You're doing everything right and still struggling with digestion. Here's the biological reason.

MTHFR is the enzyme that converts dietary folate and B12 into their active, usable forms. Your stomach lining cells, like all cells, need these active B vitamins to function properly. Without them, your cells can’t produce stomach acid efficiently, and the acid that is produced doesn’t trigger the downstream digestive cascade the way it should.

The MTHFR C677T variant, carried by roughly 40% of the population, reduces enzyme activity by 40 to 70%. That means your body is struggling to convert B vitamins even when you’re eating them. You can consume B vitamins all day and still be functionally depleted at the cellular level, which directly impairs stomach acid production and the pH-sensitive enzymes that break down protein.

When your MTHFR is underperforming, you don’t just feel bloated after meals. Your body can’t absorb the protein you’re eating because the initial breakdown step is incomplete. Food sits in your stomach longer, triggering that uncomfortable fullness and triggering reflux as your body struggles to move it forward.

HLA-DQ2 is an immune receptor that sits on the surface of your gut cells and presents antigens to your immune system. Think of it as a security checkpoint. If you carry the HLA-DQ2 haplotype, your checkpoint is configured to recognize gluten peptides as a threat. When gluten enters your small intestine, your immune system attacks the intestinal villi, the finger-like structures that absorb nutrients.

Roughly 25 to 30% of people with European ancestry carry HLA-DQ2. Not all of them develop celiac disease, but everyone who carries HLA-DQ2 and consumes gluten is triggering an immune attack on their intestinal lining. That attack damages the cells that produce stomach acid and the cells that absorb nutrients. Even if you don’t have clinical celiac disease, gluten exposure will impair your digestive capacity.

You may not have obvious GI symptoms with every gluten exposure. But over weeks and months, gluten damages the lining of your stomach and small intestine. Your stomach acid production declines, nutrient absorption drops, and you develop the constellation of symptoms: bloating, malabsorption, fatigue, and brain fog.

LCT (or MCM6) controls the production of lactase, the enzyme that breaks down lactose, the primary sugar in milk. In infancy, all humans produce lactase. But as you age, if you don’t carry the lactase persistence variant, lactase production declines. Your small intestine stops making the enzyme, so lactose sits undigested in your gut.

The LCT -13910C>T variant determines lactase persistence. If you carry the C/C genotype, roughly 65% of the global population, lactase production shuts down after childhood and remains low or absent for life. You become lactose intolerant, meaning dairy causes fermentation, gas, bloating, and often loose stools.

But here’s the overlooked consequence: undigested lactose ferments in your gut, producing gas and changing your gut pH. That altered pH affects stomach acid signaling and your ability to absorb minerals like calcium, magnesium, and iron. If you’re lactose intolerant and still consuming dairy, you’re not just getting bloated; you’re impairing mineral absorption and the pH environment your stomach needs to produce adequate acid.

FUT2 is a fucosyltransferase that adds fucose sugars to the surface of your gut cells. These sugars act as food for beneficial bacteria and as signaling molecules that shape your gut microbiome composition. If you’re a FUT2 secretor, you’re essentially feeding your beneficial bacteria the information they need to thrive. If you’re a non-secretor, your gut bacteria composition is skewed, and your B12 absorption suffers.

Non-secretor status, carried by roughly 20% of the population, alters gut microbiome diversity and impairs intrinsic factor-independent B12 absorption. That means your gut bacteria can’t produce as much B12, and the bacteria that would normally help you absorb it are underrepresented. B12 is essential for stomach acid production, nerve function, and red blood cell formation.

When FUT2 non-secretor status reduces B12 absorption, your stomach lining cells become depleted in the B vitamin that fuels stomach acid production. Over time, your acid output declines, and food doesn’t break down the way it should. You also become more susceptible to certain infections because your microbiome is less diverse and less able to defend against pathogens.

TNF (tumor necrosis factor-alpha) is a pro-inflammatory cytokine produced by immune cells in your gut. In small amounts, TNF is protective; it signals for immune activation and barrier defense. But when the TNF -308G>A variant elevates baseline TNF production, your gut is chronically inflamed. That inflammation weakens the tight junctions between intestinal cells, creating a leaky gut.

Roughly 30% of the population carries the A allele at TNF -308. Carriers of this allele produce elevated TNF-alpha, which increases intestinal permeability and allows bacterial lipopolysaccharides and partially digested food to cross the intestinal barrier. Your immune system then attacks these particles as if they were pathogens, triggering food sensitivities and further inflammation.

When your TNF is elevated and your gut is leaky, the stomach lining cells that produce acid become inflamed and dysfunctional. Your acid output drops, food isn’t broken down properly, and the food particles that do cross the leaky barrier trigger immune reactions that compound the problem. You’re caught in a cycle of inflammation, malabsorption, and food sensitivities.

SOD2 (superoxide dismutase 2) is an antioxidant enzyme that lives inside the mitochondria of your cells. It neutralizes superoxide radicals, the toxic byproducts of cellular energy production. Your stomach lining cells are among the most metabolically active cells in your body; they’re constantly producing and releasing stomach acid, which requires enormous amounts of energy. If your SOD2 is underperforming, these cells accumulate oxidative damage and can’t function.

The SOD2 Ala16Val variant, carried by roughly 30 to 40% of the population, reduces mitochondrial antioxidant capacity and leaves stomach cells vulnerable to oxidative damage. Over time, the parietal cells that produce stomach acid accumulate damage, their mitochondria become dysfunctional, and acid production declines. The stomach lining becomes inflamed and can’t regenerate efficiently.

When your SOD2 is compromised, your stomach cells are essentially getting exhausted. They can’t produce enough acid because they’re spending all their energy trying to repair oxidative damage. You may notice your symptoms worsen with stress, with foods high in oxidative load (fried foods, processed foods), or when you’re not sleeping well, all situations that increase cellular oxidative stress.