Just diluting old blood rejuvenated old mice

Researchers at the University of California, Berkeley tested the opposite idea, replacing half the blood plasma in old mice with plain saline and albumin, with no young blood or young cells involved at all.

This single dilution procedure improved muscle repair, reduced liver fat and scarring, and increased new neuron growth in the hippocampus of old mice, matching or exceeding the benefits previously seen from sharing blood with a young animal. A matching human procedure, standard therapeutic plasma exchange, also reversed the usual inhibitory effect of old blood on cell growth in lab tests.

The result reframed rejuvenation as partly about removing harmful old factors rather than only adding youthful ones, since old proteins seemed to actively suppress repair rather than young proteins actively promoting it. It was a mouse study, and the human parallel reused blood from an existing clinical procedure rather than a trial built to test rejuvenation, and human trials of plasma exchange on aging markers have since been small.

Subtraction worked as well as addition.