I have a pure hypothesis about GDF11 that might explain why:
1) It was required in mouse studies at large doses, daily
2) It's benefits wore off after about three weeks
The hypothesis is that plasma of young people contain correct levels of multiple peptides, proteins, and enzymes compatible with "youthfulness" of the entire cellular environment. When an old person is injected with plasma, they are getting that cellular environment temporarily revitalized. But, in effect, the older person is just "borrowing" these metabolites. They use them up, and after a few weeks the old person must rely on their existing cellular machinery to produce new proteins, peptides, and enzymes, and that machinery is "old". They cannot sustain the correct cellular environment for youth on their own.
Many of these metabolites are being produced by mitochondria. And since it is the mitochondria that most easily get damaged as we age, my inference is that the old person's mitochondria is largely what is responsible for failure to produce the right levels of all of the necessary metabolites. GDF11 is just one of these metabolites, and it is a very important one that apparently accounts for a large part of the benefit when a young person's plasma is transfused to an older person.
In the shortest term, they will use plasma and/or GDF11 to study these effects in humans. As a next step, they will identify other proteins, peptides, and enzymes, that allow them to get 90%+ of the benefit of transfusing plasma, without needing to use plasma. But, ultimately, the only way to "permanently" correct the defects in the older person's cellular environment would be to replace the mitochondria with newer ones. That leads us to the thread I started about mitochondrial transplants:
http://www.longecity...al-transplants/
Again, this is just hypothesis, but it seems like a reasonable course that this research will follow.