If you've been reading about MTHFR and methylation online, you've probably encountered the categories of "overmethylation" and "undermethylation" symptoms. The framework gets used as a quick diagnostic — feel anxious? overmethylating, avoid methyl donors. Feel depressed? undermethylating, take more. The reality is more complicated. The framework can be useful, but it isn't a biochemical truth — and treating it like one leads to mistakes. In this post we'll work through what's actually going on and how to use these categories appropriately.
When we talk about methylation, we're referring to the addition of a methyl group (one carbon plus three hydrogens) to another molecule. Methyl donors — folate, methylfolate, methyl B12, SAMe — supply those groups. Methylation is involved in everything from DNA expression to neurotransmitter synthesis to detoxification.
Here's the catch most popularizations miss: methylation isn't a single global on/off switch. It's an intricate, regulated process with feedback loops. Taking SAMe or methylfolate doesn't automatically increase methylation everywhere in your body uniformly. Your body responds to the level of SAMe, the level of homocysteine, environmental drivers of epigenetic changes, and various other cellular signaling pathways. Some pathways might increase, others might compensate by decreasing, the net effect depends on what your body actually needed.
So when someone says "I'm overmethylated" or "I'm undermethylated," what they usually mean is that their symptoms feel a certain way — not that any specific biochemical state has been measured. That's not a problem in itself. It just means we should treat the framework for what it is: a symptom-based heuristic, not a diagnosis.
The original idea was described in the context of mental health by Dr. William Walsh. The classic framing:
The intuition is that someone in a more "depressive, low-motivation" state might benefit from more methyl donors, while someone in a more "anxious, wired" state might benefit from fewer. There's clinical signal in this — but it's not as clean as it sounds.
Here's where it gets messy. People aren't one symptom category. Someone can be depressed and anxious and obsessive at the same time. Someone with chronic fatigue can have all four. The framework presumes a clean phenotype that often doesn't exist clinically.
And it doesn't always predict response to treatment cleanly either. In my practice, I do see general patterns — patients with the more depressive, low-energy presentation tend to do better on methyl donors, and patients with prominent anxiety tend to have a harder time tolerating them. But "tend to" is the key phrase. It's not universal. I've had anxious patients respond beautifully to low-dose methylfolate, and I've had depressed patients get more agitated on it. The principle is useful as a guide; it falls apart when treated as a rule.
To make better decisions, I lean on lab markers that give an objective handle on methylation status — independent of how the patient says they feel. The most useful:
If MCV is high and homocysteine is high in a patient with depressive symptoms, methyl donors are very likely to help. If both are normal in a patient who's anxious and wired, more methyl donors are unlikely to be the answer regardless of how the over/under framework labels them.
Use the framework as a starting hypothesis, not a final answer. Here's how I work with it:
What works for one person doesn't always work for the next, even with apparently identical genetics. The framework's value is in helping you ask the right questions, not in handing you the answer.
Overmethylation vs undermethylation is a useful symptom-based heuristic, originally described by Dr. William Walsh. It captures real patterns I see in practice — but it's not a biochemical truth, and using it as a strict diagnostic leads people astray. Pair the symptom picture with objective markers (MCV, homocysteine, folate, B12), reassess regularly, and remember that each person's biochemistry is different. The goal isn't to "be" overmethylated or undermethylated — it's to give your body what it actually needs and let it find balance.
For more on the methylation cycle, MTHFR genetics, and how to use methyl donors safely — including the specific dosing principles I use with patients — check out my book "Don't B12 Deficient". Or work with me directly to interpret your labs and personalize the approach.
Topics: MTHFR, Methylation, Overmethylation, Undermethylation, Methyl Donors, SAMe, Methylfolate, Mental Health