Serotonin acts as a neurotransmitter, a type of chemical that helps relay signals from one area of the brain to another. Although serotonin is manufactured in the brain, where it performs its primary functions, some 90% of our serotonin supply is found in the digestive tract and in blood platelets
How is serotonin made?
Serotonin is made via a unique biochemical conversion process. It begins with tryptophan, a building block to proteins. Cells that make serotonin use tryptophan hydroxylase, a chemical reactor which, when combined with tryptophan, forms 5-hydroxytryptamine, otherwise known as serotonin.
What role does serotonin play in our health?
As a neurotransmitter, serotonin helps to relay messages from one area of the brain to another. Because of the widespread distribution of its cells, it is believed to influence a variety of psychological and other body functions. Of the approximately 40 million brain cells, most are influenced either directly or indirectly by serotonin. This includes brain cells related to mood, sexual desire and function, appetite, sleep, memory and learning, temperature regulation, and some social behavior.
In terms of our body function, serotonin can also affect the functioning of our cardiovascular system, muscles, and various elements in the endocrine system. Researchers have also found evidence that serotonin may play a role in regulating milk production in the breast, and that a defect within the serotonin network may be one underlying cause of SIDS (sudden infant death syndrome).
What is the link between serotonin and depression?
There are many researchers who believe that an imbalance in serotonin levels may influence mood in a way that leads to depression. Possible problems include low brain cell production of serotonin, a lack of receptor sites able to receive the serotonin that is made, inability of serotonin to reach the receptor sites, or a shortage in tryptophan, the chemical from which serotonin is made. If any of these biochemical glitches occur, researchers believe it can lead to depression, as well as obsessive-compulsive disorder, anxiety, panic, and even excess anger.
One theory about how depression develops centers on the regeneration of brain cells -- a process that some believe is mediated by serotonin, and ongoing throughout our lives. According to Princeton neuroscientist Barry Jacobs, PhD, depression may occur when there is a suppression of new brain cells and that stress is the most important precipitator of depression. He believes that common antidepressant medications known as SSRIs, which are designed to boost serotonin levels, help kick off the production of new brain cells, which in turn allows the depression to lift.
Although it is widely believed that a serotonin deficiency plays a role in depression, there is no way to measure its levels in the living brain. Therefore, there have not been any studies proving that brain levels of this or any neurotransmitter are in short supply when depression or any mental illness develops. Blood levels of serotonin are measurable -- and have been shown to be lower in people who suffer from depression - but researchers don't know if blood levels reflect the brain's level of serotonin.
Also, researchers don't know whether the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.
Antidepressant medications that work on serotonin levels -- SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin and norepinephrine reuptake inhibitors) -- are believed to reduce symptoms of depression, but exactly how they work is not fully understood.
Can diet influence our supply of serotonin?
It can, but in a roundabout way. Unlike calcium-rich foods, which can directly increase your blood levels of this mineral, there are no foods that can directly increase your body's supply of serotonin. That said, there are foods and some nutrients that can increase levels of tryptophan, the amino acid from which serotonin is made.
Protein-rich foods, such as meat or chicken, contain high levels of tryptophans. Tryptophan appears in dairy foods, nuts, and fowl. Ironically, however, levels of both tryptophan and serotonin drop after eating a meal packed with protein. Why? According to nutritionist Elizabeth Somer, when you eat a high-protein meal, you "flood the blood with both tryptophan and its competing amino acids," all fighting for entry into the brain. That means only a small amount of tryptophan gets through -- and serotonin levels don't rise.
But eat a carbohydrate-rich meal, and your body triggers a release of insulin. This, Somer says, causes any amino acids in the blood to be absorbed into the body -- but not the brain. Except for, you guessed it -- tryptophan! It remains in the bloodstream at high levels following a carbohydrate meal, which means it can freely enter the brain and cause serotonin levels to rise, she says.
Can exercise boost serotonin levels?
Exercise can do a lot to improve your mood -- and across the board, studies have shown that regular exercise can be as effective a treatment for depression as antidepressant medication or psychotherapy. In the past, it was believed that several weeks of working out was necessary to see the effects on depression, but new research conducted at the University of Texas at Austin found that just a single 40-minute period of exercise can have an immediate effect That said, it remains unclear of the exact mechanism by which exercise accomplishes this. While some believe it affects serotonin levels, to date there are no definitive studies showing that this is the case.
Do men and women have the same amount of serotonin -- and does it act the same way in their brain and body?
Studies show that men do have slightly more serotonin than women, but the difference is thought to be negligible. Interestingly, however, a study published in September 2007 in the journal Biological Psychiatry showed there might be a huge difference in how men and women react to a reduction in serotonin -- and that may be one reason why women suffer from depression far more than men.
Using a technique called "tryptophan depletion," which reduces serotonin levels in the brain, researchers found that men became impulsive but not necessarily depressed. Women, on the other hand, experienced a marked drop in mood and became more cautious, an emotional response commonly associated with depression. While the serotonin processing system seems the same in both sexes, researchers now believe men and women may use serotonin differently.
Although studies are still in their infancy, researchers say defining these differences may be the beginning of learning why more women than men experience anxiety and mood disorders, while more men experience alcoholism, ADHD, and impulse control disorders.
There is also some evidence that female hormones may also interact with serotonin to cause some symptoms to occur or worsen during the premenstrual time, during the postpartum period, or around the time of menopause. Not coincidentally, these are all periods when sex hormones are in flux. Men, on the other hand, generally experience a steady level of sex hormones until middle age, when the decline is gradual.
Since both dementia and Alzheimer's disease are brain-related conditions, does serotonin play a role in either problem?
In much the same way that we lose bone mass as we age, some researchers believe that the activity of neurotransmitters also slows down as part of the aging process. In one international study published in 2006, doctors from several research centers around the world noted a serotonin deficiency in brains of deceased Alzheimer's patients. They hypothesized that the deficiency was because of a reduction in receptor sites -- cells capable of receiving transmissions of serotonin -- and that this in turn may be responsible for at least some of the memory-related symptoms of Alzheimer's disease. There is no evidence to show that increasing levels of serotonin will prevent Alzheimer's disease or delay the onset or progression of dementia. However, as research into this area continues, this could also change.
What is serotonin syndrome -- and is it common or dangerous?
SSRI antidepressants are generally considered safe. However, a rare side effect of SSRIs called serotonin syndrome can occur when levels of this neurochemical in the brain rise too high. It happens most often when two or more drugs that affect serotonin levels are used simultaneously. For example, if you are taking a category of migraine medicines called triptans, at the same time you are taking an SSRI drug for depression, the end result can be a serotonin overload. The same can occur when you take SSRI supplements, such as St. John's wort.
Problems are most likely to occur when you first start a medication or increase the dosage. Problems can also occur if you combine the older depression medications (known as MAOIs) with SSRIs.
Finally, recreational drugs such as ecstasy or LSD have also been linked to serotonin syndrome.
Symptoms can occur within minutes to hours and generally include restlessness, hallucinations, rapid heartbeat, increased body temperature and sweating, loss of coordination, muscle spasms, nausea, vomiting, diarrhea, and rapid changes in blood pressure .
Although not a common occurrence, it can be dangerous and is considered a medical emergency. Treatment consists of drug withdrawal, IV fluids, muscle relaxers, and drugs to block serotonin production.