Serotonin or 5-hydroxytryptamine (5-HT)
is a monoamine neurotransmitter derived from tryptophan, primarily
found in the gastrointestinal (GI) tract, platelets, and in the central
nervous system (CNS). In Gut, serotonin regulates intestinal movements,
in CNS, it regulates mood, appetite, sleep, memory and learning, etc.
1. Brain serotonin, carbohydrate-craving, obesity and depression
Serotonin-releasing brain neurons are
unique in that the amount of neurotransmitter they release is normally
controlled by food intake, according to the study by Massachusetts
Institute of Technology, Cambridge serotonin
release is also involved in such functions as sleep onset, pain
sensitivity, blood pressure regulation, and control of the mood. Hence
many patients learn to overeat carbohydrates (particularly snack foods,
like potato chips or pastries, which are rich in carbohydrates and fats)
to make themselves feel better. This tendency to use certain foods as
though they were drugs is a frequent cause of weight gain, and can also
be seen in patients who become fat when exposed to stress, or in women
with premenstrual syndrome, or in patients with "winter depression," or
in people who are attempting to give up smoking. (Nicotine, like dietary
carbohydrates, increases brain serotonin
secretion; nicotine withdrawal has the opposite effect.) It also occurs
in patients with normal-weight bulimia. Dexfenfluramine constitutes a
highly effective treatment for such patients. In addition to producing
its general satiety-promoting effect, it specifically reduces their
overconsumption of carbohydrate-rich (or carbohydrate-and fat-rich)
foods(1).
2. Brain serotonin in excessive carbohydrate snacking craving
Brain neurotransmitter serotonin plays an
essential role in a specific hunger for carbohydrate-rich foods in
animals and human beings, researchers found that consumption of a
carbohydrate-rich meal increases the synthesis and release of brain serotonin (by enhancing the brain uptake of its precursor, tryptophan). As a consequence of this increased release of serotonin, carbohydrate intake is decreased at the next meal. Consumption of protein does not increase brain serotonin
levels or decrease carbohydrate intake. A subgroup of obese individuals
who consume carbohydrate-rich snack foods at specific times of day or
evening has been identified. Such individuals do not routinely snack on
protein-rich foods, and their consumption of calories and nutrients at
meals is not excessive. Evidence is presented that carbohydrate snacking
seems to be related to a "need" to increase the level of brain serotonin;
treatment with a drug, d-1 fenfluramine, that increases serotoninergic
neurotransmission significantly decreases carbohydrate snack
consumption. Weight loss among the population of carbohydrate cravers
might be most successful if treatment includes either a diet or drugs
that increase brain serotonin activity when the need to snack on carbohydrate is most likely to occur(2).
3. Changes in mood after carbohydrate consumption
In the study to identify two groups of obese individuals who consume
excessive calories primarily as snack foods, found that using
standardized self-report questionnaires, we measured mood before
and 2 h after consumption of a high-carbohydrate lunch (104 g CHO).
Responses to the meal differed significantly: noncarbohydrate cravers
reported feeling considerably less alert, more fatigued and sleepy,
while carbohydrate cravers described little or no change in these
aspects of mood.(3)
4. Mood and carbohydrate cravings
In the study to investigate a sample of 113 males and 138 female college
students of the relationship between mood and carbohydrate cravings,
and the possible role of gender in these associations, found that
individuals classifying themselves as "carbohydrate cravers" reported
foods rich in carbohydrates, and "protein cravers" reported protein-rich
foods as being the ones they most strongly craved. Carbohydrate cravers
reported feeling distressed prior to their cravings and satisfied,
happy/good and relaxed following carbohydrate consumption. Protein
cravers reported feeling anxious or hungry prior to their cravings and
happy, normal, bored, and energetic following protein-rich food
consumption(4).
