Serotonin is a neurotransmitter that is most commonly known for its role in the brain, where it is believed to be one of the main factors influencing mood. However, it also has many other functions throughout the body, such as regulating digestion, sleep, and cardiovascular function! Read on to learn more about this fascinating neurotransmitter compound, the multiple roles in plays throughout the body, and some of the lifestyle, dietary, and other factors that can influence it!
What is Serotonin?
Serotonin – also known as 5-hydroxytryptamine, or “5-HT” for short – belongs to the family of neurotransmitters called catecholamines, and acts as a very important biochemical messenger throughout the brain and the digestive system.
Serotonin is synthesized from the amino acid tryptophan by the enzyme tryptophan hydroxylase.
In humans, it is found primarily in the central nervous system (CNS), gastrointestinal tract, and blood platelets.
It is responsible for regulating many important physiological functions throughout the body and brain, including neurotransmission, gastrointestinal motility, hemostasis, and cardiovascular integrity .
Psychological and Physiological Roles of Serotonin
1) Serotonin and Mood
Serotonin is best-known for its role in the brain, where it acts as one of the major neurotransmitters that allows neurons to communicate with each other.
While serotonin’s actions in the brain are diverse and highly complex, it is most well-known for its purported involvement in mood and emotion.
For example, serotonin is famous for its involvement in depression, and the majority of the most common and widely-used antidepressants primarily target the brain’s serotonin system. Nonetheless, it also plays a role in other mood-related processes, such as aggression, impulsivity, and social dominance .
Serotonin’s role in psychological processes is highly studied by scientists, and many studies suggest that the levels and overall activity of this key neurotransmitter can have significant effects on mood and emotion.
Researchers have a variety of ways of influencing serotonin activity. For example, acute tryptophan depletion (ATD) is one experimental method for artificially reducing serotonin levels throughout the body (because tryptophan is a necessary “ingredient”, or “metabolic precursor” for making serotonin – and the body and brain can’t produce any without it). Depleting serotonin levels via ATD has been associated with “low” moods, increased irritability, and increased aggression .
2) Heart & Cardiovascular Function
Serotonin is considered a significant circulating hormonal factor that has been implicated in normal cardiovascular function, either by acting directly on heart cells, or by stimulating chemosensitive nerves from the heart .
Patients with carcinoid tumors (a rare type of slow-growing cancer) have elevated levels of serotonin activity in the cardiovascular system, which has been associated with certain symptoms such as arrhythmia, which can contribute to blood flow blockage and valvular fibroplasia .
Additionally, growing mouse embryos that are exposed to dramatically elevated levels of serotonin (either due to high concentrations of 5-HT itself, or treated with high levels of selective serotonin reuptake inhibitors (SSRIs)) have been reported to show abnormal growth in many different parts of their developing cardiovascular system .
3) Intestinal Function
In the gastrointestinal tract, serotonin initiates responses like nausea, intestinal secretion, and peristalsis, and has also been implicated in gastroenteric diseases like irritable bowel syndrome .
The secretory effects of serotonin are believed to be mediated through a variety of different specialized 5-HT receptors, such as 5-HT2A .
4) Body Temperature and Breathing
Serotonin-producing cells in the mouse brain have been reported to play an essential role in maintaining proper body temperature, and regulating breathing .
One animal study (in TPH2-knockout rats) suggests that 5-HTP plays a crucial role in balancing the control systems for breathing and temperature, especially during early development .
Neurons that primarily use serotonin have been found in high concentrations across many different brain regions that are believed to be involved in respiratory control .
5) Muscular Control
Serotonin is believed to be responsible for controlling the contraction of vascular smooth muscle cells throughout the body – these muscle cells are particularly prominent throughout the circulatory system, where they are responsible for controlling blood flow by constricting or dilating blood vessels. It is believed that the “S2” type of serotonergic receptor, in particular, plays a key role in this function .
In fact, some of the earliest scientific investigations of serotonin (5-HT) was due to its vasoconstrictive effects – it generally wasn’t until later that its other physiological and psychological roles became apparent.
