In the end, Im not touching GDF-11 with a ten foot pole
Much of the hype around GDF11 was based around papers that have been called into serious question. See Elevated GDF11 Is a Risk Factor for Age-Related Frailty and Disease in Humans. That review was from 2016 so there was already evidence against GDF11 then.
Results since that further support GDF11 being a pro-aging factor:
Supraphysiological levels of GDF11 induce striated muscle atrophy
Exogenous GDF11 induces cardiac and skeletal muscle dysfunction and wasting
Supraphysiologic Administration of GDF11 Induces Cachexia in Part by Upregulating GDF15
GDF11 Decreases Pressure Overload–Induced Hypertrophy, but Can Cause Severe Cachexia and Premature Death
GDF11 induces kidney fibrosis, renal cell epithelial-to-mesenchymal transition, and kidney dysfunction and failure
GDF11 expressed in the adult brain negatively regulates hippocampal neurogenesis
I would be extremely careful with increasing the activity of any SMAD2/3-coupled TGFB-like ligands, as aside from muscle loss, they cause fibrosis and cellular senescence across the body. This family of ligands includes TGFB1, activin A (which is a component of the SASP), myostatin, and GDF11. Although GDF15 was thought to fall into this group, this was probably due to TGFB1 contamination of rGDF15, as TGFB1 has activity in the femto(!)molar range.
Follistatin neutralizes some of these (including myostatin, activin A, and GDF11) and follistatin overexpression in mice extends lifespan (17% increase relative to a 900 day control). While elevated circulating follistatin in humans is associated with increased risk of mortality and cardiometabolic disorders, this appears to be a compensatory response as
In addition to being acutely regulated, circulating follistatin is increased in the patients with type 2 diabetes and correlate with markers of insulin resistance such as fasting glucose, glycated hemoglobin, and 2-hour glucose during an oral glucose tolerance test (9). As circulating follistatin is increased during states of energy deprivation such as prolonged fasting (39) and exercise, conditions associated with increased gluconeogenesis, it could be speculated that circulating follistatin acts in negative feedback loop to regulate hepatic gluconeogenesis. Surprisingly, deletion of the (circulating) follistatin 315 isoform in mice results in augmented gene expression of GLUT2, PEPCK, and G6P (48), consequently suggesting that circulating follistatin suppresses gluconeogenesis. excerpt from Circulating Follistatin Is Liver-Derived and Regulated by the Glucagon-to-Insulin Ratio
As myostatin downregulation only suffices to normalize lifespan to the 900 day mark (see Haploinsufficiency of myostatin protects against aging-related declines in muscle function and enhances the longevity of mice), it’s plausible that suppression of multiple TGFB-family members mediates follistatin’s anti-aging effects. In humans, there’s evidence for myostatin suppression promoting longevity, where the hypertrophy-promoting K153R polymorphism is enriched in both Italian and Spanish centenarian cohorts.
Considering that TGFB antagonism extends lifespan in α-Klotho (−/−) mice, it’s tempting to speculate whether a similar role to follistatin is played by circulating α-Klotho, which binds to myostatin as well as its receptors, and can block transcriptional activation by multiple TGFB-family ligands (including myostatin, activin A, GDF11, and TGFB1). See Circulating α-Klotho Counteracts Transforming Growth Factor-β–Induced Sarcopenia
Lastly, looking at the intracellular effectors of TGFB-family signaling, a SMAD3 enhancer variant leads to reduced expression, and is enriched in centenarians. Preprint