Blood was drawn from the vein that drains the brain in 101 healthy men, at different times of year. The brain's turnover of serotonin was lowest in winter. On any given day it rose with the hours of bright sunlight, and it climbed quickly as the light grew stronger.
Serotonin gets called the happiness chemical, and that one finding seems to prove it. The full story is stranger and more useful. Most of our serotonin never goes near the brain, the idea that low mood means low serotonin has not held up well, and several of the tips sold in its name work differently from the way they're told. The habits behind them still do. Ahead: where the name comes from, what happened to a monkey's serotonin when he rose in rank, why the turkey dinner story is false, and what 17 large reviews found when they looked for low serotonin in depression.
What serotonin is
It was found twice before anyone looked in the brain. In the 1930s an Italian pharmacologist, Vittorio Erspamer, drew a substance from the lining of the gut that made intestines contract, and called it enteramine. In 1948 a team in Cleveland found something in blood serum that tightened blood vessels, and named it for where it came from and what it did: serum, and tone. In 1952 the two turned out to be the same molecule.
The brain came last. Betty Twarog was a graduate student in her twenties when she showed serotonin carrying nerve signals in mussels. The journal's editor left her paper unanswered until her supervisor telephoned to ask after it. She went on to find serotonin in the brain.
That order tells us something. About 95% of the body's serotonin is made outside the brain, most of it in the gut, where it keeps digestion moving. Elsewhere it helps blood to clot and has a hand in the heart, the blood vessels and the bladder. Gut bacteria join in: particular kinds prompt the gut's cells to make more of it.
It guards the door as well. When the gut meets something harmful, its cells release serotonin, the gut speeds up, and nausea and vomiting follow. Medicines that block that signal are the standard defence against the sickness of cancer treatment.
None of that supply reaches the head. The barrier between blood and brain keeps it out, so the two pools stay separate. The brain makes its own, from an amino acid in food called tryptophan.
Brains differ in how fast they make it. On scans, serotonin was being made 52% faster in men's brains than in women's, and the team behind the scans wondered whether that has a hand in depression being more common in women.
Serotonin is far older than we are. Nearly all animals carry it, insects and worms included, and a stinging nettle keeps it in its sting, where its job is to hurt.
1. Go out into the light
Light has the most direct evidence behind it. Alongside the 101 men, brain scans of 88 healthy people showed the same season at work: in autumn and winter their brains carried more of the transporter that clears serotonin away, and the fewer the hours of sunshine, the more of it there was.
It works as a treatment too. When 122 adults with depression were given a bright light box each morning, a common antidepressant, both, or dummy versions of both, the light alone and the two together eased depression more than the dummies over eight weeks.
Winter is where it has been tested most. For the low mood that comes with the dark months, bright light eased symptoms across eight sound tests, by about as much as antidepressants do in theirs. A light box for the purpose should give 10,000 lux and be free of ultraviolet, and it may not suit certain eye conditions or medicines that make the eyes sensitive to light. A that is UV-free fits on a desk. A slow artificial dawn before waking helped in those tests as well, which is what a does from the bedside.
Outdoors gives that brightness for nothing. A living room has about 50 lux and an office 320 to 500. An overcast day gives about 1,000, and full daylight out of direct sun gives 10,000 to 25,000: the strength of a light box, in the shade.
Light seems to shield mood as well. When healthy women had their tryptophan cut for a few hours in a dim room, their mood fell. In a bright room it held.
The night matters as much as the day. Serotonin is the raw material for melatonin, the hormone that brings on sleep, and the brain makes it in darkness. A bright day and a dark night are two halves of one cycle.
These rhythms take their light in through the eyes, so a hat and the shade are no obstacle.
Take the first break of your day outdoors, and stay out long enough to feel the light on your face. In the evening, turn the lamps down and put the screens away before bed. If street light reaches your bed, a keeps the room dark.
2. Move
Exercise changes the supply line. During a workout, the tryptophan in the blood rises against the amino acids that compete with it, so more of it is free to reach the brain.
Whatever the route, the effect on mood is well tested. Across 218 tests with 14,170 people with depression, walking or jogging, yoga and strength training all brought a moderate drop in symptoms, and the harder the exercise, the larger the effect.
Walk briskly today for as long as you comfortably can, and notice your mood an hour later.
3. Mind your company
Serotonin answers to our place among others. In vervet monkeys, the dominant male of a group carried about twice the blood serotonin of the males below him. When a male rose to the top his level rose, and when he was moved out of the role it fell. The chemistry followed the standing.
In people, a brain scan caught something similar from the inside. When healthy volunteers called up happy and sad memories, the happier they made themselves, the more serotonin one part of the brain was making.
