In mice and 11 patients, transplanted hearts moved toward their hosts' biological age

The mouse design has a built-in control. Each recipient kept its own heart, so the graft's methylation could be read against a native heart that had spent the same four to six months in the same body ^[4][5]. Comparing two hearts inside one animal removes differences between animals from the comparison. Crossing all three donor ages with all three recipient ages adds a second check ^[4]. If the surgery alone aged a heart, grafts would look older in every host. The shift ran in both directions instead: young hearts aged faster in old hosts, and old hearts looked younger in young ones ^[1].

The result fits earlier parabiosis work, in which two mice are joined so they share one circulation. Young blood extended the lives of older mice and improved their memory and learning, and old blood raised biological age in young mice ^[14]. Nature's account says the grafts changed to "more closely match" their hosts ^[1]. That wording leaves open whether a graft reached its host's age or only moved part of the way.

In people, the denominator is 11. Poganik, who studies ageing at Harvard ^[3], and his colleagues used historical transplants at Brigham and Women's Hospital for which biopsy samples could be accessed ^[6]. "The remarkable thing is that we found exactly the same effects that we saw in the mice," Poganik said ^[7][8]. The age gaps were not symmetric. Younger hearts went into recipients 38 to 50 years older, while older hearts went into recipients 8 to 24 years younger ^[6]. Using older donors depends on that second direction, and it rests on the smaller gaps.

The function data come from a different design. The team read one-year medical records for hundreds of recipients and found that heart rate, posterior wall thickness and exercise capacity were associated with the recipient's age more than the donor's ^[9]. Those are associations drawn from records. A recipient's age shapes the rest of the body as well as the heart, and a records comparison of this kind cannot separate the two. Poganik was careful about it himself. "Some, but not all, functional measures tracked the prediction that these older hearts in younger individuals functioned as if they were biologically younger, and vice versa," he said ^[10]. "Some things are probably simply irreversible." ^[11]

Current guidance generally recommends donors younger than 45, and older donors are also considered if they show no significant coronary artery disease ^[12]. Nature's report says the findings imply that organs could come from older people than they do now ^[18]. I think the data justify asking whether 45 is the right line. They cannot say where a new line should sit. Methylation age measures accumulated cellular damage ^[13], and the coronary disease screen that already decides whether an older heart is used measures something else ^[12]. The news account does not report graft survival or rejection by donor age, and a revised guideline would need both.

Demand is why the follow-up matters. The United States performed 4,636 heart transplants in 2024, 81.5% more than in 2013 ^[15], up from roughly 2,550 ^[17]. Hundreds of people around the world still die each year on waiting lists ^[16].