Quick answer: Eosinophil protein X, also called eosinophil-derived neurotoxin, is a protein released by eosinophils, white blood cells that live in the gut lining and are tied to allergic responses and defense against parasites. A higher stool level is generally read as a sign of more eosinophil activity in the gut. Studies show it rises with active inflammatory bowel conditions and with food allergy in infants, although neutrophil markers such as calprotectin often separate gut conditions better.
Key takeaways
Eosinophil protein X is released by eosinophils, a type of white blood cell involved in allergy and inflammation.
Small studies link higher stool levels with active inflammatory bowel disease.
Evidence in people without known gut disease is limited.
Most of the stool inflammation markers on a gut panel come from neutrophils, the first-responder white blood cells. Eosinophil protein X is different, because it comes from eosinophils, a type of white blood cell that lives in large numbers in the lining of the digestive tract and is closely tied to allergic responses and to defense against parasites. When eosinophils are activated, they release proteins stored in their granules, and EPX is one of them. Measuring it in stool offers a window into eosinophil activity in the gut, which is a different question from the neutrophil activity I describe in my article on calprotectin.
This article is educational. It explains what the marker reflects and how researchers connect it to the deeper mechanisms of aging, so you can have a more informed conversation with your own provider.
What does a higher eosinophil protein X level in stool mean?
EPX is a small protein stored in the granules of eosinophils. In 1989, Slifman and colleagues compared it side by side with eosinophil-derived neurotoxin, or EDN, and found that the two had virtually identical properties and are probably the same protein, which is why research papers use both names. In a 1999 study in the American Journal of Gastroenterology, Saitoh and colleagues measured stool EPX in 42 people with ulcerative colitis, 37 people with Crohn’s disease and 29 controls. EPX was significantly higher during active disease than during quiet periods, and it was released from eosinophils more readily and stayed more stable in stool than a related eosinophil protein, which makes it practical to measure.
A higher stool EPX result is therefore generally read as a sign of more eosinophil activity in the gut lining. That activity can relate to food-related immune reactions, parasites and other intestinal infections, or inflammatory bowel conditions, so the result raises a question rather than answering one. In a 2024 study of 197 adults with inflammatory bowel disease or irritable bowel syndrome, Ekoff and colleagues found that stool EDN was higher in inflammatory bowel disease, but neutrophil proteins such as calprotectin separated the two groups better than EDN did. That is why EPX is usually read alongside other markers rather than on its own.
How is eosinophil protein X connected to aging and root causes?
Eosinophils are not simply troublemakers. In a 2021 review in Seminars in Immunopathology, Gurtner and colleagues described how large numbers of eosinophils live in the lining of the gut, where they help maintain the intestinal barrier when inflammation damages the surface cells and help defend against bacteria that get past that barrier. A 2022 review by Salvo-Romero and colleagues added that gut eosinophils communicate with nerves and with almost every other cell type around them, and that eosinophil activation has been found in some people with irritable bowel syndrome and functional dyspepsia. That makes eosinophils a clear example of intercellular signaling, one of the core mechanisms researchers connect with aging, and it ties them to the gut-brain connection and to the way stress can be felt in the digestive tract.
The microbiome is part of the story too. In a 2016 review in the Journal of Leukocyte Biology, Rosenberg and colleagues summarized mouse studies suggesting that eosinophils help maintain balance in the gut and that their absence can change the makeup of the gut bacteria and the response to gut pathogens. The relationship appears to run in both directions, although much of this work is still in animals.
The most direct aging link comes from fat tissue rather than the gut. In a 2020 study in Nature Metabolism, Brigger and colleagues found that eosinophils in fat tissue go through major changes in distribution and function with age, in both humans and mice, and that these changes were associated with low-grade inflammation throughout the body. In aged mice, transferring eosinophils from young mice dampened that inflammation and improved physical and immune fitness. This connects eosinophils to chronic low-grade inflammation, although it is early research and it does not tell us what a stool EPX result means for aging in people.
From a root-cause perspective, I want to know why eosinophils in the gut might be activated in the first place. Food reactions, parasites and other infections, gut barrier health, the balance of the microbiome, ongoing stress, sleep and food quality are all worth considering, along with an inherited tendency toward allergy.
What does the research show about Eosinophil Protein X?
Researchers measured stool eosinophil proteins in 42 people with ulcerative colitis, 37 with Crohn’s disease and 29 controls. Eosinophil protein X was higher during active disease than in quiet phases, tracked with other stool markers of inflammation, and was more stable in stool than a related protein. The study was small and used a 48-hour stool collection. (Saitoh et al., 1999)
A two-year study followed 104 people with inflammatory bowel disease and tested stool every three months. In ulcerative colitis, a doubling of related eosinophil proteins was linked with about 27 to 31% higher risk of flare, but in Crohn’s disease levels were actually higher in people who stayed well. The study measured related eosinophil proteins rather than eosinophil protein X itself. (Amcoff et al., 2019)
The research is small and mostly in people with known inflammatory bowel disease, so a single result in someone without that history is hard to interpret.
Related reading
If you would like to explore related topics, my articles on calprotectin, secretory IgA and food sensitivities go deeper. You can also see the specialty tests I commonly recommend on my functional labs page.
Common questions about Eosinophil Protein X (EPX)
Is eosinophil protein X the same as EDN?
For practical purposes, yes. A 1989 study comparing the two found virtually identical properties and concluded that they are probably the same protein, so labs and studies may use either name.
How is EPX different from calprotectin?
Calprotectin comes mainly from neutrophils, the first-responder white blood cells, while EPX comes from eosinophils, which are more closely tied to allergic responses and parasites. Looking at both gives a fuller picture of which immune cells are active in the gut.
Which everyday factors are commonly discussed around eosinophil activity in the gut?
Food reactions, parasites and other infections, gut barrier health, the balance of the microbiome, ongoing stress and sleep are the factors most often discussed, along with an inherited tendency toward allergy.
Sources
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before making changes to your diet, exercise, supplements, or medications, especially if you are pregnant, nursing, or managing a health condition.
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