Inclusion Criteria:

• - Men or women at least 18 years of age - Precise diagnosis of celiac disease: serology (a-tTG IgA and/or DGP IgG and/or EmA) and histology (Marsh IIIa or greater) concordantly positive, confirmed at investigator's institution - Consuming a gluten-free diet for at least 2 years - Persistent gastrointestinal symptoms: global GSRS questionnaires ≥2 or ≥ 3 points for any of the 5 sub-dimensions - Signature of informed consent

  Exclusion Criteria:

  - Patients not interested or unable to comply with questionnaires and collection of samples of blood, feces, and urine - Complicated celiac disease (refractory, ulcerative jejunoileitis, lymphoma) - Concomitant pathologies that are uncompensated or untreated (Type I or II diabetes mellitus, hyperthyroidism, hypothyroidism, diarrhea due to bile salts, pancreatic insufficiency, bacterial overgrowth) - Consumption within the 2 weeks prior to study enrollment of medication that interferes with bowel functioning (antibiotics, NSAIDs, laxatives, metformin, opiates, anticholinergics [atropine, antidepressants, neuroleptics, antipsychotics, antiparkinsonians], anticonvulsants, antihistamines, antihypertensives [calcium antagonists, clonidine, diuretics, metal ions (aluminum), antacids, sucralfate, barium sulfate, bismuth, calcium, iron, heavy metals (arsenic, lead, mercury)], resins (cholestyramine), or any other medication deemed relevant by the investigator).

- Women that are pregnant or may become pregnant during the study

Most patients with celiac disease demonstrate substantial clinical improvement during the first few weeks after the onset of the consumption of a gluten-free diet. However, between 30-50% of patients with celiac disease have persistent gastrointestinal symptoms despite consuming a gluten-free diet and presenting negative antibodies. It has recently been reported that celiac disease patients treated with a gluten-free diet that still have persistent symptoms possess different intestinal microbiota patterns than patients without persistent symptoms. Furthermore, a pilot study showed that dietary supplementation with probiotics (Bifidobacterium infantis NLS super strain

• - Natren LIFESTART®) in untreated celiac disease patients was associated with a significant improvement in symptoms as compared to placebo.

Collectively, these findings contribute to the hypothesis that celiac disease patients on a gluten-free diet in whom gastrointestinal symptoms persist may benefit from the supplementation of Bifidobacterium infantis NLS super strain. Participants in this double-blinded crossover trial will be randomized to receive either placebo or Bifidobacterium infantis NLS super strain for 3 weeks, followed by a 2 week washout period, and then followed by 3 weeks of the other of either placebo or probiotic supplementation.