– Serotonin Deficiency
One of the theories about depression causes is the imbalance of neurotransmitters in the brain.
In a previous article, we discussed about how neurotransmitter imbalances can bring about depression, as well as the evidences supporting and against this hypothesis.
In this article, we discuss in greater details the neurotransmitter most often associated with depression – serotonin.
Serotonin and depression
Considered to be the brain’s “own mood-elevating and tranquilizing drug”, serotonin helps to calm the brain’s depressive stress response as well as boosts feelings of well-being. 
Deficiency of this monoamine neurotransmitter in the brain is often seen in depressed patients  and considered to be one of the primary depression causes .
Deficiency in serotonin can bring about symptoms like depression, anxiety, irritability, impatient, impulsivity, abusive-ness, short attention span, reactiveness, craving for sweets and high-carbohydrate foods, insomnia and poor dream recall. 
In fact, the lower the serotonin levels, the more severe the consequences – depressed patients have lower levels of serotonin than normal people, and those with lowest levels often have committed or attempted suicide. 
Studies found significantly low levels of serotonin in suicidal patients and autopsies showed that very low brain levels of serotonin is a biological marker of suicide . In fact, the link between low serotonin metabolism and impulsive suicide acts had been documented in the Journal of Affective Disorders in 1985. 
Causes of serotonin deficiency
Because serotonin is converted from the amino acid tryptophan, low levels of serotonin could result from stress-related hormonal changes (since stress hormones like cortisol reduces levels of tryptophan) [1, 5].
Deficiency in serotonin could also arise when there is difficulty in transporting tryptophan from the blood stream into the brain due to an overabundance of other amino acids which compete with tryptophan to cross the blood brain barrier. 
Lower levels of serotonin could also be brought about by inflammation. Chronic inflammation also increases the brain’s stress response (since the stress hormones cortisol has powerful anti-inflammatory properties) , which in turn results in the diversion of tryptophan from serotonin production and the reduced sensitivity of serotonin receptors to whatever serotonin is available .
Several lifestyle factors, like cigarette smoking, alcohol abuse, high sugar intake, use of steroid medication or oral contraceptive, and various nutritional deficiencies, can also lead to reduced serotonin levels in the brain [3, 4]. They are believed to lower serotonin levels by impairing the conversion of tryptophan to serotonin. 
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