At a Glance
- Serotonin is a chemical produced by the body from proteins in our diet.
- Although often linked to its effect on the brain and central nervous system, the vast majority of serotonin is stored and used in the digestive system, blood platelets and in bones.
- Serotonin has an extremely wide range of positive and negative effects on the body.
In general terms, serotonin is often most closely associated with mood, and in particular with depression. In reality, however, it has many different functions.
Due to the complexity of our bodies, especially the brain, and the difficulty of isolating the specific influence of a single chemical, we still do not fully understand the role of serotonin in the body in respect of all of its functions.
However, research that has been conducted to date can identify and explain some, but not all of its functions. In this article we’ll take a look at the questions: “what is serotonin and what does it do?”
What Is Serotonin?
Serotonin is a chemical which is produced by cells in the nervous system. It is made from an amino acid, the building blocks of protein, called tryptophan.
Tryptophan is one of nine essential amino acids, so called because the body is unable to manufacture it – tryptophan has to be sourced through our diet, in foods such as milk, cheese, nuts, red meat and chickpeas. (source)
Serotonin cannot be transported into or out of the brain, and since it is found in the body in the brain, the digestive system and in blood platelets, scientists have concluded that the serotonin used in the brain must be produced in the brain, and the serotonin that acts on systems outside of the central nervous system is produced at other sites.
In both cases, serotonin is produced from tryptophan, which undergoes several stages of chemical changes to produce serotonin. The chemical name for serotonin is 5-hydroxytryptamine, or 5-HT.
The serotonin is then attached to specialized molecules which transport it. As much as 90 percent of our serotonin is found in the digestive tract and the blood platelets, but is also found in the central nervous system, where it has to bind with serotonin receptor cells to become active.
Some research has identified serotonin receptors in specific parts of the brain. These include the cortex, which is responsible for personality traits, conscious thoughts and actions, the amygdala, responsible for emotions, survival instincts and memory, and the hippocampus, associated with memory and spatial awareness. (source)
Other studies, however, indicate a more widespread use of serotonin throughout the central nervous system stating that most structures within the central nervous system show evidence that serotonin plays a role in many of their functions. (source)
What Does Serotonin Do?
The main function of serotonin is to transmit messages between different nerve cells. Although that sounds fairly simple, when we consider that our nervous system in turn controls all the functions in the body, we start to see that serotonin can affect a wide number of body functions.
Add to this the complexity of the brain and nervous system, and the fact that scientists do not fully understand many of the functions of the brain. We then start to appreciate why research cannot fully explain all of the functions of serotonin.
However, there is some clear evidence that demonstrates the role of serotonin in some cases. Let’s take a look.
Serotonin is thought to be responsible, at least in part, for regulating our feelings of happiness or sadness, anxiety and overall mood. Drugs which are known to drastically change our mood, like LSD and ecstasy have been shown to dramatically increase the levels of serotonin in the body. (source)
Research has linked low levels of serotonin with a lowered mood state, but notes that this does not usually happen independently. Scientists believe that there must be some kind of interaction between serotonin and other chemicals in the body which results in low mood, although they are currently unable to identify any specific interactions. (source)
Serotonin and Depression
Depression is an illness which has many potential causal factors. This makes it difficult to identify the precise effect of serotonin on depression.
Research studies have most certainly identified a link between serotonin and depression, but the exact nature of the relationship is unclear. It is difficult to determine whether depression causes decreased levels of serotonin in the body or decreased amounts of serotonin are a factor in the onset of depression.
There is, however, a whole group of medications called selective serotonin reuptake inhibitors (SSRIs) which work by preventing or minimizing the reabsorption of serotonin from the bloodstream which are commonly used to treat anxiety, depression and obsessive-compulsive disorders, and have been shown to be clinically effective, but scientists are unable to categorically state exactly how they affect depression. (source)
In recent years, one study in mice compared one group of subjects with low serotonin levels with another group with normal levels of serotonin in a series of behavioral tests designed to evaluate the subjects for depression.
They found no signs of depression in either group, and concluded that a low level of serotonin is not a factor in the development of depression. However, much more research is needed using human subjects, before this information can be stated as fact. (source)
Serotonin and Muscle Function
Serotonin is thought to have a significant role in constricting smooth muscles, such as in the digestive system, in the cardiac muscle of the heart and other internal organs which are not under voluntary control.
More recent research indicates that serotonin can be found in skeletal muscle fibers. It has been shown to help promote muscle growth, and further research is being conducted in this area. (source)
The Role of Serotonin in Digestive Disorders
Since such a large percentage of the body’s serotonin is found in the digestive system, it is unsurprising that it also has significant effects within this region.
