What is gluten? (1)
The wheat kernel contains 8%–15% of protein, from which 10%–15% is albumin/globulin and 85%–90% is gluten. Gluten is a complex mixture of hundreds of related but distinct proteins, mainly gliadin and glutenin. Collectively, the gliadin and glutenin proteins are referred to as prolamins, which represent seed proteins insoluble in water, but extractable in aqueous ethanol and are characterized by high levels of glutamine (38%) and proline residues (20%).
Gliadin is classified according to their different primary structures into alpha, beta, gamma, and omega (α, β, γ, and ω) gliadins. The ω-5 gliadin content increases with fertilization and temperature during maturity. Gliadin contains peptide sequences (known as epitopes) that are highly resistant to gastric, pancreatic, and intestinal proteolytic digestion in the gastrointestinal tract, escaping degradation in the human gut. These proline-rich residues create tight and compact structures that can mediate the adverse immune reactions in coeliac disease.
Amylase-trypsin inhibitors (ATIs) account for approximately 2%–4% of the total protein in modern wheat (where gluten accounts for 80%–90%). Some of the ATI fractions have been reported to activate a toll-like receptor-4 dependent pathway leading to the release of pro-inflammatory cytokines from monocytes, macrophages, and dendritic cells derived from both coeliac and non-coeliac patients.
The average daily gluten intake in a Western diet is thought to range from 5 to 20 g/day. The Food and Drug Administration in the United States and the International standard, the Codex Alimentarius Commission, define gluten free as the allowable level of gluten detectable to 20 ppm. The safe threshold of gluten consumption for individuals with coeliac disease can vary significantly (10–100 mg/day).
Celiac disease. Dietary gluten unequivocally causes celiac disease, an immune-mediated disease that affects about 1% of Western populations. The immune response triggered is specific to toxic peptides within the gliadin fraction of the gluten protein and initiates an immune response causing mucosal inflammation, small intestinal villous atrophy, and increased gut permeability. There is a genetic disposition where the disease is triggered in susceptible individuals carrying the human leukocyte antigen (HLA)-DQ2 or DQ8.14.
Wheat allergy is an IgE-mediated reaction to the insoluble gliadins of wheat. The symptoms usually develop within minutes to hours after ingestion and include itching and swelling, skin rash, and life-threatening anaphylaxis. Symptoms can encompass baker’s asthma and rhinitis , atopic dermatitis, urticaria, or wheat-dependent exercise-induced anaphylaxis.
Non-coeliac gluten sensitivity is likely that this syndrome encompasses a heterogeneous group of patients, reporting a range of gastrointestinal and extraintestinal symptoms, clinical histories and characteristics.
1. J. R. Biesiekierski, What is gluten? J. Gastroenterol. Hepatol. 32 Suppl 1, 78–81 (2017).