Overview
Serotonin is one of the body's most basic primitive responses to stress and injury, it's not a "happy hormone." About 95% of it is produced in the intestine, mostly around the appendix, and only 3 to 5% in the brain. Its function is to shut down expensive biological processes during emergencies, the same way it lets rodents go into hibernation when food is scarce. The pharmaceutical industry inverted public understanding of serotonin in the 1960s by exploiting the government campaign against LSD: since LSD was demonstrated to be an anti-serotonin agent and was then criminalized as something that drives people insane, the drug companies sold the inverse story that raising serotonin would make people sane and happy. In reality, chronic serotonin excess contributes to nearly every degenerative disease: fibrosis, hardening of the arteries, osteoporosis, pulmonary artery hypertension, cataracts, migraines, depression, irritable bowel, blood clotting, and cancer. Almost every acute or degenerative disease would be alleviated by controlling serotonin.
Key Points
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Roughly 95% of the body's serotonin is produced in the intestine, not the brain. The biggest concentration is in or around the appendix, with only 3 to 5% in other organs such as the brain. The bacterial gradient of the small intestine increases parallel to the serotonin gradient, and as thyroid function falls, bacteria creep further up the small intestine and the serotonin-producing zone expands with them.
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Serotonin is a stress and emergency defense hormone, not a feel-good hormone. It is produced in response to injury, sunburn, ionizing radiation (dental x-rays alone are enough to activate it), suffocation, and chronic stress. Its function is to shift the organism away from oxidative metabolism toward glycolytic or hibernating shutdown of systems. Animals produce increasing amounts of serotonin before going into winter hibernation, and at extreme overproduction humans go into a similar torpor.
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The cultural belief that serotonin equals happiness came from reverse psychology after LSD was criminalized. Through the 1950s and early 1960s, researchers established that LSD and the ergot family worked by blocking serotonin nerves, and they were proposing LSD to treat migraines and other conditions of serotonin excess. Once the government banned LSD as a drug that supposedly made people insane, drug companies inverted the logic: if the anti-serotonin substance makes you crazy, serotonin itself must make you sane. The whole SSRI marketing edifice was built on this inversion.
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When SSRIs work, they work by lowering serotonin rather than raising it. When researchers treated vicious aggressive dogs with SSRIs, the dogs became friendly, and at the same time their measured serotonin had dropped significantly. Carcinoid patients flooded with serotonin from intestinal tumors were psychotic, anxious, and unmanageably aggressive. The mood-lift from SSRIs in humans seems to correspond to increased brain progesterone and pregnenolone produced as a response to brain irritation, not to the serotonin change itself.
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Estrogen drives serotonin up, progesterone breaks it down. Estrogen activates tryptophan hydroxylase, the rate-limiting enzyme that converts tryptophan to serotonin, and it inhibits monoamine oxidase type A, which would otherwise degrade serotonin. Progesterone reverses both of those effects. Polyunsaturated fats specifically release tryptophan into the brain to make more serotonin, and the resulting serotonin then activates the enzymes that turn PUFA into inflammatory prostaglandins.
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Chronic serotonin excess causes a long list of degenerative conditions. It causes blood clotting, vascular spasms, vascular leakiness, fibrosis of every system (scleroderma, hardening of the arteries), osteoporosis as a major immediate contributor, pulmonary artery hypertension and right-sided heart valve damage, cataracts and glaucoma in the eye, migraines, irritable bowel syndrome, increased prolactin, TSH and cortisol, and tumor growth. The carcinoid disease, an intestinal tumor that produces huge amounts of serotonin, established all of this in the 1950s.
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The lungs detoxify serotonin, and CO2 is what keeps it locked in the platelets until it gets there. Platelets pick serotonin up from the intestine and carry it to the lungs, where peroxide and specific enzymes destroy it. If a person hyperventilates and loses too much carbon dioxide, the platelets release serotonin indiscriminately into the bloodstream, causing capillary constriction, edema, and leakiness throughout the body. Hypothyroidism produces the same effect because lactic acid replaces CO2.
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Serotonin is the most potent activator of cortisol release, and forms a positive feedback loop with cortisol. Serotonin triggers ACTH and the cortisol cascade, while cortisol in turn promotes serotonin synthesis. Once this feedback loop is running, you cannot relax, your blood sugar stays elevated, your muscle melts, and estrogen rises through serotonin's activation of aromatase. Serotonin and cortisol together drive most of what is called depression: 2 out of 3 patients with clinical depression are non-suppressible on the dexamethasone test, meaning their cortisol stays high.
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Serotonin shuts off GABA production, creating chronic anxiety and inability to relax. GABA is the calming neurotransmitter, and benzodiazepines like Valium, Xanax, and Klonopin all work by activating it. When serotonin suppresses endogenous GABA, glutamate (the excitatory neurotransmitter) rises, producing chronic anxiety and overexcitation alongside the cognitive lobotomy. The combination is what produces the violent, homicidal phenotype seen in animal studies of SSRIs and in the school shooting cases mentioned in the literature.
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Anti-serotonin substances include cyproheptadine, aspirin, thyroid (especially T3), progesterone, calcium, CO2, and certain ergot derivatives. Cyproheptadine is the safest antihistamine because it is also antiserotonin. Aspirin is very powerful against serotonin and works in many of the same ways thyroid does. Lisuride, bromocriptine, ondansetron, and tianeptine (Stablon) are also anti-serotonin. Sugar helps stop the production of histamine and serotonin, while saturated fats and adequate calcium oppose it.
Notable Quotes
"95% of it, roughly, is produced in the intestine and the biggest concentration in the digestive system is in or around the appendix."
