The Impact of Nutrition on DNA Methylation: Methodological Challenges in Understanding Cause and Effect.

DNA methylation is a key epigenetic mechanism linking nutritional exposures to gene regulation and downstream phenotypes. Both undernutrition and overnutrition are associated with distinct methylation signatures, some of which persist beyond the initial exposure window and may relate to long-term metabolic, immune, and neurodevelopmental outcomes. However, the extent to which these associations reflect causal mechanisms, adaptive responses, or secondary effects remains unresolved. Here, we synthesize current evidence on how nutrition influences DNA methylation across the life course, integrating biochemical pathways, metabolic signaling, and microbiome-derived processes within a unified framework. We highlight how these diverse inputs converge on core regulatory axes, including methyl donor availability, enzyme activity, and chromatin context. We then evaluate emerging long-read sequencing, single-cell methylomics, deconvolution strategies, and multi-omic integration methodologies that are improving cellular resolution and enabling a more mechanistic interpretation of nutritional epigenetic variation. Despite these advances, major challenges remain, including tissue specificity, measurement limitations, and the difficulty of distinguishing causation from correlation in observational data. We argue that progress will depend on longitudinal and interventional study designs, improved causal inference frameworks, and integration of functional validation with high-resolution molecular profiling. Addressing these challenges will be critical for determining whether nutrition-associated methylation changes represent biomarkers, mediators, or causal drivers of disease, and for translating epigenetic insights into precision nutrition strategies.