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What happens in the gut
When someone removes grains for an extended period, several things shift:
The gut microbiome restructures. The bacterial populations that specialise in fermenting wheat starch and breaking down gluten proteins (notably certain Prevotella species) decline significantly. When gluten and refined starch suddenly return in a large dose, there is simply not enough of the right microbial machinery to process it normally. The undigested material ferments rapidly, producing gas, osmotic pressure, and loose stools. This decline is actually beneficial — these bacteria thrive on the very foods that drive insulin resistance and gut inflammation. Fewer of them means the gut is shifting towards a microbial community that supports metabolic health rather than undermining it.
The gluten response
Even in non-coeliac individuals, gluten triggers zonulin release — the protein that loosens tight junctions in the gut lining. This is not selective — zonulin release in response to gluten appears to occur in virtually everyone, not just those with coeliac disease. The reason some populations, such as traditional rice-eating cultures in Asia, do not show the same inflammatory consequences is important: rice does not contain gluten. Rice starch raises insulin, but it does not trigger zonulin. The gut-permeability problem is specific to gluten-containing grains — wheat, rye and barley — not all grains equally, and not all carbohydrates. After months of gut healing on a grain-free diet, the intestinal lining will have tightened considerably. A sudden gluten hit causes a sharp zonulin spike, fluid rushes into the intestinal lumen, and the result is rapid-onset diarrhoea — often within 1–4 hours.
The refined carbohydrate load
Pizza also delivers a large bolus of rapidly-digesting refined starch — white flour, high glycaemic, essentially sugar to the gut. After months of the gut receiving almost no fast carbohydrate, the digestive enzymes (particularly amylase) are not primed for that volume. Osmotic diarrhoea follows.
The fat combination
Pizza combines refined carbs with significant saturated fat. On a low-carb diet the gut is well-adapted to fat — but combined with the gluten and starch disruption, bile acid secretion and transit speed are both affected.
Why this is actually a positive sign
Counterintuitively, this reaction is evidence that the gut has healed and the microbiome has rebalanced away from a grain-dependent state. This is precisely why a grain-free, very low carbohydrate diet combined with intermittent fasting is so effective at restoring metabolic health. By removing the primary drivers of gut inflammation and insulin resistance simultaneously — gluten, refined starch, and constant feeding — the gut lining repairs, the microbiome rebalances, and insulin sensitivity begins to return. The body is not broken. It is responding correctly to the removal of something it was never well-designed to handle at the quantities modern diets deliver. The reaction to the pizza is a vivid, personal demonstration of exactly what grains were doing to the gut lining all along — just now the contrast is sharp enough to feel it acutely.
What About Cultures With High Wheat Diets?
Research for Italian and Mediterranean Diet High in Gluten Consumption
Studies measuring zonulin levels in healthy Italian adults do show chronically elevated baseline zonulin compared to non-gluten-eating populations. Fasano’s research group — he is the scientist who discovered zonulin — has found that gluten exposure raises zonulin in essentially everyone, but the magnitude and downstream consequences vary significantly. So yes, chronic gluten consumers likely do have a degree of chronic intestinal permeability running in the background.
But here is what makes the Italian context different
Traditional Italian wheat consumption looks almost nothing like modern Western gluten exposure:
- Traditional Italian bread is long-fermented sourdough. The extended fermentation process partially breaks down gluten proteins and significantly reduces the gliadin fraction — the specific component most responsible for zonulin release. Industrial bread, pizza dough made with fast-acting yeast, and ultra-processed wheat products do not do this.
- Traditional pasta is made from durum semolina, which has a different gluten structure and a lower glycaemic impact than soft wheat flour. It is also typically eaten al dente — which further slows digestion and reduces the glucose spike.
- Portion sizes and meal frequency matter. A traditional Italian meal pattern involves a modest pasta course as a primo — not a large bowl as a main — followed by protein and vegetables, embedded in a longer, slower meal with olive oil, which itself modulates gut permeability.
- The Mediterranean diet as a whole — rich in polyphenols, olive oil, fermented foods, and fibre — actively supports tight junction integrity, partially counteracting the zonulin effect.
The adapted gut question
There is some evidence that populations with very long ancestral exposure to wheat have subtle genetic and microbiome-level adaptations that partially buffer the zonulin response. Certain Lactobacillus strains prominent in traditional fermented food cultures can degrade gluten peptides before they reach the gut wall. This is not immunity — it is mitigation.
The modern disruption
Crucially, even in Italy, coeliac disease, non-coeliac gluten sensitivity, and IBS rates have risen sharply in the past 40 years — tracking almost exactly with the shift from traditional long-fermented, durum-based wheat foods to industrially produced fast-yeast bread, pizza, and refined pasta. The Italian gut may have co-evolved with traditional wheat over centuries, but it appears to be struggling with what industrial wheat has become just as much as everyone else’s.
The Problematic Summary
Traditional gluten-eating cultures likely carry some background intestinal permeability, partially compensated for by fermentation, food quality, meal structure, and microbiome diversity. The catastrophic rise in gluten-related pathology in those same cultures over the past 50 years suggests that compensation was always conditional — and modern industrial wheat, eaten frequently, in large quantities, without fermentation, has broken through whatever adaptive buffer existed.