Hypothesis

Lactase (LCT) activity increases intracellular galactose, driving flux through the hexosamine biosynthesis pathway and elevating O‑GlcNAc modification of MST1. This post‑translational change amplifies MST1 kinase activity, leading to FOXO3 inhibition, reduced telomerase recruitment, and accelerated telomere shortening. Conversely, protective proteins such as UBA7 ubiquitinate MST1, limiting its O‑GlcNAcylation and preserving telomere length. We therefore hypothesize that dietary lactose restriction attenuates LCT‑mediated MST1 activation, slows telomere attrition, and improves aging‑related outcomes.

Mechanistic rationale

• Lactose hydrolysis by LCT yields glucose and galactose. Galactose enters the Leloir pathway; excess galactose is diverted to UDP‑GlcNAc synthesis via the hexosamine biosynthetic pathway (HBP).
• Increased UDP‑GlcNAc raises O‑GlcNAc transferase (OGT) activity, enhancing O‑GlcNAcylation of serine/threonine residues on MST1 at sites identified in phosphoproteomic studies (e.g., Ser‑356).
• O‑GlcNAcylated MST1 displays reduced binding to inhibitory 14‑3‑3 proteins, resulting in sustained kinase activity toward downstream targets such as STK4/MST1 substrates that phosphorylate and inhibit FOXO3.
• FOXO3 suppression diminishes transcription of TERT and antioxidant genes, shortening telomeres and increasing oxidative DNA damage.
• UBA7, an E2 ubiquitin‑conjugating enzyme, promotes K48‑linked ubiquitination of MST1, targeting it for proteasomal degradation and counteracting O‑GlcNAc–mediated stabilization.

Testable predictions

1. In humans, higher self‑reported lactose intake will correlate with increased circulating O‑GlcNAc‑MST1 levels and faster leukocyte telomere length (LTL) attrition over 2‑year follow‑up.
2. A randomized crossover trial assigning participants to lactose‑free vs. regular dairy diets for 12 weeks will show:
   • ↓ plasma galactose and UDP‑GlcNAc,
   • ↓ O‑GlcNAc‑MST1,
   • ↑ FOXO3 nuclear localization,
   • ↓ rate of telomere shortening (measured by qPCR) relative to baseline.
3. Mendelian randomization instruments for LCT expression will predict O‑GlcNAc‑MST1 levels but not LTL when the HBP is pharmacologically inhibited (e.g., with azaserine), indicating mediation through the hexosamine pathway.

Analytical approach using g‑methods

We propose to emulate a target trial of lactose restriction in the UK Biobank using longitudinal dietary records (baseline and repeat assessments). Steps:

• Define eligibility: adults aged 40‑60 without diabetes or gastrointestinal disease at baseline.
• Assign time‑varying exposure: lactose intake grams/day, updated at each questionnaire wave.
• Define outcome: change in LTL (bp/year) measured at baseline and follow‑up.
• Adjust for time‑varying confounders (physical activity, protein intake, BMI, medication use) via inverse probability weighting of a marginal structural model (IPW‑MSM).
• Stabilize weights, assess positivity, and conduct sensitivity analyses for unmeasured confounding using E‑values.
• Validate the MSM with g‑computation and compare to standard regression to quantify bias from model misspecification.
• Perform subgroup analysis by sex to test the null effects reported in prior MR studies.

Falsifiability

If lactose restriction fails to alter O‑GlcNAc‑MST1, FOXO3 localization, or telomere attrition rates beyond measurement error, the hypothesis is refuted. Similarly, if IPW‑MSM estimates show no effect after controlling for confounding, or if MR‑based mediation analysis shows no indirect effect of LCT on LTL via HBP metabolites, the mechanistic chain is not supported.

References

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC12681095/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC11258487/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC11975414/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC6001810/ [5] https://pubmed.ncbi.nlm.nih.gov/41004220/