For example, animal studies have reported that intravenous injections of serotonin can induce a wide variety of responses in the cardiovascular system, leading to increases or decreases in blood pressure, depending on where and when it is active in the body [12, 13].
Serotonin can also control blood pressure using other, “indirect” mechanisms, such as by amplifying the response of other vasoactive substances such as NET, angiotensin II, and histamine .
6) Bone Growth, Development, and Maintenance
Serotonin receptors have been identified in all the major types of bone cell (osteoblasts, osteocytes, and osteoclasts) .
Recent data suggests that gut-derived serotonin may mediate the skeletal effects of LDL receptor-related protein 5.
Additionally, some evidence suggests that use of SSRIs (selective serotonin reuptake inhibitors) – which increase serotonin levels throughout the body – may be associated with reduced bone mass, increased bone loss, and increased risk of fractures .
Finally, some studies have even suggested a direct stimulatory effect of serotonin on bone formation pathways . However, other studies have reported inhibitory effects, indicating a potentially complex role of serotonin in bone formation .
7) Organ Growth & Development
According to some research, serotonin may play a critical role in the development and growth of several different major bodily organs, such as the heart and the lungs [18, 19, 20].
In both mice and people, high levels of 5-HT2B in the lungs have been associated with the development of pulmonary hypertension, further suggesting a direct influence of serotonin on lung growth and function [21, 20].
8) Involvement in Eating Disorders
5-HTP is believed to be involved in many of the psychological and behavioral symptoms involved in major eating disorders, such as anorexia. For these reasons, some researchers have suggested that tryptophan supplementation may have potential as a treatment for anorexia .
Tryptophan is a precursor of serotonin and an essential amino acid only available in the diet, it is, therefore, likely that excessive diet restriction may lead to decreased brain 5-HT stores.
9) Attention-Deficit Hyperactivity Disorder
According to one controlled trial, families were studied to determine the link between the serotonergic system and attention-deficit hyperactivity disorder (ADHD). Preliminary data suggested an important role of the 5-HT system in the development of ADHD .
There is clear evidence that dopamine and 5-hydroxytryptamine neuronal systems interact in ADHD, although the exact nature of these interactions remains unclear, and is still being actively researched .
10) Appetite, Food Cravings, and Obesity
Some researchers have noted that the amount of serotonin that certain populations of neurons release into the brain appear to be significantly determined by food intake. This has led to the hypothesis that the serotonin system may play a crucial role in the regulation of appetite and eating behavior – although the precise nature of this potential function is not yet fully known.
Some preliminary evidence suggests that the intake of carbohydrates, in particular, may affect serotonin – possibly by acting through mechanisms related to insulin secretion. Other macronutrients, such as protein, do not appear to have this effect, suggesting that this effect may be specific to certain types of nutrients or foods .
Factors That May Increase Serotonin
Several psychiatric disorders – such as depression – have been consistently associated with abnormal serotonin activity in the brain. Because of these links, researchers have paid considerable attention towards studying different ways to target the serotonin system (usually with the goal of increasing the levels or overall activity of serotonin).
For example, one of the most well-known and widely-used treatments for depression, selective serotonin reuptake inhibitors (SSRIs), significantly increase serotonin levels throughout the brain (specifically, by preventing serotonin from being “cleaned out” of neuronal synapses, which allows it to have longer and stronger effects on overall brain activity).
Medications such as SSRIs are fairly effective for treating depression, and are generally safer than many other forms of treatment .
However, SSRIs still have several considerable drawbacks. For one, many depression patients don’t respond successfully to them. Secondly, even in patients who do respond to treatment with these medications, SSRIs can frequently cause a number of significant negative side-effects, including nausea, skin reactions, weight gain, sleep disturbances, and sexual impairments .
Additionally, some preliminary evidence from animal studies (in rats) suggests that long-term use of SSRIs may worsen underlying serotonin deficiencies .
For these reasons, some people are reluctant to rely on pharmaceutical treatments, and prefer to try non-chemical treatments instead .
In the sections below, we’ll discuss some of the various lifestyle, dietary, and supplement-based approaches that have been proposed to potentially increase serotonin levels or activity. However, keep in mind that the science behind these is still mostly preliminary, and none of these lifestyle strategies or supplements have been officially approved for treating depression or any other psychiatric or medical conditions.
