Standardizing Intestinal Permeability Assessment: Optimization of Gluten Dose and Urine Collection Times for u-GIP and Lactulose:Mannitol Ratio in Healthy Volunteers

Urinary gluten immunogenic peptides (u-GIPs) have been proposed as a complementary marker to classical intestinal permeability tests based on lactulose, mannitol, and the lactulose:mannitol ratio (LMR). However, the effects of gluten dose, urine collection interval, and sampling strategy on their performance remain insufficiently defined. This study evaluated these variables to support protocol standardization. Data from four standardized protocols including 46 healthy adults exposed to 0, 2, 4, or 10 g of gluten were analyzed. All participants ingested fixed doses of lactulose and mannitol. Urine was collected cumulatively (0-6 h and 0-15 h) or by individual voids. u-GIP levels were measured by lateral-flow immunoassay, and lactulose and mannitol by ion chromatography. u-GIP excretion showed a clear dose dependence. Lactulose excretion increased transiently only at the 10 g dose during the 0-6 h interval, while mannitol excretion and LMR were unaffected. The u-GIP excretion index showed linear proportionality at the 2 g and 4 g doses but exhibited saturation kinetics at the 10 g dose. The 4 g dose showed the lowest interindividual variability. Sampling strategies yielded equivalent results. A 4 g gluten challenge combined with a 6 h urine collection demonstrated effectiveness in healthy volunteers and may be suitable for clinical application. Further research involving larger cohorts of both healthy individuals and patients with intestinal hyperpermeability is required to validate this method.