DNA methylation is the addition of methyl groups to cytosine bases — typically at CpG sites. Doesn't change the DNA sequence but profoundly affects which genes are expressed.
Mechanism — Methylation generally silences gene expression. Patterns shift predictably with age — some sites lose methylation, others gain it. Horvath and other epigenetic clocks use these patterns to estimate biological age.
Use case — Support healthy methylation with methylated B-vitamins (methylfolate, methylcobalamin), choline, betaine, glycine, and sufficient SAMe precursors. Stress and inflammation drive aberrant methylation.
Caveats — Methylation patterns are tissue-specific and dynamic. Commercial methylation tests often oversell what current science can interpret.