Your Functional Medicine Questions, Answered by Your Genes.

MTHFR is an enzyme that sits at the center of your methylation cycle, one of the most critical metabolic pathways in your body. It takes folate from your food and converts it into methylfolate, a form your cells can actually use. That methylfolate is then used to produce SAM-e, which is the donor molecule for hundreds of chemical reactions in your body, including DNA repair, neurotransmitter production, and energy metabolism. If MTHFR works well, this process hums along invisibly.

Here’s the problem: the MTHFR C677T variant, carried by roughly 40% of people with European ancestry, reduces this enzyme’s efficiency by 40-70%. That means your cells are converting B vitamins into usable energy at a fraction of the rate they should be. You can eat a perfect diet and still be functionally depleted at the cellular level because your body can’t process the nutrients you’re consuming.

This shows up as persistent fatigue that doesn’t respond to more sleep, brain fog that coffee can’t fix, and a sense that you’re running your body at half capacity. You might also notice that you’re sensitive to B vitamin supplementation in standard forms (you feel worse, not better), or that your homocysteine levels are creeping up even though you eat well. Functional medicine practitioners know about MTHFR, but they need to know if you carry the variant to prescribe the right form of supplementation.

Your VDR gene codes for the vitamin D receptor, a protein that sits on the surface of nearly every cell in your body. Vitamin D circulating in your bloodstream is useless until it binds to this receptor and enters the cell. Once inside, vitamin D controls gene expression for immune function, bone remodeling, mitochondrial biogenesis, and ATP production. Without a working VDR, vitamin D can’t do any of those jobs, no matter how much you supplement or how much sun you get.

VDR variants, common in roughly 30-50% of the population, reduce how efficiently your cells absorb vitamin D. This means your cells are functionally vitamin D deficient even when your blood levels look normal. A functional medicine lab might show your 25(OH)D at 40 ng/mL, which is considered sufficient by standard medicine, but if you carry a VDR variant, your cells are acting as if you’re deficient.

You’ll notice this as persistent fatigue despite good sleep, weak immune function with recurrent infections, poor mitochondrial energy production, and slow recovery from exercise. Your functional medicine practitioner might recommend vitamin D supplementation, which is correct, but they need to know about your VDR variant to recommend the right dose and form (often higher than standard, and sometimes with additional cofactors like magnesium and K2).

COMT is an enzyme that clears dopamine, norepinephrine, and epinephrine from your brain and body after they’ve done their job. These are your stress neurotransmitters, the chemicals that keep you alert, motivated, and responsive to threats. They’re essential, but they’re also exhausting if they stay elevated too long. COMT is the off switch. It metabolizes them quickly so your nervous system can return to baseline and you can sleep.

The COMT Val158Met variant, carried by roughly 25% of people as a homozygous slow variant, reduces how quickly you clear these neurotransmitters. Your nervous system stays activated during sleep, depleting your neurological reserves and preventing deep, restorative sleep even though you’re in bed for eight hours. You wake up tired, not refreshed.

You’ll feel this as racing thoughts at bedtime, the sense that your mind won’t shut off, light sleep that feels unrefreshing, and a tendency to feel overstimulated by noise or activity in the evening. You might also notice that you’re sensitive to stimulants like caffeine or stimulant medications, or that you feel wired and anxious when you should be relaxed. Your functional medicine practitioner needs to know about your COMT variant to design the right evening protocol, which often involves lower stimulant exposure, specific calming nutrients, and sometimes magnesium glycinate at night.

SLC6A4 codes for the serotonin transporter, a protein that sits on the surface of brain cells and recycles serotonin back into the neuron after it’s been released. Serotonin is the neurotransmitter that influences mood, pain perception, and the production of melatonin, your sleep hormone. After serotonin does its job in the synapse, the transporter pulls it back in so the cell can use it again. This recycling is efficient and elegant.

The SLC6A4 5-HTTLPR short allele, carried by roughly 40% of the population, impairs this recycling process. Your brain recycles less serotonin, leading to inconsistent serotonin availability and disrupted melatonin production even though you have no obvious mood disorder. Sleep becomes shallow and non-restorative, and you wake up in the night for no reason you can identify.

You’ll experience this as sleep that feels empty, waking at 3 or 4 AM without a clear trigger and being unable to fall back asleep, and a sense that sleep isn’t putting you back together. You might also notice mood variability, sensitivity to stressful situations, and a tendency toward rumination. Your functional medicine practitioner needs to know about this variant to recommend serotonin support through either precursors (5-HTP, L-tryptophan) or through lifestyle modulation (morning sunlight exposure, evening darkness) that boosts melatonin production.

TCF7L2 is a transcription factor that regulates genes involved in glucose metabolism, insulin secretion, and insulin sensitivity. It controls how your pancreas responds to blood sugar elevation and how sensitive your cells are to the insulin signal. When TCF7L2 is working well, your blood sugar stays stable, insulin secretion is appropriate, and your cells respond to insulin efficiently. This is foundational for energy, mood stability, and metabolic health.

TCF7L2 variants, common in the population, reduce your cells’ ability to maintain stable glucose and insulin signaling. Your blood sugar becomes less stable, your insulin becomes less effective, and your body struggles to maintain steady energy throughout the day. Standard blood tests might show normal fasting glucose and normal HbA1c, yet you’re experiencing energy crashes, mood swings, and difficulty concentrating.

You’ll notice this as energy that’s high after eating and then crashes hard two to three hours later, cravings for carbohydrates that intensify in the afternoon, difficulty losing weight despite eating well, and mood swings that correlate with meal timing. Your functional medicine practitioner needs to know about this variant to recommend appropriate macronutrient ratios, meal timing, and sometimes chromium or berberine supplementation to stabilize glucose metabolism.

APOE codes for apolipoprotein E, a protein that transports cholesterol and fat-soluble vitamins throughout your body and brain. It’s essential for maintaining brain cell membranes, clearing amyloid proteins, and supporting neurological function. Your APOE type (E2, E3, or E4) is determined by two genetic variants, and it influences how efficiently your body processes dietary fat, your cholesterol metabolism, and your risk for cognitive decline.

The APOE4 allele, present in roughly 25-30% of the population (higher in some ancestries), changes how your body processes dietary fat and cholesterol. APOE4 carriers have different cholesterol metabolism, altered brain amyloid clearance, and increased sensitivity to dietary fat composition compared to E3 carriers. This doesn’t mean you’ll develop cognitive decline, but it does mean your brain is more sensitive to what you eat.

You might notice this as difficulty concentrating or brain fog that worsens after high-fat meals, difficulty clearing brain fog with standard interventions, a sense that your cognitive function is fragile when stress is high, or family history of cognitive decline that makes you cautious. Your functional medicine practitioner needs to know about your APOE status to calibrate your diet (APOE4 carriers often do better with lower saturated fat and higher plant-based fats), recommend appropriate supplementation (sometimes higher omega-3, sometimes additional antioxidants), and monitor cognitive biomarkers.