How it fits together · lesson 5 of 11
Brain chemistry & NAD⁺ — synthesis and the longevity hype
Building dopamine and serotonin from amino acids, the NAD⁺ pathway, and a balanced look at NMN/NR.
7 min read · reviewed October 2026
Your mood and drive partly come down to two molecules — dopamine and serotonin — and your body builds them on short assembly lines from amino acids in food. Each line needs specific cofactors. If those are short, the line makes less product, which is one (not the only) reason nutrition and mood are linked.
We'll finish with NAD⁺, a molecule you met in the energy lesson that has become the centerpiece of the longevity-supplement boom — and separate its real biology from the marketing.
Building your neurotransmitters
Starts from the amino acid tryptophan → 5-HTP → serotonin. Cofactors: vitamin B6 (as P5P) for the final step, plus iron and BH4 (tetrahydrobiopterin) for the rate-limiting hydroxylation upstream. Serotonin can then become melatonin at night.
Starts from the amino acid tyrosine (or phenylalanine) → L-DOPA → dopamine. Cofactors: iron and the helper molecule BH4 (tetrahydrobiopterin) for the rate-limiting step, then vitamin B6 for the last step. Dopamine can go on to make norepinephrine.
Notice the overlap: B6, iron, and BH4 show up on both lines. This is why an iron or B6 shortfall can ripple into mood, focus, and sleep at once — the same cofactor crunch from lesson 1, now in the brain.
Does eating more tryptophan flood your brain with serotonin?
Not really — it's more complicated than 'turkey makes you sleepy.' The brain tightly controls what gets in, and the cofactors and competing amino acids matter as much as the raw starting material. Fixing a genuine cofactor deficiency (like iron or B6) helps the lines run; loading single amino acids is a much blunter, less reliable tool.
How it works →
Tryptophan competes with other large neutral amino acids for the same blood-brain-barrier transporter, so a protein-rich meal can actually lower the tryptophan ratio reaching the brain. Carbohydrate shifts the ratio favorably via insulin. The rate-limiting enzymes (tryptophan hydroxylase, tyrosine hydroxylase) need BH4 (tetrahydrobiopterin, made from GTP — a separate pteridine from folate) and iron, and feed into B6-dependent decarboxylation — so cofactor status, not just substrate, gates output.
What the studies show →
Severe deficiencies clearly impair this chemistry — iron deficiency is linked to altered dopamine signaling and restless-legs and mood symptoms, and correcting it helps. The evidence for amino-acid or precursor supplements (5-HTP, L-tyrosine, tryptophan) lifting mood in well-nourished people is weak, inconsistent, and not a substitute for evaluation of clinical depression. ⚕️ Don't self-treat mood disorders with precursors — especially alongside antidepressants (serotonin syndrome risk).
Magnesium is a required cofactor for roughly how many distinct enzymatic reactions in the body?
Somewhere between 10 and 600 reactions.
Reveal the answer →
300+ (some estimates 600+)
The classic figure is 300+ enzymes, with newer estimates citing 600+ when you count every reaction that uses Mg·ATP. It's one of the most far-reaching cofactors there is — which is why a magnesium gap shows up as such a scattered list of symptoms (cramps, poor sleep, fatigue, irritability). The exact count matters less than the lesson: shared cofactors create body-wide effects.
NAD⁺ and the NMN/NR hype — a balanced read
NAD⁺ is the electron carrier that makes your energy and powers repair-and-maintenance enzymes. Its levels fall with age, which led to a wave of supplements (NMN, NR) sold to 'restore youth.' They reliably raise NAD⁺ in your blood — but whether that translates into living longer or healthier in humans is still unproven. Promising idea, incomplete evidence.
How it works →
NAD⁺ is consumed not just as a redox carrier (NAD⁺/NADH in glycolysis, Krebs, ETC) but also by NAD⁺-spending enzymes: sirtuins (involved in stress resistance and metabolic regulation) and PARPs (DNA repair). Both cleave NAD⁺ to do their work, so heavy DNA damage or inflammation drains the pool. NMN and NR are precursors the cell can salvage back into NAD⁺, raising tissue levels in animals and circulating levels in humans.
What the studies show →
The mechanism and the rodent data are genuinely interesting: in mice, NAD⁺ precursors improve some metabolic and age-related measures. In humans, trials consistently show NR/NMN raise blood NAD⁺ and are reasonably safe short-term, but robust evidence for meaningful clinical benefit — let alone extended lifespan — is still thin and mixed. Bottom line: a plausible mechanism with strong animal support and weak human outcome data. Worth watching, not worth treating as proven; basics (sleep, exercise, not being deficient in the cofactors in this track) have far stronger evidence.
Pathway-thinking, applied
- When something feels 'off' (energy, mood, sleep), ask which pathway and which cofactor — not just which symptom.
- Cover the cofactor basics first: the B-vitamins, magnesium, iron status, selenium — they sit upstream of everything here.
- Treat readouts (homocysteine, ferritin) as gauges of a pathway, and replete to adequate — don't chase ever-higher numbers.
- Be skeptical of 'flood the system' pitches (megadose antioxidants, NAD⁺ boosters) where human outcome evidence is thin.
- Get cofactors from a varied whole-food diet first; use targeted supplements to fill verified gaps.
Browse the pathways and the cofactors each one depends on — then check your own coverage on the Nutrients page.