Serotonin is known to play a part in regulating sleep, body temperature, memory, learning, mood and appetite. In the brain, serotonin works as a neurotransmitter – a brain chemical that helps one nerve cell communicate with another.
But serotonin has other functions in your body – in fact, about 90% of your body's total serotonin is found outside the brain. Most of it – around 80% – is in the gastrointestinal tract regulating intestinal contractions, and 10% is found in the blood platelets, where it has a function in wound healing. Serotonin's role in gastrointestinal contractions helps explain why one of the commoner side-effects of serotonin supplements and medications is an upset stomach.
The chemical name of serotonin is 5-HT, or 5-hydroxytryptamine (not to be confused with 5-HTP, which is a precursor of serotonin).
Serotonin is normally synthesised from L-Tryptophan which you get from food. L-Tryptophan is converted to 5-HTP, which is then converted to serotonin, in the enterochromaffin cells surrounding the intestines, and in the brain.
Serotonin can then be converted to melatonin in the pineal gland, and this seems to be the way that serotonin influences sleep, since we know that melatonin is one of the main regulators of our sleep cycles.
It's useful to have an overview of the way that serotonin works and its many diverse functions in your body, at the very least to understand that you need to be careful before you try to manipulate serotonin levels in your own body.
Like every single other substance in your body, from the simplest ones such as sodium to the most complex DNA, the levels of serotonin need to be regulated quite tightly for optimal functioning. In other words, you don't want your serotonin levels to be too low or too high - both situations can lead to problems.
Too much serotonin can cause problems - in the extreme, it can lead to serotonin syndrome, which can be fatal.
Serotonin has become known as a "feel good" brain chemical, though this is only part of the whole picture. In fact, it's serotonin in insect venoms and poisonous plant spines that causes you pain if you get stung or pricked, so serotonin is not entirely benevolent in its effects! It's also the serotonin in pathogenic amoebae and certain seeds and fruits that causes diarrhoea – your body is trying to get rid of these stealth invaders (it's to their benefit as well to be expelled speedily and passed on).
Nevertheless, low levels of serotonin in the brain do cause problems. It's fairly certain that low serotonin is associated with clinical depression and some behavioural problems as well as with SIDS (Sudden Infant Death Syndrome) in babies. We can't easily measure the level of serotonin in a living brain (though positron emission scanning can provide relevant research data), so most of the research that tells us about serotonin has been done on animals, from roundworms to mice.
Serotonin and aggression
There is some evidence that low serotonin levels can be associated with aggression, though this relationship seems to be mediated by greater impulsiveness together with feelings of unhappiness, rather than by a direct increase in aggressiveness. Here are links to some of the relevant research studies:
- Mice that were genetically engineered to lack brain serotonin were found to show increased aggression and impaired maternal care... see Proc Natl Acad Sci U S A. 2009 June 23; 106(25): 10332–10337)
- Other experiments on animals show a complex relationship between serotonin and aggression, that varies according to the animal's social dominance status.... see Wikipedia for a discussion of this research.
- One fascinating recent study on 19 human volunteers showed that low serotonin levels did seem to increase potential aggression, by reducing the connectivity between the amygdala (which is largely responsible for generating fear in response to a threat) and the prefrontal cortex (which is responsible for our more rational capacity). In other words, serotonin may help us respond more rationally to a threat.... read more in this article in New Scientist.
Serotonin and appetite
Serotonin affects appetite but again it can be complex. In humans serotonin is released while eating, and tends to decrease appetite. This aspect of serotonin release seems to be triggered by the perception of "resource availability". This may be one of the reasons that a restricted diet that makes you feel deprived (regardless of what type of diet, or how much actual food is involved) is likely to leave you feeling unsatisfied.
Another way that serotonin is related to appetite has to do with the carbohydrates that you eat.
Serotonin is synthesised in direct proportion to the availability of its precursor, L-Tryptophan. Although L-Tryptophan is an amino acid and is mostly obtained from protein, eating a high protein meal does not increase the amount of L-Tryptophan available to the brain, because of competition for transport into the brain. For the L-Tryptophan to get into your brain, you need to eat a small amount of carbohydrate, which triggers insulin release, and so lowers the levels of other amino acids in your bloodstream.
(However, most anti-depressant medications, though they increase serotonin in the brain, tend to cause weight gain.... it's not a simple cause and effect relationship between serotonin and appetite.)
Serotonin and depression
Anti-depressants, especially the SSRI anti-depressants such as Prozac and Zoloft, affect serotonin uptake in the brain, and there's a huge amount of other evidence that suggests that clinical depression is associated with low serotonin levels. An association doesn't necessarily imply cause and effect — according to Web MD:
.....researchers don't know whether the dip in serotonin causes the depression, or the depression causes serotonin levels to drop.
We do know that some conditions associated with serotonin dysfunction are due to a lack of serotonin receptors or other problems with the serotonin transport system in the brain, rather than low serotonin levels per se. It still seems, though, that a boost in serotonin can help overcome this problem, and this could be the way that anti-depressants help people with depression when it's genetically based. If this is the case, then a natural serotonin booster could equally be of help.
This is just an overview of serotonin and how it works in your body. It's an ancient regulatory chemical, in evolutionary terms — it's found in all bilateral animals and a number of plants, and it's role has evolved in complexity to its present function in humans.
If you want to delve further into the biochemistry of serotonin and how it works in your body, Wikipedia has lots of information here, including references to many of the significant animal research studies.
For a discussion of current research and thinking on serotonin's effect on mood, this fascinating article by Dr Simon Young in the Journal of Psychiatry & Neuroscience is well worth reading.
There are several other neurotransmitters in your brain, each with its specialty role, and all of them critically important to brain functioning. This includes aspects of mood and appetite regulation – serotonin deficiency is not the only possible cause of dysfunction in these areas, so keep this in mind as you read on.
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