Sorry, Peter Thiel, young blood may not be the key to reversing aging. A study from the same team that tantalized us with promises of youth (provided we can take the blood of youths) now suggests that old blood hurts health more than young blood helps. We may be better off trying to figure out how to fix the blood we have instead of looking for fresh new sources.

Thiel, the Silicon Valley billionaire now on Donald Trump’s transition team, garnered attention (and accusations of vampirism) for being “very interested” in the potential of young blood to rejuvenate old people. This technique has worked in mice, but in many of these experiments, the old mice weren’t just getting blood. They were actually attached together and sharing organs, too, says Michael Conboy, a bioengineering researcher at UC Berkeley who was part of both a 2005 study on this technique and one published today in Nature Communications.

When people do transfusions, usually the alternative is that they’re going to die

In the new study, Conboy’s team created a device that transferred blood between mice without needing them to be attached. They used this device on four pairs of mice, giving old blood to young and vice versa until each animal had only half of their original blood. As controls, they also had four pairs of young mice exchange blood with other young mice, and old mice exchange blood with old mice. (One limitation of this research, says Saul Villeda, an anatomy professor at UC San Francisco who was not involved in the study, is that the sample size is small.) Then they studied how this affected the muscle, liver, and brain cells.

The results are mixed. Old mice became healthier with young blood, and they started seeing improvement within a day. Their muscles and livers became stronger, although there was no benefit for brain cells. But the young mice infused with old blood did much worse. Their muscle cells seemed okay, but there were bad effects on liver and brain cells and they produced a lot more of a molecule associated with inflammation.

Of course, nobody ever suggested that we give old blood to young people. But this result suggests that for practical purposes, transfusing young blood shouldn’t be a top research priority. “The low-hanging fruit to pursue in this field is what’s inhibitory in the old blood,” says Conboy. We already have technology that can separate out blood parts and return the unused part to us. If we can figure out what factors cause aging in old blood, we could use the existing technology to remove those things in the blood we already have.

There’s a difference between studies with blood and studies with plasma

Even before this study, though, taking the blood of young people was a bad idea, says Conboy. “You risk a catastrophic immune response” if the body and the blood reject each other, he adds. “When people do transfusions, usually the alternative is that they’re going to die. To do it just to work yourself up and make yourself seem younger, that’s pretty risky.”

Villeda, the UCSF professor, is more open to the young blood approach. “The paper shows that there are different approaches: one is reintroducing things that we are missing between the young and old, and the other one is targeting and blocking these other pro-aging effects,” he says. “I think doing both — drop aging factor, increase youthful factor — are equally interesting. I really think that both approaches are equally important to pursue.” He also notes that there is a difference between blood transfer and studies that inject just young plasma into old mice. Plasma might have stronger effects and be less risky, though that doesn’t mean those of us looking to stay forever young should be preying on the next generation. We still don’t know how much blood or plasma would be needed, or in what doses. “Doing this chronically?” says Villeda. “That’s absolutely not feasible.”