You eat well and still feel depleted. Your genes may be the reason.

MTHFR encodes methylenetetrahydrofolate reductase, the enzyme responsible for converting dietary folate into methylfolate, the active form your cells actually use. This enzyme sits at the center of your methylation cycle, which drives hundreds of biochemical reactions that keep your energy, mood, and cognitive function stable.

The C677T variant, carried by roughly 40% of people with European ancestry, reduces MTHFR enzyme activity by 40 to 70%. This means your cells are converting folate into usable forms at a fraction of the rate they should be. You can eat a diet rich in spinach and legumes and still be functionally folate-depleted at the cellular level. The same applies to B12 conversion. Your body can absorb B12 from food, but your MTHFR enzyme has to activate it into methylcobalamin or cyanocobalamin for use.

What this feels like day-to-day is a low-grade but persistent fatigue, difficulty concentrating, and a sense that your brain is working through fog. Some people also notice anxiety that doesn’t respond to typical interventions, or a cycle of depression that no amount of sleep seems to resolve. Your methylation cycle controls neurotransmitter production and detoxification, so when this step is compromised, everything downstream suffers.

VDR encodes the Vitamin D receptor, the protein in your cells that captures Vitamin D and activates the genes that regulate calcium absorption, immune function, and mitochondrial energy production. Vitamin D doesn’t do anything until it docks into this receptor, so a genetic variant in VDR can make you functionally Vitamin D deficient even if your serum levels look adequate.

VDR variants like FokI and BsmI, present in 30 to 50% of the population depending on ancestry, reduce your cells’ ability to bind and respond to Vitamin D. You can take high-dose Vitamin D and still not achieve the cellular response your body needs. Some variants reduce the sensitivity so much that you need two to three times the standard recommended dose just to achieve normal cellular signaling. Your mitochondria depend on Vitamin D receptor activation to produce ATP, so when this step is compromised, your energy production declines at the cellular level.

You’ll notice this as persistent fatigue despite adequate sunlight or supplementation. Your immune system also becomes reactive, making you susceptible to viral infections that last longer than they should. Some people develop a kind of metabolic stiffness, where their body doesn’t respond well to exercise or dietary changes the way other people’s bodies do. Seasonal mood changes often become more pronounced.

BCMO1 encodes beta-carotene 15,15-monooxygenase, the enzyme that converts the orange plant pigment beta-carotene into retinol, the active form of Vitamin A your body actually uses. This conversion happens in your intestinal lining and liver. If BCMO1 is compromised, you can eat kilos of carrots and sweet potatoes and still be Vitamin A deficient.

The variants R267S and A379V, present in roughly 45% of the population, significantly reduce conversion efficiency. Some people with these variants convert beta-carotene at less than 10% of the normal rate. This matters because Vitamin A is essential for vision, immune function, skin barrier integrity, and gene expression. Your body can’t make it from scratch, so you must get it from food, and if the conversion enzyme is broken, dietary plant sources become nearly useless.

You’ll experience this as unexplained vision issues, especially night vision or difficulty adjusting to darkness. Your skin may become dry or prone to breakouts because Vitamin A drives skin cell turnover. Your immune system becomes weaker, and you get more respiratory infections. Some people notice their hair becomes thinner or their nails become brittle.

FUT2 encodes a fucosyltransferase enzyme that adds fucose (a type of sugar) to carbohydrates in your saliva, digestive secretions, and intestinal lining. This enzyme controls what your gut microbiome eats. The sugars FUT2 produces feed beneficial bacteria like Faecalibacterium prausnitzii and Roseburia, species that produce butyrate, a short-chain fatty acid that fuels your intestinal cells and brain.

FUT2 variants affect whether you’re a secretor or non-secretor of these carbohydrates. Non-secretors, roughly 20% of the population, have reduced FUT2 activity and don’t produce the fucosylated glycans that feed beneficial bacteria. Your gut microbiome composition is different, and it shapes what nutrients you can extract from food. Non-secretors tend to have less stable microbiomes and are more susceptible to infections because they’re missing the community structure that protects intestinal health.

You’ll notice this as digestive inconsistency, bloating after eating carbohydrates, food sensitivities that develop seemingly randomly, and a tendency toward more frequent respiratory or gastrointestinal infections. Some people experience unexplained fatigue linked to poor nutrient absorption because their microbiome isn’t producing the right metabolites to support intestinal barrier function.

FADS1 and FADS2 encode fatty acid desaturases, enzymes that add double bonds to polyunsaturated fatty acids, allowing your body to convert the short-chain omega-3 ALA (from flax and chia) and omega-6 LA (from vegetable oils) into the long-chain forms EPA and DHA that your brain, heart, and eyes actually need. Without these enzymes working, you can’t transform plant-based omega sources into the marine forms your nervous system depends on.

The rs174537 variant in FADS1, present in 30 to 40% of people, reduces delta-5 and delta-6 desaturase activity significantly. You may convert ALA to EPA at less than half the normal rate, meaning your omega-3 status is chronically inadequate despite consuming flax oil or chia seeds. This becomes especially critical if you’re vegetarian or vegan and relying entirely on plant-based omega-3 conversion, because your engine simply can’t do the work at the speed required.

You’ll experience this as brain fog, difficulty with focus and memory, mood instability, joint stiffness, and a general sense that your mind isn’t sharp. Since EPA and DHA are essential for dopamine and serotonin production, mood resilience often suffers. Your skin may become dry or inflamed. Some people notice their joints become achy or their recovery from exercise slows.

PPARG encodes the peroxisome proliferator-activated receptor gamma, a nuclear receptor that controls how your cells switch between carbohydrate and fat burning, and how they respond to dietary nutrients. PPARG acts as a master metabolic switch, telling your cells when to store energy, when to burn it, and how to handle micronutrients like iron and fatty acids. It also regulates inflammation and immune tolerance.

Common PPARG variants, particularly Pro12Ala, affect approximately 20 to 30% of people. Carriers may have reduced ability to adapt their metabolism efficiently to dietary changes, and their cells may be more susceptible to insulin resistance and inflammatory responses to nutrient imbalances. This makes you more sensitive to the specific forms and timing of nutrients you consume. You can’t eat the same way as someone without this variant and expect the same metabolic result.

You’ll experience this as difficulty losing weight despite calorie restriction, a sense that your metabolism is slow even when eating reasonably, and blood sugar instability that shows up as energy crashes after meals. Some people notice their body composition skews toward storing fat in the midsection regardless of overall weight. You may also feel unusually sensitive to inflammatory foods or notice that your energy levels respond dramatically to when you eat, not just what you eat.