Normal Bloodwork, Still Deficient? Your Genes May Be Blocking Absorption.

MTHFR encodes an enzyme that converts dietary folate and B12 into their active, usable forms. This isn’t a minor step. Every cell in your body uses this enzyme to power the methylation cycle, which controls DNA repair, neurotransmitter synthesis, detoxification, and energy production. Without active folate and B12, these systems start to fail.

The C677T variant, carried by roughly 40% of people with European ancestry, reduces MTHFR enzyme activity by 40 to 70%. That means your cells are converting B vitamins at a fraction of the normal rate, even if your serum B12 and folate look perfectly normal on standard bloodwork. You can eat folate-rich foods and still be functionally B vitamin deficient at the cellular level.

You notice it as brain fog, fatigue that doesn’t lift with sleep, difficulty concentrating, mood instability, or slow wound healing. Your methylation cycle is starved for substrate. Your cells can’t make the neurotransmitters and antioxidants they need to function.

VDR encodes the Vitamin D receptor, a protein that sits on cell membranes and inside cells, allowing them to respond to Vitamin D. Think of it as the lock, and Vitamin D as the key. If your receptor doesn’t function properly, Vitamin D can’t deliver its signal, even if your serum levels are high.

The FokI variant and other common VDR polymorphisms affect how efficiently your cells can bind Vitamin D and activate it. Roughly 30 to 50% of people carry a variant that reduces this efficiency. You can supplement 4,000 IU daily and still have insufficient Vitamin D signaling inside your cells. Your blood test shows normal 25-hydroxyvitamin D. Your cells are still starving for its effects.

You experience this as weak bones and muscles, poor immune function, mood disorders, slower wound healing, and reduced mitochondrial energy production. Vitamin D controls over 200 genes. When your cells can’t hear Vitamin D’s signal, nearly every system suffers.

BCMO1 encodes the enzyme that converts beta-carotene from plants into retinol, the active form of Vitamin A. This conversion isn’t automatic. It requires a functioning BCMO1 enzyme. Your body doesn’t have much storage capacity for active Vitamin A, so you need this conversion pathway working efficiently every single day.

The R267S and A379V variants reduce BCMO1 activity significantly. Roughly 45% of people carry at least one copy. If you have a BCMO1 variant, eating more carrots and spinach doesn’t help. Your body can’t convert the beta-carotene into usable Vitamin A. Your serum retinol might be normal because you ate some preformed Vitamin A recently, but your tissue stores are depleting.

You notice it as poor night vision, dry skin, frequent infections, slow wound healing, and reduced fertility. Vitamin A controls immune function, skin barrier integrity, and reproductive health. When your cells can’t convert plant-based carotenoids, you need preformed Vitamin A from animal products to avoid deficiency.

HFE encodes a protein that controls how much iron your intestines absorb and how efficiently cells take it up. The C282Y variant causes iron overload. But the H63D variant, present in roughly 15 to 20% of people with European ancestry, causes the opposite problem: mild iron dysregulation and reduced absorption.

With the H63D variant, your intestines absorb iron less efficiently. Standard serum ferritin might look normal because you’re absorbing just enough to stay out of the anemia range. But your tissue iron and hemoglobin might be chronically suboptimal, starving your cells of the oxygen and enzyme cofactors iron provides. You’re not technically anemic by lab standards, but you’re functionally iron deficient.

You experience this as persistent fatigue, poor exercise tolerance, brain fog, pale or pale-yellow skin, and hair loss. Iron is essential for oxygen transport, enzyme function, and immune signaling. When your cells can’t accumulate enough iron, energy production plummets.

FUT2 encodes a protein that determines which sugars appear on the surface of your gut cells. This might sound irrelevant to nutrition, but it’s not. The sugar pattern acts as a selection signal for your gut bacteria. Different FUT2 variants recruit different bacterial populations. Some bacteria are prolific B vitamin producers. Others aren’t.

FUT2 variants affect the diversity and composition of your microbiome, which directly controls how much B vitamin your bacteria can produce for you. Roughly 20 to 30% of people carry variants associated with reduced bacterial B vitamin synthesis. Your bloodwork shows normal B vitamins, but your gut bacteria can’t produce them reliably, so serum levels depend entirely on dietary intake. The moment you have any gut dysbiosis or dietary inconsistency, your B vitamin levels crash.

You notice it as chronic fatigue, mood instability, and neural symptoms that fluctuate with diet and digestive health. B vitamins are essential for energy, mood, and nervous system function. When your microbiome can’t produce them and serum levels become unreliable, you feel it immediately.

COMT encodes an enzyme that breaks down catecholamines (dopamine, norepinephrine, epinephrine) and processes certain nutrients like B vitamins and folate. It controls the pace at which your body clears these compounds. Fast metabolizers clear them quickly. Slow metabolizers accumulate them.

The V158M variant affects COMT activity. The V allele produces a faster-acting enzyme, while the M allele produces a slower one. Roughly 30% of people are homozygous for the slow variant. If you’re a slow COMT metabolizer and you supplement high-dose B vitamins or methylated folate, you can accumulate excessive levels inside your cells, causing agitation, anxiety, insomnia, and overstimulation.

You experience this as a paradoxical worsening of symptoms when you supplement, or as racing thoughts, irritability, and sleep disruption despite normal nutrient intake. Your cells aren’t deficient in B vitamins or folate. They’re accumulating them because you can’t clear them fast enough.