I totally believe you when you say that you experienced strong side effects. All what I am saying is it's possible that you are obtaining something else other than GDF11 from buckylabs or there are some impurities causing these side effects. GDF11 is not protein bound, same for most of polypeptides. The assay in the studies I linked measures serum concentration of GDF11 after centrifugation, so we are talking non-cell bound circulating GDF11. Also Hamsterman himself said that he started using NMN around the time of starting to use GDF11.
Hope you find something that works for you man, we will all be happy for you, and keep us updated on your experiments.