In other words, there is still “insufficient evidence” for the efficacy of these supplements. Therefore, these “complementary strategies” should not be used to replace conventional treatment – especially for people who have been officially diagnosed with a specific psychiatric condition, such as depression.
These are complementary approaches only, meaning that while they may help support and enhance the effectiveness of conventional treatments, they are probably not sufficient just on their own.
As always, be sure to discuss any significant lifestyle, dietary, or other changes with your doctor first before embarking on any of the “complementary” strategies discussed throughout this post! In other words, you can consider trying the strategies listed below if you and your doctor determine that they could be appropriate for you.
Lifestyle and Behavioral Factors
Several behavioral methods and other lifestyle factors have been identified which may potentially affect the serotonin system in the brain.
Exercise has been linked to a wide variety of significant benefits to physiological and psychological health – including increased serotonin levels and positive effects on overall mood.
For example, several human studies have reported that engaging in aerobic exercise leads to increased brain levels of serotonin, and that these increases can be long-lasting if good exercise habits are maintained [29, 30].
Similar findings have been reported by animal studies. For example, even short bouts of exercise has been reported to increase serotonin activity in the brains of rats .
If possible, exercising outdoors may be even more beneficial. For example, sunlight exposure has been linked to increased serotonin levels, which may in turn translate into improved overall mood .
2) Positive Thinking
Just as low serotonin levels may negatively affect our mood, negative moods also cause serotonin levels to drop. However, some evidence suggests that using cognitive and behavioral strategies to actively induce positive moods may be helpful to maintaining healthy serotonin levels .
This “two-way influence” between serotonin levels and mood may potentially affect a range of physical and mental health issues.
For example, some researchers believe that habitual negative thoughts may play a causal role in the development or maintenance of certain mood disorders, such as depression and anxiety. Conversely, developing a habit of positive thinking may have a protective effect against these mood disorders, such as by boosting self-esteem and overall mood .
Along these lines, “cognitive” forms of therapy, such as cognitive-behavioral therapy (CBT), are believed to be safe and relatively effective treatments for depression. CBT involves identifying thought patterns that trigger negative moods, and then developing techniques to “distance” oneself from these thought patterns. In this way, the “strength” of these habitual thought patterns can be reduced over time .
Some scientific evidence suggests that implementing more positive-thinking habits in one’s day-to-day life may be a natural way to keep depression at bay, and potentially raise serotonin levels .
3) Positive Mood Induction
Somewhat similar to the potential effects of “positive thinking”, some evidence suggests that something as simple as listening to positive music, or engaging in other pleasant experiences such as walking in nature, may have an effect on the brain’s serotonin levels.
For example, some studies in humans have reported that listening to pleasant music can improve mood in the short-term. Similar effects have also been reported for other pleasant experiences, such as watching pleasant films, or listening to positive stories. Behavioral strategies such as these are sometimes referred to by researchers as “mood-induction” techniques .
Similarly, another study in 20 healthy human subjects reported that listening to pleasant music brought about a short-term boost in their serotonin levels, and improved their mood .
Additionally, partaking in fun activities such as nature walks, meditation, and massages all each been linked to potentially-beneficial effects on serotonin levels and overall mood [37, 38, 39].
However, it’s important to note that these “mood-induction” effects can go the other way as well: for example, listening to “negative” or “unpleasant” music and other stimuli have been reported to lower a person’s mood [35, 36].
4) Social Interaction
Some evidence also suggests that positive social interactions may also have a potential beneficial effect on serotonin levels and mood.
For example, the amount of positive face-to-face social interactions a person experiences has been reported to have a strong correlation with serotonin levels .
However, this relationship also goes both ways: for example, interactions with individuals exhibiting negative traits or emotional states – such as anxious or aggressive behavior – has been reported to cause people to “mirror” these emotional states, and therefore reduce their mood .
5) Healthy Sleeping Habits
Unsurprisingly, sleep is hugely important for the brain and overall psychological well-being.
