Sometimes is it a good idea to carefully read a topic before engaging in discussion. Otherwise you are discussing the same issues which already had been resolved again and again...
The paper says in C.elegans you need 29 microgram SPD/mL to extend the lifespan and lower concentration did not have such an effect (so there is no build-up i that species).
hen extrapolating to a human with just 1L of food and liquids intake this would mean about 30mg SPD per day. That's no easy target with food. You would need two glasses of grapefruit and conversion of 80% putrescine to get to that level.
I am almost tempted to say the people using 50mg SPD once a week are on the right track.
Please have a look what I wrote in this very topic. It isn't hard to get 30mg of spermidine, you can get it from about 70gr of wheat germ, for example. You can drink liters of grapefruit juice, however, and it won't have any big effect on your spermidine levels because putrescine doesn't remain intact in the gut as it is broken down by the digestive enzyme diamine oxidase.
It's funny how obsessed you guys are with grapefruits. Grapefruits don't contain any more putrescine than oranges. According to some sources they contain a bit more, according to other sources they contain a bit less[2]. It's a wish-wash really. And it is completely irrelevant anyway, because as I've said, pustrescine doesn't stay intact in the human gut.
I find 70gr of wheat germ quite high on a daily basis.
But wouldn't intestinal DAO not also react with spermidine?
You seem to be right about putrescine from juice,here a report where they have done measurements with radiolabels in rats:
http://www.ncbi.nlm..../pubmed/8912017"Our results suggest that intestinal diamine oxidase clears the blood from diamines and prevents luminal uptake of putrescine."
Pimagedine is an inhibitor of diamine oxidase. See also
http://europepmc.org...act/MED/1814681 for others. I don't think that this is really an option to enhance the putrescine uptake, also considering you see 50% reduced DAO in Crohn's Disease. Just discussing this idea here.
Competition with histamine would be another idea, but I think there are are some issues as well.
Last idea: as with all enzymes, there is a.) a limited amount of enzyme activity and b.) a certain turnover rate. Once you add more substrate, the enzyme would be overloaded and putrescine would become bioavailable, but needs to be converted (SAMe?) before it's finally cleared e.g. from plasma. The question is whether an overload of the digestive DAO enzymes is possible? If not, this whole supplementing idea would not work in human, than it's really ornithine or arginine or just high protein diet/vitamins for de-novo synthesis.
I haven't yet found real data in human (looking for a simple putrescine blood plasma curve after 240ml grapefruit).Also thanks for the SAMe literature reference.