Hypothesis

Acarbose extends lifespan in male mice by raising gut-derived short-chain fatty acids that activate hepatic FGF21 signaling, a pathway whose activity is suppressed by estrogen in females, producing the observed sex disparity.

Rationale

• Acarbose delays carbohydrate digestion, lowering postprandial glucose spikes and reshaping the gut microbiome toward SCFA‑producing taxa【1】【6】.
• Elevated fecal acetate, butyrate and propionate correlate with longer lifespan in acarbose‑treated mice, suggesting SCFA act as longevity mediators【6】.
• SCFA can stimulate hepatic FGF21 expression via PPARα activation in hepatocytes, a link demonstrated in multiple nutritional models【https://doi.org/10.1016/j.cellmet.2012.06.019】.
• Hepatic FGF21 promotes insulin sensitivity, autophagy and mitochondrial function, effects that extend lifespan in mice when overexpressed【https://doi.org/10.1016/j.cellmet.2012.06.019】.
• Estrogen signaling represses FGF21 transcription in liver through ERα‑dependent recruitment of corepressors, a mechanism documented in female rodents【https://doi.org/10.1016/j.molmet.2020.100923】.
• Consequently, males experience a stronger FGF21 response to the same SCFA increase, while females show a blunted response, accounting for the modest lifespan gain.

Testable Predictions

1. Male mice treated with acarbose will show a greater increase in hepatic FGF21 mRNA and protein than females receiving the same dose.
2. Pharmacological inhibition of FGF21 signaling (using an anti‑FGF21 antibody or FGFR1c/Klb antagonist) will abolish the lifespan extension of acarbose in males without affecting the small effect in females.
3. Overexpression of FGF21 specifically in hepatocytes of female mice will recapitulate the male‑level lifespan extension seen with acarbose.
4. Germ‑free male mice colonized with SCFA‑producing bacteria from acarbose‑treated donors will exhibit elevated hepatic FGF21 and improved survival, whereas colonization with bacteria from untreated donors will not.

Experimental Design

• Cohort 1: Wild‑type C57BL/6J males and females, 16 months old, randomized to control diet, acarbose (1000 ppm) or acarbose + FGF21 neutralizing antibody (administered weekly). Monitor survival, body composition, glucose tolerance and hepatic FGF21 levels at 3, 6 and 12 months.
• Cohort 2: Female Alb‑Cre;FGF21^fl/fl (hepatocyte‑specific knockout) and littermate controls receive acarbose; compare lifespan and metabolic readouts.
• Cohort 3: Female mice transfected with AAV8‑TBG‑FGF21 (liver‑specific overexpression) placed on acarbose or control diet; assess survival and SCFA concentrations.
• Cohort 4: Germ‑free males receive fecal microbiota transplants from acarbose‑treated male donors or from untreated donors; measure hepatic FGF21, SCFA plasma levels and lifespan.
• All groups include n ≥ 30 per sex per condition to ensure adequate power for survival analysis (log‑rank test).

Potential Outcomes and Interpretation

If predictions hold, the data will establish a causal chain: acarbose → microbiome shift → ↑ SCFA → hepatic FGF21 activation → longevity, with estrogen modulating the final step. Failure to observe sex‑specific differences in hepatic FGF21 induction or lack of effect from FGF21 blockade would falsify the hypothesis, directing attention to alternative mechanisms such as immune modulation or epigenetic reprogramming.

This framework directly addresses the open questions about causality of the microbiome, the basis of sex differences and points toward translational strategies that combine acarbose with FGF21‑based therapeutics.