Having a healthy and well-balanced sleep schedule has been reported to be important for maintaining serotonin balance throughout the brain. It is believed that at least part of the reason for this is because the brain produces and releases significant amounts of serotonin during sleep .
Conversely, sleep deprivation can “de-sensitize” serotonin receptors. This, in turn, may disrupt the activity of the brain’s serotonin system in general – and sleep deprivation has even been associated with an increased likelihood of depression .
Dietary Factors and Supplements
In addition to some of the general lifestyle-related and other “behavior-based” techniques described above, researchers have also identified a number of dietary factors and supplement-based approaches that may also have an influence on a person’s overall levels of serotonin.
Once again, however, it is important to note that there is still “insufficient evidence” for the efficacy of these supplements, and much more follow-up research in healthy human populations will be needed to confirm their effects. As always, be sure to discuss any dietary changes or new supplements with your doctor first, as this is the best way to avoid any potential negative interactions with other lifestyle- or health-related factors.
1) Omega-3 Fatty Acids
According to some preliminary research, certain omega-3 polyunsaturated fatty acids (PUFAs), such as EPA and DHA, may have noteworthy effects on the brain’s levels of serotonin .
Although the mechanisms involved are still being studied, some researchers have proposed that these compounds may act by increasing serotonin production, or stimulating serotonin receptors .
EPA and DHA are commonly taken in the form of dietary supplements, such as fish oil. However, there are also many foods that are rich in these compounds, including :
- Fatty fish (such as salmon and sardines)
2) Vitamin D
Vitamin D is one of the important “ingredients” (metabolic precursors) that the brain needs in order to produce serotonin.
Specifically, vitamin D is required for the conversion of tryptophan into active serotonin (5-HT). In fact, nutritional deficiencies of vitamin D have been associated with significantly reduced brain levels of serotonin, and has been hypothesized to be one potential contributing factor to a variety of psychiatric conditions (such as depression and ADHD) .
Vitamin D can be consumed as a simple dietary supplement. Of course, it can also be obtained directly from the diet by eating enough foods that are rich in this important vitamin. Some of the dietary sources that are particularly rich in vitamin D include :
- Fish and shellfish
Milk also contains a protein called alpha-lactalbumin. Some early research has linked increased intake of this compound with increased levels of tryptophan, which could theoretically further contribute to increased serotonin levels .
According to some early studies, the amount of serotonin that the brain’s neurons release may be directly affected by specific types of foods or macronutrients. In fact, some researchers believe that this diet-serotonin interaction may be part of the reason that depression patients often report experiencing increased cravings for sugars and other carbohydrates .
Specifically, carbohydrates and sweet foods can trigger the increased secretion of insulin, which in turn stimulates serotonin release .
Additionally, some researchers have suggested that carbohydrates may contribute to increased serotonin levels by increasing the ability of certain other dietary compounds, such as tryptophan, to cross the blood-brain barrier (BBB). Theoretically, this could make more tryptophan available to the brain, which, in turn, could convert into more active serotonin .
However, much more research will still be needed to confirm these dietary interactions, and to figure out exactly how they potentially affect serotonin levels.
It’s also important to note that it probably wouldn’t be a good idea to rely on sugars and other carbohydrates to try to affect serotonin levels, since eating too much of these foods could also contribute to weight gain and obesity!
Finally, some research also suggests that certain herbs and other natural plant-based compounds may have the potential to affect a person’s serotonin levels:
- St. John’s Wort: One animal study has reported that St. John’s Wort stimulated the production of serotonin in the brains of rats (specifically, by inhibiting the TDO (Trp 2,3-dioxygenase) enzyme) .
- Curcumin: According to a few early animal studies in rats, curcumin – also known as the common spice turmeric – has been reported to help stimulate the release of serotonin and dopamine in the brains of rats, and may even have a subtle “anti-depressive” effect [51, 52].
However, these findings are still highly preliminary, and much more research will be needed to confirm the potential effects of these herbs when used as supplements in healthy human users.
It is also important to note that even “natural,” plant-based compounds can be “over-dosed,” which can cause a variety of negative side-effects. This is a big part of the reason why it’s always important to discuss any new supplements or dietary changes with your doctor first!