Spermidine and Autophagy: A Natural Fasting Mimetic by the Numbers

What is spermidine and why does longevity science care about it?

Spermidine is a natural polyamine present in all living cells. It is synthesised from putrescine in a reaction catalysed by spermidine synthase. Spermidine concentrations decline with age in every organism studied — from yeast to humans — coinciding with a weakening of autophagy. This observation raised the question: can the spermidine deficit be replenished through diet or supplements, thereby slowing one of the key steps in cellular ageing?

The richest dietary sources of spermidine are wheat germ (up to 243 mg/kg), natto (Japanese fermented soybeans, ~150 mg/kg), aged cheeses, mushrooms, and peas. A Western diet typically contains 7–25 mg of spermidine per day — considerably less than traditional diets high in legumes and fermented foods.

What did the 20-year mortality follow-up show?

The most widely cited human study is the prospective cohort work by Kiechl and colleagues (American Journal of Clinical Nutrition, 2018). It enrolled 829 participants aged 45–84 years with a 20-year follow-up period (1995–2015), during which 341 deaths were recorded.

Key results: all-cause mortality decreased consistently from the lowest to the highest tertile of spermidine intake — from 40.5 to 23.7 to 15.1 events per 1,000 person-years respectively. The age-, sex-, and calorie-adjusted hazard ratio (HR) was 0.74 (95% CI: 0.66–0.83; p<0.001) per 1 SD increase in spermidine intake. After additional adjustment for lifestyle and other dietary factors, the HR remained significant: 0.76 (95% CI: 0.67–0.86; p<0.001).

The authors calculated that the difference in mortality between the extreme tertiles of spermidine intake is comparable to the effect of a 5.7-year difference in biological age. These are observational data: they do not prove that spermidine reduced mortality, but show an association that persisted after most confounders were adjusted for.

How does spermidine trigger autophagy?

Autophagy is the cellular self-cleaning process in which damaged organelles and protein aggregates are packaged into autophagosomes and degraded by lysosomes. Impaired autophagy is linked to the accumulation of "cellular debris" that accelerates ageing. This mechanism explains part of the benefit from fasting and caloric restriction.

The work of Hofer and colleagues (Autophagy, 2024) revealed the molecular chain. During fasting or rapamycin treatment in yeast, flies, mice, and human volunteers, a rapid rise in endogenous spermidine occurs. Spermidine activates hypusination of the translation factor EIF5A — a post-translational modification required for synthesis of the transcription factor TFEB. TFEB, in turn, is the master "switch" for autophagy genes.

When researchers pharmacologically blocked spermidine synthesis (using an ODC1 inhibitor), the anti-ageing effects of both fasting and rapamycin were substantially diminished across all model organisms. The authors conclude that spermidine functions as an obligatory downstream effector of the anti-ageing action of fasting and rapamycin, rather than merely a mimetic.

What do clinical trials in humans show?

The review by Yu and colleagues (General Psychiatry, 2025) systematised 4 randomised controlled trials of spermidine on cognitive function in older adults. The picture is heterogeneous:

• Wirth et al. (2018): 1.2 mg/day for 3 months — moderate memory improvement with a mean effect size in the intervention group versus placebo.
• Schwarz et al. (2022): 0.9 mg/day for 12 months — no significant cognitive changes found; exploratory analysis suggested a possible anti-inflammatory effect.
• Pekar et al. (2021, 2023): 3.3 mg/day in dementia patients — improvement on the MMSE scale of approximately 2–5 points.

Doses, populations, and durations differ across these trials, making synthesis difficult. The data are most optimistic for people with early cognitive impairment, while the effect in healthy adults remains uncertain. Meanwhile in Denmark, the large placebo-controlled RCT POLYCAD (187 patients with CHD, aged 65+, 24 mg/day spermidine, 48 weeks) is ongoing, with completion expected in 2026.