You're eating protein and still depleted. Your genes may be blocking tryptophan.

MTHFR is an enzyme that produces methylfolate, the active form of folate your cells actually use. Methylation is not just about one reaction. It’s a cycle that charges up dozens of biochemical processes, including the conversion of tryptophan into serotonin. When methylation stalls, the whole pathway slows down.

The MTHFR C677T variant, carried by roughly 40% of people with European ancestry, reduces the enzyme’s activity by 40-70%. This means you’re converting B vitamins into their active forms at a fraction of the rate you should be. Even with normal dietary intake of folate and B12, your cells are running on fumes. You can eat a perfect diet and still be functionally depleted at the cellular level because the methylation cycle can’t charge up.

What does this feel like? Tryptophan sits in your bloodstream and can’t get converted. You feel unmotivated, your mood dips in the afternoon, sleep comes slowly, and no amount of rest fixes the underlying exhaustion. Your mind feels foggy. You might crave carbs because your body is looking for a serotonin boost it’s not getting from tryptophan.

COMT is an enzyme that breaks down dopamine, norepinephrine, and estrogen. It doesn’t directly convert tryptophan, but it regulates the broader neurotransmitter environment your tryptophan metabolites have to work in. There are two common variants that matter: the Val158Met (also called V158M). If you carry the Met variant (slow COMT), you clear dopamine slowly and tend to accumulate it. If you carry the Val variant (fast COMT), you chew through dopamine quickly.

Roughly 30-40% of people carry variants that affect COMT speed. The mismatch between your COMT speed and your lifestyle determines whether tryptophan-derived serotonin has a calm neurotransmitter environment to work in or whether it’s being drowned out by dopamine or starved by dopamine deficiency. Fast COMT people burn through dopamine and need more stimulation; slow COMT people overstimulate easily and need less.

How does this sabotage tryptophan? If you’re slow COMT, excessive caffeine or stimulation overwhelms your serotonin system and you feel anxious or jittery despite eating tryptophan. If you’re fast COMT, you burn through dopamine and serotonin quickly, leaving you chasing stimulation and unable to feel calm or sleep well. Either way, tryptophan conversion suffers because the neurotransmitter balance is off.

SOD2 is superoxide dismutase 2, a mitochondrial antioxidant enzyme. It neutralizes free radicals before they damage your cells. Tryptophan metabolites, especially those in the kynurenine pathway (the other fate of tryptophan, besides serotonin), are vulnerable to oxidative stress. If SOD2 is weak, these metabolites get oxidized and become toxic instead of useful.

The SOD2 Ala16Val variant, present in roughly 40-50% of people, reduces the enzyme’s activity. This means your cells are generating metabolic byproducts from tryptophan faster than you can neutralize them, leading to a buildup of oxidative stress that blocks further tryptophan conversion. You end up in a cycle: tryptophan comes in, starts to convert, hits oxidative damage, and the process stalls. Your mitochondria, which need the most protection, get the most damage.

What happens next? You feel depleted because your mitochondria aren’t producing energy efficiently. Tryptophan conversion falters because the enzymes involved are getting damaged by free radicals. You might also notice joint or muscle soreness that doesn’t match your activity level, or brain fog that gets worse when you’re stressed (stress increases oxidative load).

VDR is the receptor that lets your cells detect vitamin D and activate vitamin D-responsive genes. One of those genes codes for intestinal tight junction proteins that control what gets absorbed. Another regulates hepcidin, which controls iron absorption (iron is needed for tryptophan conversion enzymes). A third affects the expression of amino acid transporters, including those for tryptophan.

The VDR BsmI, FokI, and TaqI variants are common, present in roughly 30-50% of people. These variants reduce your cells’ sensitivity to vitamin D, meaning you need more of it circulating to activate the same number of vitamin D-responsive genes, and even high supplementation leaves you functionally deficient. This triggers a cascade: your intestinal barrier loses structural integrity, tryptophan absorption drops, iron handling gets disrupted, and the downstream conversion pathway stalls at multiple points.

What does this look like? You might have a permeable gut (gas, bloating, food sensitivities), poor tryptophan absorption (low mood despite eating protein), and iron dysregulation (fatigue, pale nails, restless legs). Your vitamin D levels might be normal on paper, but your cells aren’t responding. Tryptophan is available, but it’s not getting absorbed at the intestinal level.

FADS1 and FADS2 are desaturase enzymes that convert short-chain plant omega-3s (ALA) and omega-6s (LA) into long-chain forms (EPA, DHA). These long-chain omega-3s are structural components of mitochondrial and cellular membranes. They’re also precursors to signaling molecules called resolvins and protectins that regulate inflammation and protect neurons.

The FADS1 rs174537 variant, present in roughly 30-40% of people, reduces desaturase activity by 30-50%. This means even if you eat flaxseeds, chia, or walnuts, your body can’t convert them into EPA and DHA, leaving your mitochondria and neuronal membranes starved for the fats they need to function. Tryptophan conversion happens in mitochondria. If those mitochondria are built from a suboptimal fat structure, the enzymes involved work poorly.

How does this show up? You feel brain fog, especially after eating carbs (signal that your neuronal membranes aren’t stable). Your mood is unstable. Sleep is fragmented. You might have dry skin or other signs of membrane instability. Tryptophan is present, but the cellular infrastructure needed to convert it efficiently is compromised.

HFE regulates hepcidin, a hormone that controls iron absorption and storage. If HFE is working normally, you absorb iron appropriately and store it safely. Several HFE variants disrupt this. The most common in European ancestry is H63D, present in roughly 15-20% of people. C282Y (the hemochromatosis variant) is rarer but causes severe iron overload if homozygous.

The H63D variant, even in a single copy, reduces iron absorption and leads to lower ferritin levels, meaning fewer iron stores available for the heme-dependent enzymes that must convert tryptophan. You might have normal serum iron or hemoglobin on standard bloodwork, but ferritin is low, and your cells are iron-starved. Tryptophan hydroxylase, the enzyme that initiates tryptophan conversion into serotonin, is heme-dependent. Without iron, it doesn’t work.

What happens? You feel exhausted despite sleeping. Your mood is flat. Nails are pale or ridged. You might get restless legs at night. You eat red meat but don’t absorb it well. Standard iron tests come back normal because doctors look at hemoglobin first, and they miss the low ferritin that signals cellular iron deficiency. Tryptophan piles up unconverted.