Parabiosis
Parabiosis means surgically joining the circulations of two animals so they share one blood supply; joining a young animal to an old one is called heterochronic parabiosis. It is the most striking evidence that aging is partly carried in the blood, but it is mouse work, the two lifespan studies disagree, and in many tissues the stronger signal runs the wrong way: old blood harms young tissue more than young blood helps old tissue.
There is nothing here to act on. People cannot be joined to a younger partner, and commercial "young plasma" infusions have no proven benefit.
What the experiment is
In a parabiosis experiment, two mice are surgically joined so their blood vessels connect and their blood mixes. When one partner is young and the other old, the old animal's tissues partially rejuvenate, and this happens at some expense to the young one.[1] It is the most striking evidence that aging is partly a signalling state, carried by factors in the blood, and not just accumulated wear inside each cell. The wider picture is under altered intercellular communication.
Old blood does more harm than young blood does good
Joined animals share organs as well as blood, so a cleaner version of the experiment exchanges blood between young and old mice without joining them. A single exchange affects muscle, liver and the hippocampus, a memory centre of the brain, within days. It also settles the direction of the effect: in many tissues, the inhibitory effect of old blood is larger than the benefit of young blood, and injury makes the negative effect worse.[2] Pro-aging signals in old blood actively suppress healthy function, in other words, rather than merely reflecting passive decline.
Some of those signals have names. Old blood carries a recognisable set of pro-inflammatory proteins that rise with age. In mice, several of them (the chemokine CCL11, beta-2-microglobulin and transforming growth factor-beta1) have been shown to reduce the birth of new neurons and impair memory when young animals share an old animal's circulation.[3]
Two lifespan studies, two different answers
In the more recent study, old female mice joined to young partners for three months and then separated lived a median of about six weeks longer than old mice joined to old partners, and their blood and liver still tested epigenetically younger two months after separation.[4]
An earlier study by a second lab, using a similar three-month design, found only a small gain in the old animals that was not statistically significant, while the young partners died sooner.[5] Taken together, the two studies suggest a lifespan effect that is small at best in old mice, and a real cost to young ones.
What it does not show
All of this is animal work. People cannot be joined to a younger partner, so there is no human version of the experiment. The closest human tests remove plasma, with or without replacing it, and they are mostly small, mostly track biomarkers rather than health, and point in different directions; they are covered under plasma exchange. The idea that removing or diluting the factors in old blood might restore regenerative capacity is the rationale for those trials, not something parabiosis has shown in people.
Commercial "young plasma" infusions are a different thing again: they transfuse plasma from young donors, and they have no proven benefit.
What to do
- Read parabiosis results as biology, not as a treatment. They show that blood carries signals that affect aging in mice; they do not show that any blood product slows aging in people.
- Don't buy young-plasma infusions. See plasma exchange for the evidence and the regulator's warning.
- For the circulating environment, the levers with human evidence are everyday ones: exercise and diet modestly lower some inflammatory markers in trials, and the evidence for sleep is thinner; see altered intercellular communication.
Further reading
- Rebo J, et al. A single heterochronic blood exchange reveals rapid inhibition of multiple tissues by old blood. Nat Commun 2016.[6]
- Zhang B, et al. Multi-omic rejuvenation and life span extension on exposure to youthful circulation. Nat Aging 2023 — female mice.[7]
- Yankova T, et al. Three Month Heterochronic Parabiosis Has a Deleterious Effect on the Lifespan of Young Animals, Without a Positive Effect for Old Animals. Rejuvenation Res 2022.[8]
- Gulej R, et al. Impacts of systemic milieu on cerebrovascular and brain aging: insights from heterochronic parabiosis, blood exchange, and plasma transfer experiments. Geroscience 2025.[9]