Hypothesis

Acarbose extends lifespan primarily through a gut‑to‑brain signal: increased colonic short‑chain fatty acids (SCFAs) stimulate vagal afferents, which then activate hypothalamic AMPK‑FOXO pathways to drive systemic longevity mechanisms. This cascade is male‑biased because androgen signaling up‑regulates the SCFA receptor FFAR3 (Gpr41) on vagal neurons, whereas estrogen in females suppresses FFAR3 expression, attenuating the signal despite similar SCFA production and systemic metabolic improvements.

Testable Predictions

• Vagotomy abolishes longevity: Male mice receiving acarbose + subdiaphragmatic vagotomy will show no lifespan extension, while intestinal alpha‑glucosidase inhibition (measured by post‑prandial glucose) remains intact.
• Microbiome depletion blocks effect: Antibiotic‑treated males on acarbose will fail to increase circulating SCFAs and will not exhibit lifespan gain, confirming microbiota dependence.
• FFAR3 loss phenocopies vagotomy: Male FFAR3‑KO mice on acarbose will have normal SCFA levels but lack vagal activation (reduced c‑Fos in nodose ganglion) and show no lifespan extension.
• Estrogen modulation: Ovariectomized females given acarbose will regain male‑like lifespan extension; conversely, testosterone‑treated females will not further increase benefit beyond baseline, indicating hormone‑dependent receptor regulation.
• Hypothalamic AMPK activation is downstream: Pharmacological inhibition of hypothalamic AMPK in males will block acarbose‑induced FOXO nuclear translocation and lifespan extension, despite preserved vagal firing.

Mechanistic Reasoning

Acarbose’s inhibition of intestinal α‑glucosidases increases colonic carbohydrate availability, boosting bacterial fermentation and SCFA production (acetate, propionate, butyrate) acarbose improves cognitive function in Alzheimer's disease mouse models. SCFAs activate FFAR3 on vagal afferent terminals, increasing action potential firing toward the nucleus tractus solitarius. This signal propagates to the paraventricular hypothalamus, where it stimulates AMPK via CaMKKβ, leading to FOXO‑mediated transcription of stress‑resistance and autophagy genes acarbose alters the gut microbiome and increases SCFAs.

Sex differences arise from hormonal regulation of FFAR3: androgen response elements in the Gpr41 promoter enhance transcription in males, while estrogen receptor‑α binds upstream repressive elements in females, lowering receptor density IGF-1 decreased significantly in both sexes. Consequently, equivalent SCFA rises generate stronger vagal‑hypothalamic signaling in males, driving longevity; females experience weight loss and metabolic improvements via peripheral pathways but lack the central neuroendocrine amplification, explaining their modest lifespan gain females show greater body weight reduction yet minimal lifespan extension.

Experimental Design (Outline)

1. Groups: Male and female C57BL/6 mice; acarbose vs control; +/- vagotomy, +/- broad‑spectrum antibiotics, +/- FFAR3 KO, +/- hormone manipulation.
2. Readouts: Survival curves, fecal SCFA quantification (GC‑MS), vagal afferent activity (in vivo electrophysiology or c‑Fos), hypothalamic p‑AMPK/FOXO levels, metabolic phenotypes (body weight, glucose tolerance).
3. Analysis: Cox proportional hazards for lifespan; two‑way ANOVA for mechanistic endpoints; mediation analysis to test whether vagal activity mediates the effect of SCFAs on longevity.

If vagal or FFAR3 interruption eliminates lifespan extension while preserving peripheral metabolic changes, the hypothesis is supported; failure to do so would falsify the gut‑brain causal direction.