You're Eating Right But Still Deficient. Here's Why.

MTHFR codes for an enzyme that sits at the center of your methylation cycle, the biochemical process responsible for converting nutrients, making neurotransmitters, repairing DNA, and regulating inflammation. When this enzyme works properly, you convert dietary folate and B12 into their active forms, which your cells then use for hundreds of downstream functions.

The MTHFR C677T variant, carried by roughly 40% of people with European ancestry, reduces this enzyme’s activity by 40 to 70%. That means your cells are converting B vitamins and processing methylation at a fraction of the rate they should be. You can eat a perfect diet and still be functionally depleted at the cellular level, with your standard bloodwork showing nothing wrong.

The downstream effect is brutal: brain fog, fatigue, poor concentration, mood instability, and slow wound healing. Many people with MTHFR variants also develop muscle pain, low immune function, and anxiety, because methylation failure cascades into problems with neurotransmitter production and inflammation control.

VDR codes for the vitamin D receptor, the protein your cells use to actually absorb and respond to vitamin D. Having enough vitamin D in your blood means nothing if your cells cannot take it up and use it. VDR variants determine your cellular sensitivity to vitamin D.

The most common VDR variants (BsmI, FokI, TaqI) are carried by roughly 30 to 50% of the population, depending on ancestry. People with certain VDR variants have functionally lower vitamin D despite normal or even high blood levels, because their cells are less efficient at using it. They also have reduced mitochondrial function and energy production.

You experience this as persistent fatigue, weak bones, poor immune function, muscle weakness, and slow recovery from exercise. Despite taking 2,000 or 4,000 IU of vitamin D daily and seeing normal blood levels, you still feel depleted. The problem is not your intake; it’s your cellular ability to use what you’re taking.

BCMO1 codes for an enzyme that converts beta-carotene (the orange pigment in plants like carrots, sweet potatoes, and leafy greens) into retinol, the form of vitamin A your body actually uses. If you cannot make this conversion, eating plant-based vitamin A sources does almost nothing for you.

Approximately 45% of the population carries a BCMO1 variant (R267S or A379V) that impairs this conversion. If you have one, your body converts beta-carotene to retinol at a fraction of the normal rate. You can eat orange vegetables every day and still be vitamin A deficient.

Vitamin A is essential for immune function, eye health, skin health, and reproductive health. Deficiency shows up as frequent infections, poor night vision, dry skin and hair, slow wound healing, and fertility problems. Many people with BCMO1 variants feel confused about why a diet full of vegetables is not preventing these issues.

HFE codes for a protein that tells your intestines how much iron to absorb. It is the gatekeeper of iron regulation. Some HFE variants increase iron absorption too much (hemochromatosis), while others dysregulate it in subtler ways that still cause problems.

The HFE H63D variant, present in roughly 15 to 20% of people with European ancestry, is associated with mild iron dysregulation and increased iron loss. People with this variant often have lower ferritin levels and higher risk of iron-deficiency anemia, especially if they donate blood frequently, menstruate heavily, or follow a vegetarian diet. You may absorb and retain less iron than standard recommendations suggest you need.

Low iron causes fatigue, poor concentration, hair loss, weak immunity, and pale or brittle nails. Many people with HFE variants feel perpetually tired despite eating iron-rich foods, because their absorption is compromised. Women especially are often told their iron is just low, without anyone checking whether a genetic variant is the root cause.

FUT2 codes for an enzyme that determines whether you are a secretor or non-secretor, which controls how you absorb B12 and impacts your gut microbiome composition. Secretors release ABO blood group antigens into their saliva and digestive tract, which shapes what bacteria colonize the gut and how efficiently you absorb certain nutrients.

Approximately 30% of the population are FUT2 non-secretors, with lower B12 absorption and a less diverse gut microbiome. Non-secretors are more susceptible to certain bacterial infections and have naturally lower B12 status even with adequate dietary intake. This is not a dietary problem; it is a genetic one.

The result is fatigue, poor memory, mood instability, numbness in the extremities, and slow wound healing. Many non-secretors take standard B12 supplements and feel no better, because standard dosing assumes you have normal absorption. You are absorbing a fraction of what you are taking.

COMT codes for an enzyme that breaks down dopamine, norepinephrine, and estrogen. The speed at which you metabolize these neurotransmitters determines how much stimulation you can tolerate, how easily you feel overwhelmed, and how long hormones stay active in your body.

The COMT V158M variant (and related SNPs) is carried by roughly 25 to 30% of the population. If you have the slow-metabolizer allele, you process dopamine and estrogen more slowly. You are more sensitive to stimulants like caffeine, more prone to anxiety and overwhelm, and more vulnerable to hormonal imbalances. If you have the fast-metabolizer allele, you need more dopamine and norepinephrine stimulation to feel focused and motivated.

Slow metabolizers taking high-dose B vitamins, caffeine, or stimulating herbs feel jittery and anxious. Fast metabolizers taking the same supplements feel nothing. This is why nutrient needs are so different from person to person, and why the same supplement helps one person and hurts another.