[Ray Peat — Serotonin and Endotoxin]
"Serotonin is a hormone of withdrawal, anxiety, and suppressing metabolism, lowering your demands. It's an emergency anti-stress hormone that is necessary in context, but when you're chronically stressed, it's what causes stomach ulcers, contributes to fibrosis of every system, scleroderma, for example, hardening of the arteries, osteoporosis."
[Ray Peat — Ask Your Herb Doctor (KMUD): Milk]
"Everything is herded away from understanding that just about every acute or degenerative disease would be alleviated by controlling serotonin."
[Ray Peat — Serotonin and Endotoxin]
"It's known to cause blood clotting, spasms of coronary arteries, other arteries, inflammation. The latest horrible thing is that it's a major factor in osteoporosis."
[Ray Peat — Ask Your Herb Doctor (KMUD): Evidence-Based Medicine]
"I think the antidepressants that work are, in the long run, shifting the balance away from serotonin."
[Ray Peat — Ask Your Herb Doctor (KMUD): Serotonin, Endotoxins & Stress]
Important Things To Consider
5-HTP is more dangerous than melatonin. 5-HTP turns massively into serotonin, and a single dose can create cataracts almost instantly. People prescribed fish oil and 5-HTP for eye conditions like cataracts or glaucoma are being given exactly the inputs that drive the disease, since fish oils break down to increase serotonin formation in the eye. 5-HTP also suppresses immunity and stimulates cortisol.
Whey protein powder concentrates tryptophan and is highly oxidized. Tryptophan is the precursor amino acid for serotonin, and the amino acids in whey are mainly tryptophan. Adults have very low requirements for tryptophan, methionine, and cysteine. Animal studies show that animals nearly devoid of methionine live 30% to 40% longer, and similar effects are seen with tryptophan restriction. Gelatin lacks those amino acids and is safe in very large amounts.
Several common fruits are high in serotonin or tryptophan. Bananas are high in serotonin content, to the point where doctors used to suspect intestinal tumors when high serotonin showed up in the urine of banana eaters. Prunes, plums, kiwis, and pineapple (high in tryptophan) are similar. Watermelons, grapes, and citrus, especially oranges, are safer choices.
Slow-digesting fibers feed bacteria that produce serotonin higher up the intestine. Raw vegetables, crisp apples, crisp pears, and pectin-rich fruits are excellent bacterial food and poor human food. As thyroid function falls, peristalsis slows, bacteria creep further up the small intestine, and the serotonin-producing zone expands with them. Carrots are the exception because they contain antifungal and antibacterial defenses that act like an antibiotic on the intestine.
SSRI withdrawal must be supported, not done abruptly. Stopping SSRIs without support can produce extreme sadness, deep withdrawal, and mood volatility because the chronic serotonin promotion has interfered with cellular respiration and depleted thyroid-driven oxidative metabolism. Withdrawal works better with thyroid, progesterone, coffee, vitamin D, calcium, sugar, and elimination of polyunsaturated fats to reactivate oxidative metabolism while suppressing glycolysis.
Melatonin partially detoxifies serotonin but has its own endocrine costs. The pineal converts serotonin into melatonin under adrenergic stress signaling at night, and melatonin can act as an anti-serotonin in some contexts. In animal studies, however, supplemental melatonin lowered both progesterone and thyroid while increasing estrogen, so it should not be a long-term solution.
Hyperventilation and hypothyroidism both produce systemic serotonin poisoning. Estrogen makes women hyperventilate and become slightly alkalotic, which releases serotonin and histamine, contracts the breathing muscles, and progresses from flemminess through asthma to terminal lung fibrosis. Hypothyroid people fail to produce enough CO2, lactic acid rises, and the same cascade happens systemically.
Serotonin syndrome is potentially lethal and is treated with cyproheptadine. If serotonin gets high enough fast enough, you can die from it. Untreated serotonin syndrome carries roughly 40% lethality, and even with treatment about 10%. The FDA banned over-the-counter sale of L-tryptophan in the early 90s after an epidemic of eosinophilic myalgia tied to tryptophan supplementation. Many SSRIs now carry black-box warnings about combining them with anything that inhibits the enzymes that degrade serotonin.
Cyproheptadine is sedating and impairs sleep quality despite knocking you out. Cyproheptadine is a non-selective serotonin antagonist originally marketed as an antihistamine. A 2.5 milligram tablet, the lowest dose available, is enough to put most people to sleep, and they wake up feeling drugged. Starting with 0.5mg or less is reasonable.
High-dose melatonin can paradoxically raise serotonin. Supplements selling 5, 10, or even 15 milligrams of melatonin are obscene. Endogenous nightly production is around 500 micrograms. Excess melatonin shuts off the enzyme that converts serotonin to melatonin via negative feedback, so serotonin builds up. The result is intense nightmares indistinguishable from PTSD-style nightmares, which respond to serotonin antagonists.
Endurance exercise mechanically triggers serotonin release in the gut. The bouncing and twisting of running stretches the intestine, which the gut interprets as trauma, releasing histamine, serotonin, and nitric oxide to speed transit. This is the literal cause of the marathon runner's "runs," and is why some long distance runners wear diapers. The combined effect of mechanical gut irritation, blood being shunted away from the intestines, and elevated lipolysis makes endurance exercise one of the more efficient ways to flood the body with serotonin.
Light and darkness regulate serotonin in the opposite direction from what most people are told. Serotonin in the blood is highest at midnight in the winter, and the worst time for depression, strokes, and inflammation is around midnight in winter. Morning light treatments are sold as raising serotonin to make people feel good, butlight tends to support progesterone while darkness drives estrogen, serotonin, and melatonin.