You're Eating Well and Still Bloated and Exhausted. Here's Why.

Lactase is an enzyme produced in your small intestine that breaks down lactose, the sugar in milk. Your LCT gene controls whether your body keeps making this enzyme into adulthood. In most mammals, lactase production drops sharply after weaning. Humans are unusual; some of us keep making lactase throughout life, and some don’t.

If you carry the C/C genotype at rs4988235, you are lactase non-persistent, meaning your lactase production declines after childhood. This genotype is present in roughly 65% of the global population, and about 30% of people with European ancestry. When you eat dairy, the undigested lactose ferments in your colon, producing gas, bloating, and cramping within 30 minutes to two hours of consumption.

But here’s the catch: lactose malabsorption also prevents you from absorbing calcium and other nutrients efficiently. Your gut bacteria ferment the lactose, producing short-chain fatty acids that trigger cramping and urgency. Meanwhile, the malabsorption of nutrients from dairy can contribute to fatigue, bone loss, and deficiencies in magnesium and vitamin D, all of which worsen energy and worsen bloating by affecting gut motility.

The FUT2 gene controls whether you secrete ABO blood group antigens into your saliva, mucus, and digestive tract. These antigens shape your gut microbiome composition. If you are a non-secretor (rs601338 homozygous variant), your gut bacteria look different from secretors’, and that difference matters enormously for digestion and energy.

Non-secretor status is present in roughly 20% of the population. Non-secretors have reduced populations of beneficial Bacteroides and altered microbiome diversity, which impairs fermentation of dietary fiber and reduces production of short-chain fatty acids that fuel your intestinal lining. You also absorb B12 less efficiently because your reduced microbial community doesn’t produce as much of the compounds that help you absorb it from food.

Without adequate B12 absorption, your energy crashes. Your mitochondria can’t produce ATP efficiently. Fatigue deepens, and because your gut motility is slowed by B12 deficiency, food sits longer in your stomach and intestines, causing bloating. It’s a cascade: bad microbiome leads to poor B12 absorption, which causes fatigue and slows digestion, which causes bloating.

HLA-DQ2 is an immune antigen presentation protein encoded by two separate genetic loci. If you carry the DQA1*05 allele together with the DQB1*02 allele, you have HLA-DQ2.5, which is necessary for celiac disease. This haplotype is found in roughly 25-30% of people with European ancestry.

When you eat gluten, your immune system recognizes gluten peptides and presents them to your T cells using the HLA-DQ2 molecule. If you have HLA-DQ2, your immune system mounts an attack on the villi lining your small intestine, destroying the cells responsible for nutrient absorption. This happens even if you never get a positive celiac test, because the damage occurs at the tissue level, not necessarily in your bloodstream.

The result is catastrophic malabsorption. You eat, but your body cannot extract nutrients efficiently. Minerals vanish. Iron, B12, folate, and magnesium are lost. Your energy production collapses. Meanwhile, the inflammation in your intestinal lining triggers bloating, gas, and often diarrhea or constipation as your gut tries to move undigested material through.

MTHFR encodes methylenetetrahydrofolate reductase, an enzyme that converts dietary folate into its active form, methylfolate, which your cells need for DNA synthesis, methylation reactions, and energy production. If you carry the C677T or A1298C variant, your enzyme works less efficiently.

The C677T variant is carried by roughly 30-40% of European populations, and it reduces MTHFR enzyme activity by 40-70%. If you have this variant, your cells cannot convert dietary B vitamins efficiently, leaving you functionally depleted in methylfolate, methylcobalamin, and choline, even if you eat a diet rich in leafy greens. Your mitochondria cannot produce energy. Your gut cells cannot repair themselves. Your intestinal lining becomes leaky.

Without efficient methylation, your gut lining degrades. Intestinal permeability increases, allowing partially digested food particles and bacterial endotoxins to cross into your bloodstream. Your immune system reacts, triggering inflammation and bloating. Your energy production stalls because you lack the cofactors needed for the methylation cycle. Fatigue compounds the bloating because your depleted mitochondria cannot generate enough ATP to power even normal digestion.

TNF encodes tumor necrosis factor-alpha, a powerful inflammatory cytokine that signals your immune system. The -308G>A variant at rs1800629 causes higher baseline TNF-alpha production. This variant is carried by roughly 30% of the population.

If you carry the A allele, your baseline TNF-alpha is elevated, and your immune system stays in a primed inflammatory state even when there’s no active threat. In your gut, elevated TNF-alpha increases intestinal permeability. It damages the tight junctions between your intestinal cells, making your gut leaky. Food particles and bacterial lipopolysaccharides cross the barrier and trigger immune activation. Your immune system reacts to these particles with inflammation, bloating, and sometimes diarrhea.

The inflammation also slows your gut motility. Food moves more slowly through your intestines, fermenting as it sits, producing gas and bloating. Additionally, chronic elevated TNF-alpha exhausts your mitochondria, consuming ATP in the process of managing inflammation, which leaves you profoundly fatigued. You feel tired because your immune system is burning energy to contain an inflammatory state that started in your gut.

SLC6A4 encodes the serotonin transporter, a protein that recycles serotonin from the synapse back into the neuron so it can be reused or broken down. Roughly 95% of your body’s serotonin is produced in your gut, where it controls muscle contractions, intestinal motility, and the sensation of fullness. Roughly 40% of the population carries at least one short allele of the 5-HTTLPR polymorphism.

If you carry the short allele, your serotonin transporter works less efficiently, meaning serotonin accumulates in your gut synapses and your gut becomes hypersensitive to normal food movement and gas production. Normal intestinal contractions feel exaggerated. Normal amounts of gas feel painful. Your gut sensitivity is heightened, and you perceive more discomfort from normal digestive processes. At the same time, impaired serotonin recycling slows your gut motility because serotonin is being sequestered instead of being available for the next wave of contractions.

The result is a double hit: your gut is hypersensitive, so normal digestion feels uncomfortable, and your gut motility is slowed, so food sits longer, ferments more, and produces more gas. The discomfort and bloating trigger stress, which suppresses appetite and digestion further. Additionally, impaired central serotonin recycling contributes to mood disorders and anxiety, which worsen IBS and gut hypersensitivity further.