You're Taking Iron and Still Exhausted. Here's the Genetic Reason.

HFE is your body’s iron thermostat. It encodes a protein that tells your intestines how much iron to absorb and tells your liver how much to store. When HFE is working normally, it communicates with hepcidin, a hormone that controls iron absorption. If iron is low, hepcidin drops and you absorb more. If iron is high, hepcidin rises and you absorb less. It’s a beautiful feedback loop.

The most well-known HFE variant is C282Y, which causes hemochromatosis (iron overload), a serious condition where your body absorbs too much iron. But there’s another variant, H63D, that affects roughly 15 to 20 percent of people with European ancestry. H63D doesn’t cause dangerous overload, but it disrupts the fine-tuned regulation of iron absorption. If you carry H63D, your body may struggle to absorb iron efficiently, even when you need it. You might have normal serum iron but consistently low ferritin, or fluctuating levels that don’t respond to supplementation.

You notice this in your energy levels. You take iron supplements and they don’t seem to stick. Your ferritin stays stubbornly low. You feel the fatigue most in the afternoons and after exercise. Your nails are brittle. You might have restless legs at night. A standard iron panel says you’re borderline, so your doctor tells you to try a little more dietary iron. But the problem isn’t how much you’re eating. It’s that your HFE variant is telling your gut to close the iron door.

TMPRSS6 encodes a protein that directly regulates hepcidin, the hormone that controls iron absorption in your intestines. It’s a gene that fine-tunes your iron uptake. When TMPRSS6 is working well, it adjusts hepcidin levels based on your iron stores, so you absorb more iron when you need it and less when you don’t.

The TMPRSS6 rs855791 variant is carried by roughly 45 percent of the population. People with this variant have chronically lower hepcidin signaling, which means their body absorbs iron less efficiently and their ferritin stays lower than optimal. You might feel fine if you’re eating iron-rich foods consistently, but the moment you skip a few days or eat a plant-based meal, your iron status plummets. Your body isn’t buffering iron the way it should.

You experience this as fatigue that gets worse over time. You feel it as a slow decline in energy, especially toward the end of the week if your diet has been lighter in meat. Your ferritin drops faster than it should when you take a break from supplements. You might feel particularly tired on days when you skip breakfast, because your body missed an iron-absorption window. Exercise makes you more exhausted because your muscle tissue is hungry for oxygen and iron isn’t supplying it.

MTHFR codes for an enzyme that converts folate and B12 into their active forms, which your body uses for methylation, DNA synthesis, and mitochondrial energy production. Iron depends on these B vitamins to work. Your mitochondria need active B12 to generate ATP and use iron to carry oxygen. If MTHFR is working well, you convert dietary folate and B12 into usable forms automatically.

The MTHFR C677T variant is carried by roughly 40 percent of people with European ancestry. This variant reduces the enzyme’s efficiency by 40 to 70 percent, which means you can eat plenty of folate and B12 and still have functionally low levels of the active forms your cells need. You’re not actually deficient in these vitamins, but your body can’t convert them. The consequence cascades: your mitochondria can’t function optimally, so they can’t use iron efficiently, and your energy crashes.

You feel this as exhaustion that doesn’t improve with rest. You take B vitamins and they don’t help because they’re in the wrong form. Your brain fog is persistent. You might have mild anemia on bloodwork without an obvious cause, because your cells aren’t synthesizing hemoglobin properly. You feel better briefly after eating but crash again quickly, because your mitochondrial energy production is compromised.

VDR encodes the vitamin D receptor, a protein that sits on the surface of your cells and receives the vitamin D signal. When vitamin D binds to VDR, it turns on a cascade of cellular processes, including the production of hepcidin, which regulates iron absorption. Without a working VDR, your body can’t respond to vitamin D properly, even if your vitamin D levels are high.

The VDR BsmI, FokI, and TaqI variants are common, affecting roughly 30 to 50 percent of the population depending on ancestry. People with certain VDR variants have reduced cellular responsiveness to vitamin D, which impairs hepcidin regulation and indirectly reduces iron absorption. You might supplement with vitamin D, have normal or even high blood levels, and still experience the cellular effects of vitamin D deficiency, including poor iron absorption and low energy.

You notice this because taking vitamin D supplements doesn’t give you the expected boost in energy or mood. Your ferritin stays low despite iron supplementation. You feel more fatigued in winter months even if you’re supplementing with D3. Your bones and muscles feel weaker than they should. You might have slow recovery after exercise or illness.

BCMO1 encodes an enzyme that converts beta-carotene from vegetables and plants into retinol, the active form of vitamin A. Vitamin A is essential for regulating hepcidin and iron absorption. It’s also required for hemoglobin synthesis. If you can’t convert beta-carotene efficiently, you develop a functional vitamin A deficiency even if you eat plenty of carrots and spinach.

The BCMO1 R267S and A379V variants are carried by roughly 45 percent of the population. People with these variants have significantly reduced conversion efficiency, which means plant-based beta-carotene is poorly absorbed and converted to usable vitamin A. You might consider yourself well-nourished because you eat lots of vegetables, but your body isn’t actually able to use them for vitamin A. The consequence is low hepcidin production and reduced iron absorption.

You experience this as persistent fatigue despite eating well. You take iron supplements and your ferritin doesn’t rise. You feel worse on a plant-based or low-meat diet. Your skin might be pale or have a yellowish tone. Night vision is worse than it should be. Your immune system seems weaker. You might recover slowly from infections.

FUT2 encodes an enzyme that determines the sugars on the surface of your intestinal cells. These sugars are food for your gut bacteria. Which bacteria colonize your gut depends partly on what sugars are available. Your microbiome composition, in turn, affects your ability to absorb nutrients, including iron, and to synthesize B vitamins and other cofactors iron needs to work.

FUT2 variants are common, and the specific alleles matter for nutrient absorption. People with certain FUT2 variants have gut microbiome composition that is less efficient at producing short-chain fatty acids (which feed intestinal cells) and less able to synthesize B vitamins and other nutrients that support iron absorption. Your digestion might feel fine, but your microbiome isn’t optimized for nutrient extraction.

You notice this as persistent low iron despite good dietary intake and no obvious digestive complaints. You might have mild bloating or gas even when eating iron-rich foods. Your energy dips more on days when you haven’t eaten fermented foods or taken probiotics. You feel better when you’re consistent with fiber intake. You might notice that iron supplements work better when you also take a probiotic.