T

he author of a study published Tuesday cautioned against the idea that transfusions of “young” blood can reverse the aging process, a notion that has captured the public imagination and motivated startups.

In the new research, published in Nature Communications, scientists replaced half of the blood in old mice with blood from young animals, and did the opposite swap in young mice, giving them old blood.

They found that the younger mice experienced symptoms associated with aging — they stopped making new neurons in a part of the brain associated with memory formation. And the old mice seemed younger in some respects — their muscles recovered better after being injured.

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Senior coauthor Irina Conboy, an assistant professor of bioengineering at the University of California Berkeley, said that the negative effects of old blood on the young were more pronounced, and that she did this study in part to tear down what she perceives as a public narrative holding up transfusions of “young” blood as a rejuvenative cure-all.

But Amy Wagers, a Harvard professor who has published previous research with Conboy, said that the young blood did have a positive effect in the older mice, and that this study does not rule out the benefits of younger blood.

Dr. Jesse Karmazin, cofounder of Ambrosia LLC, a Monterey, Calif., company running a clinical trial to study the effects of giving older people doses of younger people’s blood plasma, said that “this elegant paper is further evidence that young plasma rejuvenates older animals.”

Much of the earlier research on the effects of differently aged blood on mice was conducted by sewing together the circulations of two mice of different ages, beginning with a paper Conboy wrote in 2005 with Wagers and others. But conjoined mice share more than blood, making it difficult to determine what changes to the mice were caused by the differently aged blood.

Conboy said that that future research should focus on determining what might be in old blood that causes detrimental effects. Conboy said that there are a handful of proteins in old blood that might be contributing to the aging process, but that she can’t disclose what they are because of patents she filed.

Wagers said that this new study can’t say definitively that there’s something in the old blood that had deleterious effects on young mice. Maybe the old blood simply diluted the young blood, preventing it from doing its job.

However, Wagers praised the study for offering a new way to study the effects of blood transfusions, which is more controlled and precise than sewing two mice together.

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