Serotonin is a common neurotransmitter found in our brains. Neurotransmitters are special chemicals in our brain that help 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. The average adult has between five and 10 milligrams of serotonin in the body.

Q. 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 heart, 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).

Q. 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 of the newest theories about depression 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, such as Celexa, Lexapro, Prozac, and Paxil — 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. And while blood levels of serotonin are measurable — and have been shown to be lower in people who suffer from depression — what doctors still don’t know for certain is whether or not the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.

Antidepressant medications that work on serotonin levels — medications known as 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 yet fully understood.

Q. 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.

Eating a carbohydrate-rich meal will have your body trigger a release of insulin. This in turn causes any amino acids in the blood to be absorbed into the body except for 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.

Getting an adequate supply of vitamin B-6, which can influence the rate at which tryptophan is converted to serotonin.

Q. 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 40 minutes of regular exercise can have an immediate effect on mood.

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.

Q. 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.

Q. What is serotonin syndrome and how common is it?

SSRI antidepressants are generally considered safe; however, a rare side effect of SSRIs is serotonin syndrome. Serotonin syndrome is a condition that occurs when levels of this neurochemical 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.

Your can learn more about serotonin syndrome here.