Spinach triggers eczema primarily through its high histamine content — it is one of the highest-histamine vegetables. Histamine is the same chemical the body releases during allergic reactions, and eating high-histamine foods adds directly to the body's histamine load, making skin itchier and more inflamed.

Around one third of people with eczema have reduced ability to break down histamine, making them especially sensitive to high-histamine foods like spinach. Spinach's histamine content rises rapidly during storage — spinach that has been sitting in a fridge for several days may have significantly more histamine than when it was first bought. Frozen spinach, which is blanched and frozen quickly after harvest, may actually contain less histamine than 'fresh' supermarket spinach that has spent days in transit and refrigeration.