Serotonin is one of the brain’s neurotransmitters and affects or regulates several bodily functions including appetite, memory, mood, body temperature, moral judgment and behavior, libido, learning, pain perception, and sleep. Moreover, most of the body’s serotonin is found outside the nervous system, including in the intestinal wall and blood platelets.
Several diseases are associated with altered function or levels of serotonin in the brain, including fibromyalgia¹, certain types of depression², anxiety, obsessive-compulsive disorder, and migraine. Often, the cause of reduced serotonin activity is uncertain, but both genetic and environmental factors play a role.
Although the previously dominant “serotonin hypothesis” for depression has been challenged in recent times, serotonin still plays an important role in our understanding of mental health. Recent research suggests that the relationship between serotonin and mental disorders is more complex than previously thought. While moving away from the idea that depression is primarily caused by serotonin deficiency, researchers now recognize that serotonin is part of an intricate interplay with other neurotransmitters, hormones, and environmental factors.
Serotonin continues to influence mood, sleep, and appetite, and can contribute to the symptom picture in various mental disorders. Modern treatment methods therefore consider serotonin’s role as part of a larger neurobiological system, rather than focusing solely on increasing serotonin levels.
Serotonin Affects More Than Just Mood
The interaction between different parts of the brain, the various neurotransmitters, and how this affects us mentally, psychologically, and physically is very complex, and knowledge in this area is still relatively limited. However, medications that affect serotonin function in the brain are among the most sold pharmaceuticals and have positive effects on many of the diseases mentioned above.
What happens in the brain when we lack serotonin? To find out, researchers have used a method where tryptophan, the amino acid from which serotonin is converted, is excluded from the diet. Acute tryptophan deficiency increases impulsivity, irritability, aggression, and also reduces mood. One of the reasons for increased impulsivity and aggression during serotonin deficiency is due to reduced activity in the serotonin neurons that run between the frontal lobe and amygdala. The amygdala is an important center for emotion generation, and through serotonin neurons, the frontal lobe promotes impulse control and down-regulation of emotional reactions. With reduced serotonin activity, the frontal lobe’s ability to control emotions and impulses will therefore be weakened³.
How does the brain react to increased serotonin? Tryptophan supplementation in people who easily become irritated reduces the tendency to argue while improving mood and making behavior friendlier⁴,⁵. This in turn affects our physical health. Hostility, the opposite of friendliness, is a known risk factor for coronary heart disease and also for increased general mortality⁶.
Serotonin and Morality
The serotonin system is widespread in areas of the brain that regulate moral judgment and moral behavior. In 2010, an interesting study was conducted to investigate whether increased serotonin affects moral judgment and behavior in healthy individuals. The test group received a 30mg Citalopram tablet, a medicine that increases serotonin activity, while the control group received placebo medicine. 1.5 hours later, participants were tested on various moral dilemmas. The researchers summarized the results this way: In participants with increased serotonin activity, they were “more likely to ‘turn the other cheek’ and forgive unfair behavior” compared to the control group. Furthermore, the test group showed increased resistance to harming others⁷.
Is it only medications that can affect the brain’s serotonin? Interestingly, several lifestyle factors influence serotonin activity in the brain, and we shall look at some of them.
Sunlight
Daylight stimulates the formation of serotonin in the brain. Even on a cloudy day, it is often brighter outdoors than in a normally lit room. However, large portions of the population spend most of the day indoors throughout most of the year, which seems to have increased over the past generations. Previously, it was quite common to spend more time outdoors, often in connection with work. In our time, most people work indoors, where we also spend much of our leisure time. Figures suggest that we are indoors approximately 90% (21.5 hours) of the day⁸.
One of the causes of winter depression is thought to be reduced exposure to daylight, which becomes increasingly scarce the further north in the country we are. Light therapy, for example 30-60 min/day, has positive effects on both winter depression and other types of depression⁹ and increases serotonin levels within minutes/hours¹⁰. The effect of light treatment for winter depression can be experienced after just a few days. Another advantage of being out in sunshine is that the body produces vitamin D, which is important for tryptophan to be converted to serotonin.
It is particularly beneficial to expose oneself to sunlight in the morning. The secretion of the sleep hormone melatonin ceases when we are exposed to daylight. At the same time, light leads to increased levels of serotonin, which is one of several neurotransmitters that contribute to wakefulness.
Physical Activity
Studies show that physical activity increases the level of tryptophan in the brain, the precursor to serotonin⁶. Furthermore, animal experiments and human studies seem to show that physical activity directly stimulates increased serotonin in parts of the brain. One also sees that physical activity has positive effects on several conditions associated with reduced serotonin activity, such as depression and fibromyalgia. Physical activity also increases other important neurotransmitters in the brain such as noradrenaline and dopamine.
