Circulating factors affecting human health/longevity
Oxytocin

It has been shown that oxytocin signaling helps in muscle regeneration by activating MAP Kinase/ERK pathway in skeletal muscles.[1] 

Oxytocin is also known to promote adult neurogenesis.[2]

Myostatin

It is a paralog of GDF and like GDF prevents hypertrophy of cardiac muscles.[3]

Leptin

Though leptin is not really connected to ageing but it regulates appetite and thereby the metabolism [4]. It may contribute indirectly to ageing by reducing food intake and preventing obesity and other metabolic syndromes.  

This review summarizes different soluble small (synthetic) molecules that have potential to induce regeneration.