipient. The study authors note that "the biological age of experimentally transplanted hearts rapidly assimilates to that of the recipients." Researchers analyzed this adaptation by measuring accumulated cellular damage in both mouse models and human transplants. They looked at DNA methylation levels, which are used as markers of biological aging, in over 320,000 genetic regions. The results showed that the biological age of the transplanted hearts changed to match that of the recipients, with young hearts aging when placed in older bodies and older hearts rejuvenating when placed in younger ones. In human studies, researchers examined heart transplant records from Brigham and Women's Hospital in Boston, focusing on 11 cases where there was a significant age difference between donor and recipient. They found that aging-related markers in the transplanted hearts changed rapidly. The lead author of the study, Jesse Poganik, noted that similar effects were seen in both mice and humans. Additional analysis of medical records showed that factors like heart rate, heart wall thickness, and exercise capacity were more closely related to the recipient's age than the donor's. However, the study found no detectable change in the overall biological age of the recipients, indicating that the effects are limited to the transplanted tissue. Poganik explains that while some functional measures support the idea that older hearts in younger individuals function as if they were younger, this effect may not be fully reversible. Further research on larger groups is needed, but these findings could eventually allow for a broader selection of older donors, helping to reduce the shortage of available organs.