So how are methyl groups in our body depleted? Your methyl groups are depleted by consuming a poor diet loaded with "white death" - white sugar, white flour, and white salt, "bad oils" - trans-fatty acids and hydrogenated oils, produce loaded with pesticides and herbicides, junk food, drinks loaded with caffeine, high fructose corn syrup and acid, genetically-modified foods, and stress!
But what helps create methyl groups? Proper Stomach acid (aka HCL). This is WHY it is vital for everyone to have the proper amount of stomach acid (HCL)! We create massive amounts of stomach acid when we are young but as we age, we make less and less. Again, further decreases in stomach acid are caused by excess stress and a poor diet. Methyl groups are VITAL for normal cell replication at the level of the DNA! They literally turn genes "on" or "off". When you have depleted methyl groups, bad genes like cancer causing oncogenes are turned "on" and good genes, like cancer preventing tumor-suppressor gens, are turned "off". Methyl groups are also vital for phase II liver detoxification, protein methylation, homocysteine metabolism (increasing the methyl groups decreases inflammation), neurotransmittter synthesis, and nucleic acid synthesis.
Inadequate methylation capacity can lead to ADD/ADHD, birth defects, depression, cognitive decline, diseases, and cancer. Impaired methylation has even been associated with autism.
So a person has depleted methyl groups and all of a sudden they come in contact with a toxin, a dental infection, a severe physical or emotional stress, and...BOOM... they express the bad gene. The next thing you know, they develop a neurodegenerative disorder like Parkinson's, ALS, or Alzheimer's, they develop an autoimmune condition, or they develop cancer. None of these conditions drop out of the sky and land on a person; there has to be a cause, and a common cause is depleted methyl groups.