You eat normally and suddenly look six months pregnant. Your genes may explain why.

Lactase is an enzyme that breaks down lactose, the main sugar in milk and dairy products. Most humans produce lactase only during infancy. After weaning, lactase production naturally declines in roughly 65% of the global population. Your LCT gene controls whether your body keeps making lactase into adulthood or whether it shuts down production in childhood.

If you carry the C/C variant at rs4988235, you are lactase non-persistent, meaning your intestines stopped producing lactase efficiently years ago. You’re in good company: this is the most common variant globally and affects roughly 30% of people with European ancestry. When you consume dairy, your body cannot break down the lactose, and it ferments in your intestines, creating gas, bloating, and rapid abdominal distension.

You eat a bowl of yogurt or a slice of pizza and feel fine for five minutes. Then the bloating hits hard. Your stomach expands visibly. You feel pressure and discomfort. The gas production can last for hours. This isn’t a food allergy or intolerance; it’s a straightforward enzyme deficiency encoded in your DNA.

Your FUT2 gene codes for a fucosyltransferase enzyme that puts specific sugar molecules on the surface of cells lining your intestines. This might sound trivial, but it’s actually a chemical signal that tells your gut bacteria which strains are welcome and which should leave. If you’re a secretor (the common form), your intestinal lining broadcasts a welcoming signal. If you’re a non-secretor (rs601338), your gut is essentially silent on that channel.

Roughly 20% of people are non-secretors, and your microbiome composition is completely different as a result. Non-secretors tend to have less diverse gut bacteria and are more prone to bloating-inducing strains that produce excess gas during digestion. You might have good bacteria in your gut, but if you’re a non-secretor, the balance is skewed toward organisms that ferment food aggressively and produce methane and hydrogen gas.

For non-secretors, the same meal that produces manageable bloating in a secretor can cause acute abdominal distension. You’re not eating differently; your bacterial ecosystem is wired to respond more aggressively to food. Add this to lactose intolerance or gluten sensitivity, and bloating becomes severe.

HLA-DQ2 is part of your immune system’s antigen presentation machinery. Think of it as a display case in the window of your gut-lining cells. When gluten peptides are present, HLA-DQ2 can pick them up and show them to immune cells. If your immune system recognizes those gluten peptides as a threat, it launches an attack on your intestinal lining. This is the hallmark of celiac disease.

Roughly 25-30% of people with European ancestry carry HLA-DQ2, and it’s necessary for celiac disease to develop. If you have HLA-DQ2, your gut is immunologically primed to see gluten as an invader, and even small amounts trigger inflammation and intestinal damage. Importantly, not everyone with HLA-DQ2 develops celiac disease, but everyone with celiac disease has HLA-DQ2 or HLA-DQ8.

When you eat gluten and carry HLA-DQ2, your intestines become inflamed. That inflammation causes swelling of the intestinal wall, which narrows the passages food travels through and traps gas. You look and feel bloated because your intestines are literally swollen from an immune attack. The bloating is a sign of active intestinal damage.

MTHFR is an enzyme that catalyzes methylation, a foundational chemical reaction that happens billions of times per day in every cell in your body. One of its key jobs is converting dietary folate into methylfolate, the form your cells can actually use. If your MTHFR enzyme works efficiently, this conversion happens seamlessly. If you carry the C677T variant, the enzyme’s efficiency drops by roughly 40-70%.

The C677T variant is carried by approximately 40% of the population, making it extremely common. With a C677T variant, your cells are chronically depleted in methylfolate even when you eat plenty of folate-rich foods, and this depletion ripples through your entire system, including your digestive system. Your gut cells need adequate methylation to maintain barrier function and regulate inflammation.

When methylation capacity is low, your intestinal lining becomes more permeable, inflammation increases, and your gut motility can become sluggish. You eat food that triggers inflammation, and without adequate methylation to calm that inflammation, bloating becomes pronounced. You might also feel foggy, tired, or anxious in addition to the bloating, which are all signs of systemic methylation problems.

TNF stands for tumor necrosis factor-alpha, and it’s one of your body’s most powerful inflammatory signaling molecules. A little TNF is necessary to fight infection and repair tissue. But too much TNF, especially in your gut, causes chronic inflammation, intestinal permeability (leaky gut), and excessive immune activation.

Your TNF gene has a variant at position -308 (rs1800629). If you carry the A allele, your cells produce elevated levels of TNF-alpha even at baseline. Roughly 30% of people carry at least one A allele. With elevated TNF-alpha production, your intestines stay in a state of low-grade inflammation, and any food that triggers even mild immune activation causes disproportionate inflammatory response and acute bloating.

You eat something slightly irritating (spicy food, a food you’re mildly sensitive to, something high in histamine), and your TNF-alpha levels spike. Your intestinal lining becomes more permeable. Gas is trapped. Fluid accumulates in your intestinal tissues. Within minutes, you’re visibly bloated and uncomfortable. The bloating can last for hours because the inflammatory cascade takes time to resolve.

SLC6A4 codes for the serotonin transporter, a molecular pump that recycles serotonin out of the synaptic space after it’s been released. Most people think of serotonin as a brain chemical, but roughly 95% of your body’s serotonin is actually in your gut. Your intestines use serotonin to coordinate muscle contractions (peristalsis) that move food forward through your system.

The 5-HTTLPR promoter region of SLC6A4 comes in short and long variants. If you carry the short allele (S/S or S/L), your serotonin transporter is less efficient at recycling serotonin. Roughly 40% of people carry at least one short allele. With short SLC6A4 alleles, serotonin recycles slowly, which disrupts the normal rhythm of intestinal muscle contractions and can slow gut transit time, trapping food and gas in your intestines for hours.

You eat a meal and your gut doesn’t move it along at the normal pace. Food sits in your intestines longer, giving bacteria and enzymes more time to ferment it and produce gas. That gas has nowhere to go because your gut motility is sluggish. Within 20 minutes of eating, you’re dramatically bloated. The bloating often worsens throughout the day as more food accumulates. You might also experience constipation, brain fog, or anxiety, since low serotonin turnover affects your brain as well.