These days, it feels like nearly everyone has some kind of mysterious food intolerance. This can manifest itself in a variety of ways: You suddenly become constipated for three days or come down with a killer headache but can’t quite pinpoint what the culprit is — until it strikes again out of (seemingly) nowhere. An unlucky handful will break out in hives or develop a skin rash.

While it usually is an actual intolerance — or full-blown allergy — to certain foods or food groups at play, there’s another under-the-radar possibility. For some, these seemingly disparate symptoms may actually stem from a histamine intolerance, which is when your body has trouble flushing histamine compounds out of your system.

While the condition isn’t super common — we’re talking approximately 1% of the population — interest and awareness is increasing. Roughly a dozen books about histamine intolerance were published in 2019 alone and hashtags like #histaminefree are gaining traction on social media.

It’s super hard to diagnose, perhaps even worse to live with (you can’t have avocados, booze, or chocolate!), and looks almost exactly like an allergy. But if you’ve been dealing with these kind of undiagnosable issues, answers are everything. Here, everything you want to know about histamine intolerances. Starting with...

Umm, what exactly is a histamine?

Histamine is ingested via food and stored in nearly all tissues of the body. When released, it plays a role in a ton of different actions that all keep your organs functioning and body working healthily — things like helping the smooth muscle tissue of the lungs, uterus, and stomach contract (that keeps your organs working); dilating blood vessels; stimulating gastric acid secretion in the stomach; accelerating your heart rate.

Histamine is usually a good thing, says Sara Axelrod, M.D., allergist-immunologist at ENT and Allergy Associates in East Brunswick, NJ. It helps immune cells travel into injured tissue to heal an injury or infection, for example.

The problem starts when someone isn't able to metabolize the compound. “People with histamine intolerance are low in the essential enzymes [called diamine oxidase] that help break down histamine in the body. It starts to build up faster than it can be broken down and causes unwanted symptoms,” explains Becky Campbell, doctor of natural medicine and author of The 4-Phase Histamine Reset Plan.

It sounds like (and often looks like) a food allergy or intolerance. But actually, if you have a sensitivity or intolerance to certain fare, your immune system is triggered by your digestive system, which is what causes your reaction, Dr. Axelrod explains. A histamine intolerance, on the other hand, is specifically the build up of histamines in the body. This distinction is important medically, but mostly, it matters to sufferers because histamines span a huge array of foods rather than being focused to a specific group, like an allergy to shellfish or intolerance to gluten.

What are the symptoms of a histamine intolerance?

A histamine intolerance looks like a lot like seasonal allergies — if you eat histamine-rich food or drinks, you may experience hives, itchy or flushed skin, red eyes, facial swelling, runny nose and congestion, headaches, or asthma attacks. Other symptoms can be more severe, like a drop in blood pressure, heart palpitations, and anxiety or panic attacks.

Gastrointestinal issues are also super common: One 2019 study in Intestinal Research surveyed some 60 sufferers on the topic and they reported bloating was the most common and serious symptom, followed by diarrhea, abdominal pain, and constipation.

We know — these symptoms are all over the place. That’s because there are histamine receptors all over the body. Because the symptoms are so random, sufferers are often sent to multiple specialists, Campbell adds.

And because of this bounce-around, histamine intolerance is usually attributed to other diseases like an allergy or food intolerance, mastocytosis (a rare condition where mast cells accumulate in the skin or organs), psychosomatic diseases (physical symptoms that manifest from stress and anxiety), anorexia nervosa, or adverse drug reactions, says research out of Comenius University in Bratislava, Slovakia.

What foods are high in histamine?

Because histamine levels increase with maturation, fermented foods and drinks are often the worst offenders reports a landmark review in The American Journal of Clinical Nutrition. Think: aged cheeses, yogurt, kefir, sauerkraut, processed meats, canned and pickled foods, smoked meats, vinegars, and alcoholic drinks (especially wine, champagne, and beer).

Other foods high in histamine include avocados (say it ain’t so!), legumes like chickpeas and lentils, strawberries, citrus fruits, chocolate, certain spices like curry and cinnamon, nuts like cashews and walnuts, tomatoes, bananas, eggplant, and spinach.

“Most of these foods are very high in nutritional value, so this can be confusing as to why you have to remove them. For those that can’t break down histamine efficiently, though, the risks outweigh the benefits,” Campbell says.

A life without avocados, though? There’s a whole slew of foods you can still nosh on, Dr. Axelrod reassures, including fresh meat and fish, non-citrus fruits, eggs, gluten-free grains like quinoa and rice, almond milk, coconut milk, all fresh veggies (except eggplant and spinach), and olive oil.

How is a histamine intolerance treated?

There’s no reliable lab test or procedure that can pinpoint histamines as the issue. The intolerance won’t show up on an allergy test — that is, if you go to the allergist, your skin and blood test will both come back negative. (However, this is a super important step since ruling out other triggers or underlying diseases are necessary for diagnosis, Dr. Axelrod adds.)

Some medications can help. Sometimes, an OTC antihistamine is enough to decrease the histamine load in the body and relieve symptoms, Dr. Axelrod says. But often, people see months of symptoms with no relief despite medicine. Newer research, like a small 2019 study in Food Science and Biotechnology, has found when sufferers supplement with diamine oxidase — the compound that helps most of us metabolize histamine but which sufferers are low in — their symptoms improve significantly.

But really, the only way to figure out the culprit is steering clear of the offending foods — all of them, both experts agree.

“Figuring out what causes your symptoms is not an easy process. It takes a lot of trial and error and adding and subtracting certain foods in order to make sure you’re covering all of your bases and not missing anything,” Campbell says.

How to determine if you have a histamine intolerance

As with all health issues, if you think you might have a histamine intolerance, it’s best to talk with your allergist about the best treatment plan. But generally, the only way to diagnose it is an elimination diet, where you avoid all aggravating foods for a set period of time, then slowly reintroduce them one by one to see how your body reacts.

For a histamine intolerance, here’s what you can expect for the elimination diet: It takes about three to four weeks for histamines to clear out the tissue in your body, so you want to completely eliminate high-histamine foods for at least 21 days. Then, slowly reintroduce a single food for one week at a time. If you have no reaction, add in another. If you do have a reaction, you’ll have to detox your body again (that means spending another three to four weeks off all histamine-rich foods), then adding another back in.