Most of the serotonin in the digestive system is thought to play a key role in regulating the body’s bowel movement and function. As well as playing its part in normal bowel function, serotonin also has an important protective role in the gut.
The body produces increased levels of serotonin in response to the detection of a toxic substance in the digestive tract, which speeds up the digestive process to decrease the amount of the substance that is absorbed into the bloodstream, and to expel the substance as diarrhea. (source)
Research indicates that low levels of serotonin are associated with constipation, while high serotonin levels have been observed in diarrhea and celiac disease.
Serotonin also appears to play a role in irritable bowel syndrome (IBS), a condition which can present with symptoms of either constipation or diarrhea.
Some research suggests that abnormal levels of serotonin after consuming food may be related to the symptoms of IBS, with IBS patients presenting with constipation having low levels of serotonin, and IBS patients who present with diarrhea having high levels of serotonin. (source)
There is still much about our sleep patterns that are not fully understood. However, according to research from Harvard Medical School, the hypothalamus is the main center of the brain responsible for driving our sleep response. (source)
Historically, scientists thought that serotonin was only responsible for initiating and maintaining sleep. However, more recent research has shown that serotonin can also inhibit sleep, depending on how it is used in the brain. (source)
Serotonin and Pain
Recent research has investigated the relationship between serotonin and pain threshold – the amount of pain our bodies are able to cope with before we experience the sensation of pain.
One clinical trial reported that limiting the available tryptophan, and therefore decreasing the amount of serotonin in the body resulted in a significant increase in sensitivity to pain. (source)
However, not all research on the subject of pain agrees, especially with reference to specific pain-inducing health conditions.
For example, some studies have found that low levels of serotonin have been associated with conditions such as fibromyalgia (source), whereas high levels of serotonin has been correlated with chronic regional pain syndrome, commonly known as CRPS. (source)
While this evidence seems to be contradictory, the one thing these two conditions have in common is that neither of them are fully understood by medical science.
Pain is Complicated
Pain is an incredibly complex issue that some experts devote their careers to in an effort to discover more about pain. One of the reasons why pain is so complicated is that it’s a phenomenon that involves many pathways, and the amount of pain we experience depends on our circumstances.
Anxiety, for example, plays a significant role in our pain perception. To illustrate this, consider shutting your hand in your car door.
Painful? Of course it is.
But the brain moderates our response to the pain according to our mood and situation. If we were on our way to the dentist for a root canal and we’re panicking about it, the pain is likely to be much more intense than if we were just on our way to collect our prize from a winning lottery ticket. This data is well supported in research. (source)
What we can say is that although the relationship between pain and serotonin might be complicated and far from fully understood, there seems to be a definite link; some medications which act to keep as much serotonin and norepinephrine, a hormone and neurotransmitter, as active as possible in the body have been proven to reduce chronic pain in some conditions. (source)
Serotonin and Bone Health
Serotonin has been closely linked with bone health, although studies have produced conflicting results.
Some studies have shown that high levels of serotonin in blood serum are associated with low bone mineral density. They suggest that over a long term this can lead to osteoporosis – a condition where the honeycomb structure of the bones is broken down, resulting in weak brittle bones which are more prone to fractures when exposed to excessive forces. (source)
Other studies, however, indicate that long term high levels of serotonin, a condition called hyperserotonemia, results in increased bone mass density in non-human subjects. (source)
Further research has attempted to explain the conflicting results, and has identified that a microsystem involving serotonin exists within bones. Some research suggests that serotonin increases the production of osteoclasts – bone cells responsible for bone reabsorption, but it can also stimulate the production of osteoblasts – the cells that build and reshape bones. However, there is a need for further research in this area. (source)
Helps Heal Wounds
Serotonin is released from platelets in the blood, and has the effect of constricting the tiny blood vessels – arterioles and capillaries – near the surface of the skin when we suffer a wound. This reduces the flow of blood and makes it easier for the platelets to form a blood clot to seal the wound ready for repair. (source)
Regulates Sexual Function
Sexual behavior is heavily influenced by the release of several chemicals, but particularly by dopamine and serotonin. While dopamine stimulates desire for sex, serotonin generally inhibits sexual activity.
Studies indicate that when serotonin levels are low, libido increases, and conversely, high levels of serotonin is associated with a reduced sexual drive. This is particularly evident in patients on SSRI medication; with some studies indicating the occurrence of sexual dysfunction as being between 30 and 60 percent. (source)
Scientists have identified a clear link between serotonin and nausea. In response to toxic substances, or something that irritates the digestive system, serotonin stimulates particular nerve fibers that trigger nausea and cause us to vomit in an attempt to expel the undesirable substance.
