Is dietary spermidine just as good as a supplement?

Dietary spermidine is associated in observational studies with lower blood pressure, fewer cardiovascular diseases and better cognitive scores in older age. These are encouraging associations, but not proof that spermidine itself is the cause. Food sources such as wheat germ, mushrooms and legumes deliver spermidine together with dozens of other substances, and it is difficult to determine what exactly is responsible for the effect.

The big question mark with supplements is absorption into the blood. In a small study involving five healthy men, the amount of spermidine in the blood did not rise measurably at all after taking a spermidine supplement. That is a serious signal: if the substance does not reach your blood, it is questionable whether it works in your tissues. The same study did observe reduced inflammatory markers in the blood, but the supplement contained multiple substances at the same time, so spermidine itself cannot be credited for that.

Animal research shows that spermidine activates cellular 'self-cleaning' (autophagy) and protects the heart against ageing and high blood pressure. That is biologically plausible and consistent, but it is animal research. Whether that effect can be replicated in humans via a supplement has not been demonstrated.

Interestingly, your gut bacteria themselves also produce spermidine. Certain bacteria and yeasts produce spermidine in the gut and in this way influence the gut wall. Whether the gut microbiome can be steered via probiotics to produce more spermidine has not yet been systematically studied in humans.

A direct comparison between dietary spermidine and supplements simply does not exist. Reviews conclude that larger, well-designed studies are needed to establish bioavailability, efficacy and optimal dosage. Spermidine is considered safe for the time being, but formal safety data from large studies are still lacking.