In a study in 2013, a team at the Harvard Stem Cell Institute reported (1) that combining the circulatory systems of an older mouse and a younger mouse in a surgical procedure called parabiosis improved the structure and function of the enlarged hearts of older mice, otherwise known as cardiac hypertrophy, which is a common cause of heart failure in humans. The hearts became smaller and there was molecular remodeling. They then found that the blood of older mice had less of a protein growth factor called GDF11 than blood of younger mice, and that administering GDF11 to older mice had similar effects as administering blood. GDF11 is also found in human blood.
In a separate paper (2) the same team showed that GDF11 reversed the structure and function of aged muscle to that of younger muscle and increased strength and exercise capacity. In a third paper (3) they showed that young blood, as well as GDF11 alone, improved the vascular structure of the brain and caused the creation of new nerve cells.
In yet another study at Stanford University (4) researchers found that administering young blood to aged mice can reverse pre-existing effects of brain aging at the “molecular, structural, functional and cognitive level.” Synaptic plasticity and dendritic spine density increased in the hippocampus of aged mice. Cognitive abilities improved in fear conditioning and spatial learning and memory. The study used the blood of young mice and did not try to identify any specific factors.