Methylation is a chemical reaction that happens in all of the cells in our body, and is integral to our metabolic function. Without methylation, our body would cease to function.
Quoting Chris Kresser, ‘methylation happens more than a billion times per second in our body’. The amount of times that our body needs methylation goes to show its importance.
But what does methylation being a chemical reaction actually mean specifically? Well methylation occurs when one molecule passes a methyl group to another molecule.

Note that a methyl group is a carbon atom linked to three hydrogen atoms. Think of a Methane molecule that is missing one of its hydrogen atoms.
Note that the speed at which methylation occurs is dependent on enzymes.
In biological systems, like the human body, we need methylation to handle heavy metals, regulate gene expressions, regulate the function of proteins, and the processing of RNA.
Methylation is also required for the production of creatine, carnitine, CoQ10, phosphatidylcholine, melatonin, and many other substances in the body.
Methylation controls sulfur metabolism. And Sulfur metabolism influences the body’s demand for methyl groups, glutathione, and sulfur metabolites like cysteine, taurine, and sulfate.