Your Stools Are Greasy. Your Genes May Be Blocking Fat Digestion.

MTHFR encodes the methylenetetrahydrofolate reductase enzyme, which converts dietary folate and B vitamins into the methylated forms your cells actually use. This enzyme is essential for DNA synthesis, immune function, and the production of compounds that protect your intestinal lining from damage.

The MTHFR C677T variant, present in roughly 35-40% of people of European ancestry, reduces enzyme efficiency by 30-40%. This means your cells are producing less methylated folate, the active form your gut barrier needs to repair itself. If you also have reduced intestinal blood flow or chronic inflammation, the damage accumulates faster than it can be repaired.

When your gut lining is compromised, nutrients cannot cross the intestinal barrier properly. Fats slip through unabsorbed, causing greasy stools. At the same time, you become deficient in fat-soluble vitamins like A, D, E, and K because they are being lost in the stool. You may feel weak, bruise easily, or develop bone aches from vitamin K and D deficiency.

HLA-DQ2 encodes a protein on your immune cells that presents antigens to your T-cells, triggering immune responses. In people with HLA-DQ2, this protein binds gluten peptides with high affinity, causing your immune system to treat gluten as a threat even if you have never been formally diagnosed with celiac disease.

HLA-DQ2 is present in approximately 25-30% of people with European ancestry. Having the gene does not guarantee celiac disease, but without this gene, celiac disease is extremely rare. If you have HLA-DQ2 and greasy stools, undiagnosed or non-responsive celiac disease is one of the most common causes.

When your immune system attacks gluten-containing foods, it damages the villi in your small intestine, the finger-like projections that absorb nutrients and fat. As the villi flatten, your absorptive surface shrinks. Fat passes through unabsorbed, causing greasy stools, along with deficiency in vitamins A, D, E, and K. You may also experience bloating, brain fog, and weight loss.

The LCT gene regulates lactase, the enzyme that breaks down lactose (milk sugar) in your small intestine. In infants and young children, lactase is universally produced. But in roughly 65% of the global population, lactase production declines sharply after age 3-5, a normal biological process called lactase non-persistence.

If you carry the LCT C/C genotype (present in roughly 30% of European ancestry and much higher in other populations), your body naturally stops producing lactase as you age. You are not intolerant; your body is working as designed. When you consume lactose, your small intestine cannot break it down, so it passes into your colon where bacteria ferment it, producing gas, bloating, diarrhea, and greasy stools.

Given the overlap with other malabsorption causes, greasy stools from lactose intolerance can be confusing. You may eliminate dairy and feel better, leading you to assume you have a dairy allergy. You do not. Your body simply does not produce lactase. The greasy stools disappear when lactose is avoided, but they return if you reintroduce dairy without lactase enzyme supplementation.

FUT2 encodes a fucosyltransferase enzyme that decorates the surface of your intestinal epithelial cells with fucose molecules. These molecules determine what your gut microbiome looks like and how efficiently your intestines absorb vitamin B12, a nutrient essential for energy production and nervous system function.

The FUT2 non-secretor variant, present in roughly 20% of the population, means your gut environment favors different bacterial species and your intestinal cells absorb less dietary B12. Non-secretor status shifts your microbiome composition toward species that are less efficient at producing compounds that stabilize your intestinal barrier. Your gut becomes more permeable, fats slip through unabsorbed, and you become B12 deficient.

When you are FUT2 non-secretor and have a dysbiotic microbiome, your intestinal barrier becomes increasingly leaky. Undigested food particles and bacterial lipopolysaccharides cross into your bloodstream, triggering systemic inflammation. Greasy stools persist despite dietary changes because the underlying cause is dysbiosis and increased intestinal permeability, not food intolerance alone.

TNF encodes tumor necrosis factor alpha, a cytokine that orchestrates inflammation throughout your immune system. In small amounts, TNF is protective and helps clear pathogens. In excess, TNF damages your intestinal barrier by loosening the tight junctions between epithelial cells, increasing intestinal permeability and allowing fats and undigested food to pass through.

The TNF -308G>A variant, present in roughly 30% of the population, increases TNF production in response to gut triggers. People with this variant produce significantly more TNF-alpha when exposed to potential intestinal pathogens, food antigens, or dysbiotic bacteria. If you also carry variants in other genes that impair immune regulation or barrier function, elevated TNF amplifies the damage.

When TNF is chronically elevated, your intestinal lining becomes progressively more permeable. Fats and fat-soluble vitamins slip through unabsorbed. You develop greasy stools, lose weight despite eating enough, and may develop nutritional deficiencies. The inflammation also triggers visceral pain, bloating, and changes in bowel motility.

SOD2 encodes superoxide dismutase 2, an enzyme that neutralizes superoxide radicals produced in your mitochondria during energy production. Your intestinal epithelial cells are metabolically active; they are constantly producing energy to power nutrient transport and barrier maintenance. If SOD2 is not functioning optimally, reactive oxygen species accumulate, damaging your intestinal cells and impairing their ability to absorb and transport nutrients.

The SOD2 Ala16Val variant, present in roughly 40-50% of the population depending on ancestry, reduces SOD2 activity and increases oxidative stress in mitochondria. People with this variant accumulate more free radicals in their intestinal epithelial cells, causing progressive damage to the transporters that absorb fats and fat-soluble vitamins. Over time, your intestinal barrier becomes leaky and your nutrient absorption declines.

When SOD2 function is impaired, your intestinal epithelial cells cannot maintain the energy-dependent pumps that actively transport nutrients. Fats pass through unabsorbed. You become deficient in vitamins A, D, E, and K. You may also experience fatigue because your mitochondria throughout your body are under constant oxidative stress. Adding high-dose supplementation without addressing the underlying oxidative stress can actually worsen symptoms by increasing free radical production.