You have bloating and brain fog. Your genes may be driving SIBO.

FUT2 encodes a fucosyltransferase, an enzyme that decorates the lining of your small intestine with specific sugar patterns. These patterns are like flags that tell your gut bacteria which ones are welcome and which ones get suppressed. Your immune system reads these flags and enforces the microbiome you’ve inherited.

If you carry the non-secretor variant of FUT2, roughly 20% of the population does, your intestinal lining doesn’t display these bacterial-recognition flags the way it should. Your gut loses its ability to selectively encourage beneficial bacteria and suppress pathogenic or overgrowth-prone species. This opens the door for opportunistic bacteria to flourish in your small intestine.

You likely experience chronic bloating that doesn’t improve with standard probiotics, because the beneficial bacteria you’re taking can’t establish themselves without the right molecular welcome signals. You may also have B12 absorption issues, since FUT2 variants reduce intrinsic factor production and affect how B12 is extracted from food.

MTHFR catalyzes the conversion of folate into its active form, methylfolate, which your cells use to produce neurotransmitters, stabilize DNA, and repair tissue. Your gut depends heavily on this methylation cycle to regenerate intestinal cells and produce serotonin, which drives motility.

The MTHFR C677T variant, carried by approximately 40% of the population, reduces this enzyme’s efficiency by 40 to 70%. Your cells are producing methylated compounds at a fraction of the rate they should, leaving you depleted in the exact neurochemical your gut needs to move food forward. Without adequate methylation, your intestinal cells also repair more slowly, making your barrier weaker.

You experience constipation or irregular motility (bacteria thrive in slow-moving sections), low serotonin symptoms like anxiety or low mood, and a gut barrier that lets bacteria slip across the epithelium more easily. SIBO becomes likely because motility is sluggish and the immune barrier is compromised.

VDR encodes the vitamin D receptor, a nuclear receptor that sits in your intestinal cells and immune cells, reading vitamin D signals and turning on genes that maintain your barrier function and immune tolerance. Without proper VDR signaling, your gut either overreacts to harmless bacteria or fails to suppress pathogenic overgrowth.

Common VDR variants (FokI, BsmI, ApaI) reduce the receptor’s sensitivity, so even adequate vitamin D levels don’t trigger the protective genes you need. Your immune system loses the ability to distinguish between commensal bacteria that belong and pathogenic overgrowth, making SIBO more likely and harder to clear. Roughly 40% of people carry at least one VDR variant that impairs this function.

You often have a permeable gut barrier, low stomach acid, and inadequate secretory IgA production (your first-line immune defense in the gut). You may get frequent infections or food sensitivities because your immune gate isn’t working. SIBO establishes itself because your gut can’t mount a targeted immune response.

SLC6A4 encodes the serotonin transporter, a protein that sits on nerve cells and muscle cells in your intestinal wall. After serotonin does its job signaling nerve cells to contract and push food forward, this transporter recycles the serotonin back into the cell so it can be used again. Your gut is responsible for 95% of your body’s serotonin production, and SLC6A4 controls how efficiently that serotonin works.

If you carry the short allele of the 5-HTTLPR variant in SLC6A4, roughly 40% of the population does, your serotonin transporter works too efficiently. Serotonin is recycled too quickly, leaving less available in the intestinal space to drive gut contractions and coordinate normal motility. Your small intestine doesn’t move food forward quickly enough.

You experience constipation alternating with loose stool, bloating that worsens with stress (stress increases serotonin reuptake), and visceral pain or sensitivity to gut distension. Bacteria colonize the slow-moving sections, SIBO blooms, and the gas production worsens the sensation of distension,a vicious cycle.

NOD2 is an intracellular receptor in your intestinal immune cells that recognizes specific patterns on bacterial cell walls. When NOD2 detects these patterns, it triggers your innate immune system to produce antimicrobial peptides and coordinate immune surveillance. This system keeps opportunistic bacteria in check and maintains the balance between tolerance and defense.

NOD2 variants (R702W, G908R, 1007fs frameshift), carried by roughly 7 to 10% of people with European ancestry, break this recognition system. Your intestinal immune cells lose the ability to sense when bacteria are overgrowing and fail to trigger the antimicrobial response needed to suppress them. The overgrowth goes undetected and unchecked.

You have a history of recurrent SIBO or Crohn’s-like symptoms, chronic low-grade inflammation even when your CRP is normal, and poor response to standard SIBO treatments because your immune system isn’t actually fighting back. Your gut treats the bacterial overgrowth as normal, so it keeps spreading.

TNF encodes tumor necrosis factor-alpha, a master inflammatory cytokine that coordinates immune responses and is essential for controlling infections. However, TNF is a two-edged tool: the right amount protects you, too much damages your intestinal barrier. Your intestinal cells are exquisitely sensitive to TNF levels.

The TNF -308G>A variant, carried by roughly 30% of the population, increases TNF production. Your immune system produces more TNF in response to bacterial or dietary triggers, creating chronic low-grade inflammation that increases intestinal permeability and weakens the barrier. This increased permeability allows bacterial lipopolysaccharides and toxins to cross into your bloodstream, triggering systemic inflammation.

You experience leaky gut symptoms: food sensitivities that developed suddenly, chronic joint or muscle pain alongside your SIBO, and autoimmune-like reactions that standard testing doesn’t explain. The elevated TNF makes your barrier more permeable to bacterial overgrowth while also creating the inflammation that perpetuates SIBO.