Spend an hour this week with someone who leaves you feeling larger, and notice what the next day feels like.
4. Let go of the turkey story
Turkey, bananas and "serotonin foods" are the most repeated advice, and the least true. Bananas and chocolate do contain serotonin, and it stays outside the brain. Turkey does contain tryptophan, along with a crowd of other amino acids that compete for the same door, so a high-protein meal leaves the brain's share no higher. Pure tryptophan can raise brain serotonin. Food rich in it does not. (Tryptophan was first isolated in 1901, from the protein in milk, and its name carries the Greek for "to appear".)
Carbohydrate is the one food that shifts the odds. When eight people ate a carb-heavy breakfast one day and a protein-heavy one another, the share of tryptophan among its competitors in their blood ended up 54% apart, higher after the carbs. That's the grain of truth in comfort eating.
Mood goes its own way, though. When carbs were tested against dummy drinks, they gave mood no lift at any point, and within the hour people felt more tired and less alert.
Food still matters, because tryptophan has to come from somewhere, and meat, fish and eggs are full of it. Making serotonin takes helpers too: the first and slowest step needs iron, and the next needs vitamin B6. Red meat and liver are rich in iron, and pork, poultry and fish carry B6. So there is no case here against a low-carb, keto or carnivore plate: mood did not follow the carbs, and the tryptophan, iron and B6 all come with the meat. The point is that no single meal is a mood switch.
Eat proper meals built around meat, fish or eggs, and cross "serotonin foods" off the shopping list.
5. Question the low-serotonin story
For decades, low mood was explained as a shortage of serotonin, and the explanation had a famous medicine beside it: Prozac reached American pharmacies in January 1988, and within a year its sales had reached $350 million.
When 17 large reviews of the evidence were pulled together, covering blood, spinal fluid, brain scans and genes, they found no consistent sign that depression goes with lower serotonin.
Deliberately lowering it tells the same story. Across 45 tests, cutting off tryptophan for a few hours did not lower mood in healthy people. It did in people who had recovered from depression.
What does lead to it is less tidy. There is no single cause: a bereavement, an illness, money or work trouble, and facing them alone, all raise the odds, and so can the genes we inherit.
None of that means antidepressants are useless. Across 522 tests with 116,477 people, all 21 that were tested did better than a dummy pill. A medicine can help without the illness being a shortage of what it acts on. If you take one, a headline is no reason to stop. Stopping one suddenly can bring withdrawal symptoms of its own, so it is never a step to take alone.
6. Be careful with the pills
Tryptophan and its close relative 5-HTP are sold as serotonin in a capsule. Of 108 tests of them for depression, only two were sound enough to count, with 64 people between them. Those two pointed to a benefit, on far too little to rely on.
St John's wort is the one to watch. Taken alongside an antidepressant, or another medicine that acts on serotonin, it can push serotonin's effects too high, and the side effects can be serious. It also interferes with the blood thinner warfarin and with some statins.
Too much serotonin has a name, serotonin syndrome, and it is rare and serious: a fast heartbeat, sweating, shaking, twitching muscles, confusion or agitation. Any of those after mixing serotonin products means getting urgent help.
If you take any medicine, read up on a supplement's clashes before you buy it, and never add one of these to an antidepressant on your own.
7. Skip the serotonin test
Kits that claim to measure your serotonin from a urine sample have been sold since 2001. A review of the evidence behind them found no published work showing that they tell anything about an individual's brain or mood.
Keep the money, and spend the hour outdoors.
When it's more than a low day
A low mood often lifts by itself. If it has been there for most of the day, every day, for more than two weeks, or the things you used to enjoy have gone flat, reach out for help. Light and walking sit well beside treatment, and they are no replacement for it.
Start with the light
Serotonin is real, and it matters. It is also one messenger among many, and most of it is busy in the gut.
The light, the walk and the good company work whether or not serotonin gets the credit. In those 101 men, the brightest days were the busiest ones for serotonin. Start there: tomorrow morning, take your breakfast outside.
Wondering what to put on the plate instead? Foods That Lift Your Mood, and the Ones That Drag It Down — the foods with a tested link to mood, and the ones that pull it down.
Is it your energy that's low, more than your mood? Unstoppable Energy: 14 Ways to Stay Focused and Strong Every Day — why tiredness is a reading your brain sets, and what moves it.
Know someone who goes quiet when the days get short? Send them this, and ask them out for a walk while it's still light.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional. If your mood has been low for weeks or you're having thoughts of harming yourself, reach out for help today.
Affiliate Disclosure: This article contains affiliate links. As an Amazon Associate, All For Life earns from qualifying purchases, at no extra cost to you. We only link to products we would recommend without the commission.
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