Many antiemetic drugs intended to reduce or prevent nausea and vomiting act by blocking serotonin receptors, and so decrease the level of serotonin which is active in the body. (source)
This is a medical condition which results from over-stimulation of the central nervous system, including serotonin receptors. It can occur through taking medication, supplements or illegal drugs, or a combination of these which leads to serotonin levels becoming too high.
For this reason, all medications that result in increased levels of serotonin carry a warning not to mix them with any other type of medication or substance that have a similar effect – for example, migraine medication and St John’s Wort – a common supplement which is taken to help manage depression and anxiety.
Similarly, some types of tumors cause increases in serotonin, and in these cases it is not appropriate for people to take medication like SSRIs or St John’s Wort.
Serotonin syndrome results in dramatically increased activity within the nervous system, and can lead to seizures and loss of consciousness. It can potentially be life threatening. The symptoms of serotonin syndrome become apparent after several hours of ingesting the substance and include:
- Increased heart rate
- Irregular heartbeat
- Increased blood pressure
- Dilation of the pupils
- Loss of muscle coordination
If serotonin syndrome is suspected, immediate medical attention is necessary. It can be easily diagnosed, and while less severe cases may simply require the patient to stop taking the combination of medication which caused serotonin syndrome, more severe cases may need hospital treatment to monitor and restore serotonin levels to normal.
Natural Serotonin Boosters
There are several ways in which we can naturally increase low levels of serotonin.
Considering that we need the amino acid tryptophan from our diet, it makes sense that if we increase the amount of foods high in tryptophan in our diet, we can also increase our levels of serotonin.
However, when it comes to the brain – as usual – it’s not that straightforward.
For tryptophan in the diet to be used to produce serotonin in the brain, it relies on particular transporter molecules. These transporter molecules are able to carry several different types of amino acids.
This creates a high level of competition between the various types of amino acids for the transporter molecules, and the other types of amino acids are generally found in more abundant amounts than tryptophan.
As a result, far less tryptophan gets transported to the brain compared to other amino acids. Essentially, this means we need to consume foods that are higher in tryptophan than in other types amino acids in order to promote increases of serotonin levels in the brain. (source)
A treatment which has become a popular and recognized way to manage conditions such as seasonal affective disorder (SAD), a disorder which results in low mood and depressive symptoms during the winter months in people who do not experience any symptoms throughout the rest of the year.
Light therapy involves being exposed to an artificial light source that mimics natural light. It is thought to work by affecting various chemicals in the brain linked to mood.
Research indicates that it is a useful treatment for depression, with some studies suggesting that light therapy can be as effective as antidepressant medication. (source)
Research has long shown that exercise has a significant positive effect on mood, and some studies indicate it can increase the levels of serotonin in the brain.
Clinical research has proven that exercise results in a significant increase in the release and synthesis of serotonin, and levels of tryptophan in the brain following exercise. (source)
The best effect of exercise in improving mood has been observed in research when people take part in regular, consistent exercise with which they are familiar.
It appears that trying new forms of exercise which require skills that are new to the participant and need to be developed with regular practice can create higher levels of anxiety which can be unhelpful in using exercise to improve depressive symptoms. (source)
Serotonin is a chemical produced by the body from a specific amino acid, tryptophan. While serotonin has traditionally been associated with brain function, and does play a key role in regulating mood and depression, it is found primarily in the digestive system, blood platelets and in bone.
Serotonin has a significant role in the function of all muscle tissue, and has been identified as important in regulating bowel function and protecting the digestive system from toxins and irritants, causing vomiting and diarrhea to expel irritants if necessary.
Serotonin is also thought to help control sleep, and has been shown to be able to initiate, maintain and inhibit sleep, depending on how it is used by the body.
Serotonin has been shown to have a significant effect on increasing pain threshold and mediating the body’s response to pain.
While serotonin has been shown to decrease one density in some studies, other research indicates high levels of serotonin are linked with increased bone density. Too much serotonin active in the body can result in serotonin syndrome, which can require immediate medical attention.
Research studies generally agree that serotonin plays an important role in blood clotting and wound healing, and that it inhibits sexual function.
It is possible to increase low levels of serotonin in several ways, including the use of medication, light therapy, exercise and diet, although diet is not the method of choice when the goal is to increase the levels of serotonin in the brain.
While not all research is in agreement about serotonin, what is abundantly clear is that serotonin plays an extremely important role in a wide variety of human body functions, and has a significant effect on many different systems within the body.
As brain function and the impact of hormones and other chemicals is yet to be fully understood, it is unsurprising that in many cases, the role of serotonin is also not fully understood.