Today, most of us have the opportunity to live a life without regular physical activity, and many take advantage of this. In previous generations, physical activity was often a necessary part of everyday life. Researchers suggest that the reduction in physical activity in the population is a contributing factor to the increase in depression we see in our part of the world¹¹.
Researchers believe that the first six years of life are especially important for the development of the brain’s serotonin system. Sunlight and activity are two important physical factors that activate and stimulate this part of the nervous system. A lifestyle where children spend much time indoors with sedentary activities such as gaming in dimly lit rooms is thought to have a negative effect on the development of this part of the nervous system¹².
Nutrition
Serotonin is formed from the amino acid tryptophan, which is one of the amino acids found least in proteins. One might therefore think that the higher the intake of proteins, the more serotonin the body could produce. The reason why increased protein intake doesn’t necessarily lead to more serotonin in the brain is partly because tryptophan competes with several other amino acids for transport proteins that carry them across the blood-brain barrier. A diet focused on increasing the ratio between tryptophan and competing amino acids can therefore be expected to have a beneficial effect on serotonin production. Examples of protein-rich vegetable foods that have a relatively high ratio between tryptophan and competing amino acids are chia seeds (24%), tofu (9%), flaxseed (9%), and quinoa (9%). For animal sources such as chicken and beef, the ratio is 7% and 4% respectively.
Additionally, a diet with a high content of carbohydrates will increase insulin secretion and thus lead competing amino acids (but to a small degree tryptophan) into muscle tissue. This increases the ratio between tryptophan and other amino acids in the blood and thus also in the brain¹³. Experiments have shown that after a low-carb meal with turkey (the food many have thought is one of the best sources to increase serotonin), the ratio between tryptophan and other amino acids in the blood is reduced by approximately 50% compared to a meal rich in carbohydrates¹⁴. When the brain in this way has less access to tryptophan, it leads to reduced production of serotonin.
Furthermore, there are several nutrients that are important for the conversion of tryptophan to serotonin including omega-3, iron, folic acid, vitamin B12, B6, vitamin C, and as mentioned earlier, vitamin D. Deficiency of one or more of these nutrients can prevent normal serotonin production. For example, folic acid deficiency in depressed individuals can lead to reduced effect of medications meant to increase serotonin activity (“happy pills”)¹⁵.
Thoughts and Impressions
In a study, participants were asked to recall positive, negative, or neutral memories. Upon subsequent examination of the brain, researchers found increased production of serotonin in the group that recalled positive memories and reduced production in those who recalled negative memories¹⁶. Researchers assume there is a two-way communication between serotonin and mood, where serotonin can affect mood but where mood and thoughts can also affect serotonin activity.
Music therapy has also shown to increase serotonin activity within a few minutes, notably if one listens to music perceived as pleasant¹⁷.
Other Factors
Studies have shown that stress reduces serotonin activity¹⁸. Rotating shift work is a form of stress that leads to disturbances in circadian rhythm and has shown to reduce serotonin levels in the brain¹⁹.
Massage of both babies and adults has shown to increase serotonin levels temporarily by over 30%, and there is some documentation that this can reduce the frequency of migraine attacks²⁰,²¹ in people with migraine.
In women, estrogen stimulates the secretion of several neurotransmitters, including serotonin. When estrogen levels fall during parts of the menstrual cycle and also during menopause, this can lead to changes in the level of serotonin and other neurotransmitters. This is thought to be a contributing cause to some of the symptoms that women may experience during these periods. However, one study has shown that increased intake of carbohydrates in the last part of the menstrual cycle (days 21-28) can improve psychological symptoms²². Increased intake of carbohydrates increases, as described above, the level of tryptophan, the precursor to serotonin.
Intake of caffeine and nicotine initially increases serotonin in the brain. However, the brain seems to adapt to this and reduces the conversion of tryptophan to serotonin²³. When people who are dependent on caffeine or nicotine no longer consume it regularly or at all, they develop some withdrawal symptoms and symptoms of reduced serotonin levels, which usually last a few days. Typical symptoms related to serotonin deficiency are increased irritability and reduced mood.
Regarding alcohol, a Swedish study has shown that sustained high alcohol intake over time reduces serotonin function in the brain by up to 50%. In women, the change took only four years, while men reached 50% reduction after approximately 12 years²⁴.
Conclusion
As we have seen, serotonin affects many different aspects of human beings, such as physical, psychological, and mental health in addition to morality. At the same time, serotonin can be influenced through our own lifestyle. Many associate the positive effect of healthy living habits primarily with healthy physical health. However, WHO defines good health as not only concerning the absence of disease but the presence of physical, mental, and social well-being²⁵. A healthy and balanced lifestyle is thus not only helpful in preventing disease. It can also help promote well-being.
Do you want to learn more?
- Read the article “11 Tips to Calm Amygdala” that explores natural ways to calm amygdala reactivity
- Learn how to forgive other people and improve wellbeing and mental health in the article “Healing of the Mind: Learn How to Forgive”
