A Diagnosis Without a Biomarker
Non-coeliac gluten sensitivity (NCGS) describes individuals who report gastrointestinal and extra-intestinal symptoms after gluten ingestion, yet test negative for coeliac disease (normal tTG-IgA, no villous atrophy) and wheat allergy (negative IgE to wheat). Without a validated biomarker, diagnosis depends entirely on symptomatic response to gluten elimination and rechallenge — a methodological minefield.
The Salerno expert criteria (2015) attempted to standardise diagnosis by requiring a double-blind, placebo-controlled crossover challenge with gluten. Under these strict conditions, the picture becomes murkier than patient self-reports suggest. In a landmark Italian trial by Di Sabatino et al. (2015), only about one-third of self-identified NCGS patients could reliably distinguish gluten capsules from placebo. Other studies report nocebo rates — symptom worsening on placebo — as high as 40 percent.
The Fructan Confounding Problem
A pivotal 2018 study by Skodje et al. in Gastroenterology challenged the gluten hypothesis directly. In a double-blind crossover trial, IBS patients with self-reported NCGS were randomised to muesli bars containing isolated gluten, fructans (a FODMAP carbohydrate present in wheat), or placebo. Fructans — not gluten — produced significantly more symptoms than placebo. Gluten did not differ from placebo. This suggests that for many NCGS patients, the fermentable carbohydrate fraction of wheat, rather than the gluten protein, drives symptoms.
Beyond Gluten: Wheat ATIs
Wheat contains more than gluten. Amylase-trypsin inhibitors (ATIs) are pest-resistance proteins that activate toll-like receptor 4 on monocytes, macrophages, and dendritic cells, triggering an innate immune response independent of the adaptive immune cascade seen in coeliac disease. ATIs are present in modern wheat varieties and are not removed by fermentation or sourdough processing to the same degree as FODMAPs. They may explain the subset of NCGS patients whose symptoms have a genuine immunological basis, distinct from both coeliac disease and FODMAP sensitivity.
Innate Immunity, Not Adaptive
Biopsy studies in confirmed NCGS patients (by Sapone et al. and others) show increased intraepithelial lymphocytes and upregulation of innate immune markers (TLR2, TLR4) without the adaptive Th1 response or anti-tTG antibodies that characterise coeliac disease. Intestinal permeability may be increased through claudin-dependent pathways rather than the zonulin pathway dominant in coeliac disease.
Where This Leaves Patients
NCGS is almost certainly a heterogeneous entity: some patients react to fructans, some to ATIs, some experience a genuine nocebo response, and a small subset may have an as-yet-uncharacterised immune-mediated wheat sensitivity. The clinical implication is that blanket gluten avoidance may be unnecessary for many — a structured FODMAP elimination that separately tests wheat fructans against gluten-containing but low-FODMAP foods can clarify the true trigger. Sourdough fermentation, which substantially reduces both fructans and ATI bioactivity, is tolerated by some self-identified NCGS patients and is worth trialling before lifelong dietary restriction.