Taking Vitamin D but Still Weak? Your Genes May Be the Answer.

Your VDR gene codes for the vitamin D receptor, a protein that sits on the surface and inside your muscle cells and acts as the lock that lets vitamin D do its job. When vitamin D binds to VDR, it triggers calcium signaling, protein synthesis, and the hormonal cascades that build muscle strength and improve recovery.

VDR comes in several variants (BsmI, FokI, and TaqI polymorphisms). Roughly 30 to 50% of people carry at least one variant. If you have a VDR variant, your cells have fewer receptors or lower-affinity receptors, meaning vitamin D has a harder time getting in. You could have serum vitamin D at 60 ng/mL (well above the normal threshold) and still have a functional deficiency at the cellular level because your muscles can’t actually use it.

This shows up in training as slower strength gains, persistent muscle fatigue, and recovery that doesn’t improve even when you increase your vitamin D dose. Your muscles literally can’t access the vitamin D they need for protein synthesis and calcium handling during contraction.

MTHFR is the enzyme that converts dietary folate and B12 into their active, usable forms: methylfolate and methylcobalamin. This is not optional. Every cell in your body, especially muscle cells, needs active B vitamins to copy DNA, synthesize protein, and regenerate after training.

The MTHFR C677T variant, carried by roughly 40% of people with European ancestry, reduces the enzyme’s efficiency by 40 to 70%. You can eat a diet rich in leafy greens and take B vitamins, and your muscles still won’t have enough active B vitamins to synthesize new protein or clear homocysteine efficiently. Homocysteine accumulates in your blood and damages the endothelium (blood vessel lining), reducing oxygen delivery to muscles during exercise.

You’ll notice this as slower muscle growth despite consistent training, persistent fatigue that doesn’t improve with sleep, and a sense that your aerobic capacity is lower than it should be. Your muscles are starving for the building blocks they need.

FADS1 codes for an enzyme that converts short-chain omega-3 and omega-6 fats (from plant sources like flax and walnuts) into long-chain forms (EPA and DHA). These long-chain omega-3 fats are structural components of muscle cell membranes and are critical for reducing exercise-induced inflammation and supporting muscle protein synthesis.

The FADS1 rs174537 variant, present in roughly 30 to 40% of the population, reduces delta-5 and delta-6 desaturase activity. If you have this variant, your body cannot efficiently convert plant-based omega-3 (ALA) into EPA and DHA, the forms your muscles actually use for repair and growth. You could eat walnuts and flax every day and still have functionally low muscle EPA and DHA.

You’ll experience this as excessive muscle soreness after training, slower recovery between sessions, and elevated systemic inflammation markers. Your muscle membranes are stiff, and your repair signaling is blunted.

BCMO1 converts beta-carotene (the orange pigment in carrots, sweet potatoes, and leafy greens) into retinol, the active form of vitamin A. Vitamin A is essential for muscle growth, tissue repair, immune function, and vision. Your body cannot store beta-carotene and use it directly; it must convert it first.

The BCMO1 R267S and A379V variants, carried by roughly 45% of people, impair this conversion step. If you have a BCMO1 variant, eating plenty of orange and dark green vegetables does not guarantee adequate cellular vitamin A.** Your body can absorb the beta-carotene, but cannot efficiently convert it into the retinol your muscles need for growth and recovery.