You're Eating Right and Still Exhausted. Here's Why.

MTHFR produces an enzyme that converts dietary folate and B12 into the active forms your cells actually use. This process, called methylation, fuels ATP production (your cellular energy currency) and builds neurotransmitters like serotonin and dopamine. Without active methylation, your mitochondria can’t generate energy efficiently.

The C677T variant, carried by roughly 40% of people with European ancestry, reduces this enzyme’s activity by 40 to 70 percent. You might be eating plenty of dark leafy greens and nutritional yeast. But your cells are converting those nutrients into usable energy at a fraction of the normal rate. You can follow a perfect vegan diet and still be functionally B12 and folate deficient at the cellular level.

You feel it as constant fatigue that doesn’t improve with rest. Your brain feels slow in the afternoon. You recover slowly from workouts. Your mood dips without obvious reason. Normal bloodwork shows folate and B12 in range, but you still feel empty.

VDR encodes the receptor on your cells that grabs vitamin D from your bloodstream and lets it do its job. Without functional VDR, vitamin D can’t activate the genes that build mitochondria, regulate calcium, or drive ATP production. Vitamin D deficiency doesn’t show up as low mood or weak bones first; it shows up as fatigue.

Common VDR variants (BsmI, FokI, TaqI) affect roughly 30 to 50 percent of the population and reduce your cells’ ability to absorb and utilize vitamin D. You can take vitamin D supplements and still have functional vitamin D deficiency because your cells can’t actually use it. Vegans are already at higher risk for low vitamin D because plant foods don’t contain it, and sun exposure alone is often insufficient.

You feel it as a specific kind of exhaustion: low motivation, heavy limbs, especially in winter. Your energy crashes in the afternoon. You feel cold easily. Supplementing with standard vitamin D makes you feel slightly better but not dramatically, because your cells still can’t absorb it effectively.

HFE controls hepcidin, the hormone that tells your intestines how much iron to absorb and your tissues when to release it from storage. Iron is essential for hemoglobin (oxygen transport) and cytochrome c oxidase (mitochondrial ATP production). Without proper iron regulation, you’re either absorbing too much (rare in vegans, common with HFE C282Y) or too little (common with H63D).

The H63D variant, carried by roughly 15 to 20 percent of people with European ancestry, is associated with mild iron dysregulation and reduced iron absorption. Plant-based iron (nonheme iron) is already less bioavailable than animal iron. If your HFE variant also reduces absorption, you’re fighting a compounding problem. Your bloodwork might show normal iron and ferritin, but your cells still may not have enough iron to run mitochondrial enzymes efficiently.

You feel it as fatigue that gets worse with activity. You’re short of breath after climbing stairs. Your heart races at rest sometimes. Your hair and nails are brittle. You feel cold or dizzy when you stand up quickly.

TMPRSS6 encodes a protease that fine-tunes your body’s iron-sensing system. It tells your intestines how aggressively to absorb iron based on your current stores. A variant in TMPRSS6 (rs855791) disrupts this signal, making your body more conservative about iron absorption even when stores are low.

Roughly 45 percent of people carry this variant. If you’re vegan and carry it, your already-challenged nonheme iron absorption becomes even more restricted. Your body essentially underestimates your iron need and tells your intestines not to absorb it. You can eat iron-rich plant foods at every meal and still develop iron-deficiency anemia because your cells aren’t signaling for absorption appropriately.

You feel it as progressive fatigue over months. Your energy seems to decline even though nothing in your diet has changed. You’re more irritable, less able to focus. Your legs feel heavy during exercise. Cold hands and feet become noticeable.

BCMO1 encodes the enzyme that converts beta-carotene (the orange pigment in carrots, sweet potatoes, and dark leafy greens) into retinol, the active form of vitamin A your cells use. This conversion is critical for mitochondrial energy production, immune function, and vision. Plant foods contain beta-carotene. Animals store retinol. Vegans depend entirely on BCMO1 working efficiently.

The R267S and A379V variants in BCMO1, carried by roughly 45 percent of the population, reduce conversion efficiency significantly. Some research suggests conversion drops to 10 to 20 percent of normal in people with multiple variants. You can eat beta-carotene daily and still develop functional vitamin A deficiency because your body simply can’t convert plant forms to usable retinol.

You feel it as low energy combined with vision issues in dim light. Your immune system feels weak; you catch every cold. Your skin becomes dry and rough. You might have difficulty recovering from infections.

FUT2 encodes a protein that determines which sugars appear on the surface of your gut cells and in your saliva. This seemingly small detail dramatically affects which bacteria colonize your microbiome and whether you can absorb B12 efficiently. FUT2 variants create a microbiome less capable of producing or cycling B12, and also reduce your intestinal cells’ ability to absorb it.

Common FUT2 variants affect roughly 40 percent of the population. Vegans already face B12 challenges because B12 is primarily available from animal sources or fortified foods. If your FUT2 limits your absorption, you’re doubly vulnerable. Your microbiome may be less capable of producing B12 internally, and your intestinal cells may be less capable of absorbing supplemental B12.

You feel it as fatigue combined with cognitive slowing, sometimes with mild tingling in your hands or feet. Your energy crashes despite eating well and sleeping enough. Your mood may dip. You feel foggy in the morning.