The Other Histamine Enzyme: Meet HNMT

HNMT: The Histamine Enzyme Nobody’s Talking About

If you’ve looked into histamine intolerance before, you’ve probably heard all about DAO.

DAO is the enzyme that breaks down histamine in your gut. It’s the star of the show when it comes to reactions after meals, like hives, bloating, or heart palpitations. If your symptoms go beyond digestion, it could be time to look at a different enzyme: HNMT, or Histamine-N-Methyltransferase.

Symptoms of low HNMT:

• Brain fog

• Anxiety

• Sleep issues

• Hormone fluctuations

• Skin irritation

• Symptoms that flare up even without eating a high-histamine food

What is HNMT?

HNMT is the enzyme responsible for breaking down histamine inside your cells, especially in the brain, lungs, and other soft tissues. Unlike DAO, which works in the gut and bloodstream, HNMT works inside the tissues themselves. Think of it as intracellular cleanup. DAO issues tend to show up quickly (like post-meal reactions), but HNMT dysfunction can cause more lingering and systemic symptoms, especially those involving mood, sleep, and cognitive function.

Why would this enzyme be low or sluggish?

• Genetics. Some people have SNPs (gene variations) that reduce HNMT activity

• Chronic stress and poor sleep, which deplete nutrients and increases histamine

• Oxidative stress from inflammation, infections, or toxins

• Elevated histamine from environmental, immune, or food triggers

• Hormone changes. Estrogen in particular can increase histamine levels

• Poor liver function, where HNMT does a lot of its work

• Nutrient deficiencies: B12, folate, magnesium, and SAMe, which support methylation (the process HNMT relies on)

• Use of certain medications

Plus, some compounds can directly interfere with HNMT. This includes alcohol, which competes for the same enzyme systems, and possibly some plant compounds. For example, secondary metabolites found in Nigella sativa (black seed oil) and Curcuma xanthorrhiza (a turmeric relative) were found to act as HNMT inhibitors in one study. That doesn’t mean everyone should avoid them, but it may explain why curcumin supplements or even turmeric-heavy meals make some people feel worse, especially if they’re already overloaded with histamine. If something “healthy” keeps making your symptoms flare, it’s worth considering whether it could be impacting histamine clearance.

How to support HNMT

Even if you’ve never done genetic testing, there’s a lot you can do to support this enzyme.

• Focus on liver support and gentle detox practices. Your liver is a key player in methylation and histamine clearance

• Avoid alcohol

• Make sure you’re eating enough protein and supporting digestion

• Get your B vitamins in, especially B12 and folate (ideally in their active forms like methylcobalamin and methylfolate)

• Magnesium and SAMe* can also help, but talk to a practitioner before jumping into supplements. *SAMe may not be well tolerated by those with slow COMT; it can increase stimulation and anxiety in some people.

• Reduce exposure to toxins and mold, which place extra burden on methylation and detox pathways

• Support nervous system regulation. Stress slows everything down, including methylation

TL;DR

You don’t have to know your exact SNPs to start seeing improvements. Supporting HNMT can be especially helpful for people who’ve tried low-histamine diets or DAO supplements without much relief. So, if histamine intolerance is on your radar but the usual tricks aren’t working, it might be time to go a little deeper and see if supporting HNMT is helpful for you.

I have a free histamine & hormones guide if you'd like to learn more about how they interact, including information on the OG histamine enzyme, DAO.

Disclaimer: I do not diagnose, treat, prevent, or cure any disease or condition. Nothing I share is intended to substitute for the advice, treatment or diagnosis of a qualified licensed physician. I may not make any medical diagnoses or claim, nor substitute for your personal physician’s care. It is my role to partner with you to provide ongoing support and accountability in an opt-in model of self-care and any changes should be done under the supervision of a licensed physician.