Spermidine

Compared with a placebo, spermidine supplementation significantly increased spermine levels in the plasma, but it did not affect spermidine or putrescine levels. - study mdpi.com

"Oral spermidine intake of 15 mg/d for 5 days significantly increased spermine levels in the plasma but did not affect spermidine or putrescine levels. Our data strongly suggest that dietary spermidine is presystemically converted into spermine, which then occurs in the systemic circulation. Consequently, we postulate that the in vitro and clinical effects of spermidine are (at least in part) not attributable to spermidine itself but rather to its metabolite, spermine. It is rather unlikely that spermidine supplements with doses <15 mg/d exert any effec” - study mdpi.com