You have SIBO symptoms, your genes may be the root cause.

FUT2 is a fucosyltransferase enzyme that sits on the surface of your intestinal cells and determines which nutrients and antigenic signals are available to your resident bacteria. Think of it as a gatekeeper that tells your microbiome community who belongs and who doesn’t. In people with the FUT2 gene functioning normally, this gatekeeper maintains a specific microbial ecology in the small intestine that naturally resists pathogenic overgrowth.

If you carry the non-secretor variant (roughly 20% of the population), your intestinal cells don’t produce these fucosylated antigens at all. Your small intestine becomes a much more permissive environment for bacterial overgrowth because the usual ecological boundaries that control bacterial populations are blurred. Non-secretors also have reduced capacity to absorb vitamin B12 directly from food, which further compromises immune function in the gut.

You experience this as a chronically inflamed small intestine that struggles to maintain its own microbial balance. Your gut never seems to have a stable bacterial population. Even after antimicrobial treatment, whatever dysbiotic population you had tends to return because the underlying genetic barrier function remains compromised.

NOD2 is an innate immune receptor that sits inside your intestinal cells and literally recognizes specific bacterial cell wall components, like a security camera that only activates when it sees a specific badge. When NOD2 detects pathogenic bacteria, it triggers an immune response that eliminates them before they can establish overgrowth. This system is your gut’s first line of defense against dysbiosis.

If you carry one of the NOD2 variants (R702W, G908R, or 1007fs), each found in roughly 7-10% of people with European ancestry, your intestinal cells become essentially blind to certain bacterial invaders, so pathogenic organisms can establish themselves unchecked. Your immune system doesn’t see them as threats because NOD2 isn’t signaling correctly. SIBO becomes much more likely because the bacteria you should be eliminating are flying under your immune system’s radar.

You experience this as an inflamed small intestine that struggles to clear bacterial blooms. You may notice that your SIBO symptoms are worse after eating foods with certain bacterial triggers, but your immune system never mounts the defense it should. Recurrent SIBO is very common with NOD2 variants because the bacteria never get truly eliminated.

TNF (tumor necrosis factor-alpha) is a master inflammation signaler in your gut. Your cells use it to coordinate immune responses and maintain the tight junctions that hold your intestinal barrier together. When TNF signaling is balanced, your intestinal lining stays sealed and impermeable to bacterial lipopolysaccharides and other inflammatory molecules that would otherwise leak through.

If you carry the TNF -308A allele (found in roughly 30% of the population), you produce elevated TNF-alpha levels, which paradoxically increases intestinal permeability instead of protecting it. Your tight junctions loosen, your intestinal barrier becomes leaky, and bacterial toxins from SIBO organisms seep into your bloodstream. This triggers systemic inflammation and makes SIBO symptoms much more severe than they should be from bacterial overgrowth alone.

You experience this as extreme bloating, joint pain, brain fog, and fatigue that seem disproportionate to what bacterial overgrowth alone would cause. You may have food sensitivities that seem to develop or worsen after SIBO onset because your compromised barrier is letting bacterial antigens through. Your immune system stays in a constant state of alert even after you’ve treated the bacteria.

SLC6A4 encodes the serotonin transporter, the protein that recycles serotonin back into nerve cells after it’s been released in your gut. This is critical because roughly 95% of your body’s serotonin is actually in your gut, not your brain. Serotonin controls intestinal motility, the smooth muscle contractions that move food and bacteria through your small intestine. When serotonin recycling is efficient, your small intestine contracts in coordinated waves that propel bacteria downward into the colon where they belong.

If you carry the short allele variant of SLC6A4 (5-HTTLPR, found in roughly 40% of people), your serotonin transporter is less efficient, so serotonin accumulates in the synaptic space and your gut nerve cells become desensitized to it. Your small intestine loses its ability to generate strong, coordinated contractions. Bacterial populations stall in your small intestine instead of being cleared naturally. You’ve created the perfect environment for SIBO without a single dietary mistake.

You experience this as chronic constipation alternating with diarrhea, extreme bloating after meals, and a sense that food just sits in your stomach and small intestine. Your gut-brain axis is compromised, so you may also have anxiety or mood sensitivity to food choices and stress.

VDR is the vitamin D receptor, the protein that lets your cells actually respond to vitamin D. Vitamin D doesn’t work on its own; it has to bind to VDR to have any effect. In your gut, vitamin D is essential for maintaining immune tolerance, reducing excessive inflammation, and strengthening your intestinal barrier. When VDR signaling is working well, you maintain a healthy balance between fighting pathogenic bacteria and not overreacting to harmless commensals.

If you carry unfavorable VDR variants (there are multiple, but the most studied is the FokI ‘f’ allele), your cells don’t respond efficiently to vitamin D, so even if your blood vitamin D levels are technically normal, your gut tissue is functionally vitamin D deficient. Your immune system can’t properly regulate itself in the small intestine. You develop excessive inflammatory responses that damage your intestinal barrier while simultaneously being unable to mount effective responses against pathogenic bacteria like those causing SIBO.

You experience this as a gut that seems simultaneously inflamed and unable to defend itself. You may have elevated vitamin D levels on bloodwork but still have ongoing gut inflammation. SIBO symptoms may be worse during winter months or in people with darker skin tones or limited sun exposure because VDR variants make you more dependent on very high vitamin D levels to achieve functional sufficiency.

MTHFR encodes an enzyme that converts folate into methylenetetrahydrofolate, a critical substrate in your methylation cycle. Your methylation cycle is how your body processes and eliminates toxic metabolites, including those produced by pathogenic bacteria in your small intestine. When MTHFR is working well, your cells can quickly neutralize bacterial endotoxins and inflammatory molecules that SIBO organisms produce.

If you carry the MTHFR C677T variant (found in roughly 40% of the population), your enzyme’s activity is reduced by 40-70%, which means your cells process bacterial toxins far more slowly than they should. SIBO organisms pump out lipopolysaccharides and other inflammatory metabolites, but your body can’t clear them efficiently. You accumulate toxic bacterial byproducts in your bloodstream. The inflammatory burden from SIBO becomes much more severe than the bacterial count alone would predict.

You experience this as extreme fatigue, brain fog, joint and muscle pain, and unexpectedly severe symptoms compared to what SIBO testing suggests. You may feel toxic or poisoned after eating, because you literally have more bacterial metabolites circulating. People with MTHFR variants often have worse SIBO symptoms and slower recovery because their detoxification capacity can’t keep up.