5. Effects of protein and carbohydrate meals on mood and performance: interactions with sex and age
In the study to investigate the normal adult subjects (n = 184) consumed
a high-protein or
high-carbohydrate meal and were
tested two hours later their mood and performance, found that females,
but not males, reported greater sleepiness after a carbohydrate
as opposed to a protein meal. Male subjects, but not females, reported
greater calmness after a carbohydrate as opposed to a protein meal. When
meals were eaten for breakfast (but not for lunch) individuals 40
yr of age or older felt more tense and less calm after a protein than
after a carbohydrate meal. Although older subjects reported subjective
discomfort after a morning protein meal, they displayed objective
performance impairments after a carbohydrate lunch. Subjects 40 yr of
age or older were impaired on a test of sustained selective attention
(dichotic shadowing) after consuming a high-carbohydrate lunch(5).
6. Mood, performance, and pain sensitivity: changes induced by food constituents
In the study to examine the behavioral effects of the dietary constituents tryptophan
and tyrosine on human mood, sensorimotor performance and pain
sensitivity, found that tryptophan and tyrosine are neurotransmitter precursors present in
varying amount in protein-containing foods. Tryptophan (50 mg/kg)
increased subjective drowsiness and fatigue but unlike many hypnotics
did not impair sensorimotor performance. Tryptophan also decreased human
pain sensitivity in a manner that was more specific than certain
analgesic drugs(6).
7. Effects of dietary neurotransmitter precursors on human behavior
In a double-blind, crossover study the possible effects of tryptophan and tyrosine on human behavior, single
oral doses of these substances and matched placebos were administered to
20 men, found that
tryptophan increased subjective fatigue and decreased self-ratings of
vigor and alertness, but did not impair performance on any of the tests.
Tyrosine produced no effects in our young population compared with
placebo, but did decrease reaction time relative to tryptophan. It may
be concluded that tryptophan has significant sedative-like properties,
but unlike other sedatives may not impair performance(7).
8. Serotonin: influences on male sexual behavior
Serotonin
(5-HT) is primarily inhibitory, although stimulation of 5-HT(2C)
receptors increases erections and inhibits ejaculation, whereas
stimulation of 5-HT(1A) receptors has the opposite effects: facilitation
of ejaculation and, in some circumstances, inhibition of erection. 5-HT
is released in the anterior lateral hypothalamus at the time of
ejaculation. Microinjections of selective serotonin
reuptake inhibitors there delay the onset of copulation and delay
ejaculation after copulation begins, according to the study by State
University of New York(8).
9. Neuroactive steroid effects on cognitive functions with a focus on the serotonin and GABA systems
In the study to review the neuroactive steroid effects on serotonin and GABA systems, along with the subsequent effects on cognitive functions, showed that Estrogen alone, or in combination with progesterone, affects the brain serotonin
system differently in different parts of the brain, which can at least
partly explain the opposite effects on mood of those hormones. Many of
the progesterone effects in the brain are mediated by its metabolite
allopregnanolone. Allopregnanolone, by changing GABA(A) receptor
expression or sensitivity, is involved in premenstrual mood changes; and
it also induces cognitive deficits, such as spatial-learning impairment(9).
10. Serotonin, aging and cognitive functions
Serotonin and acetylcholine interact to allow normal cognitive functions in the brain. Thus, a partial reduction in both cholinergic and serotonergic functions will cause severe memory impairment in young as well as in aged rats, according to the dtudy by Department of Neurobiology, Weizmann Institute(10).
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Sources
(1) http://www.ncbi.nlm.nih.gov/pubmed/8697046
(2) http://www.ncbi.nlm.nih.gov/pubmed/6381575
(3) http://www.ncbi.nlm.nih.gov/pubmed/3788829
(4) http://www.ncbi.nlm.nih.gov/pubmed/11237349
(5) http://www.ncbi.nlm.nih.gov/pubmed/6764932
(6) http://www.ncbi.nlm.nih.gov/pubmed/6764930
(7) http://www.ncbi.nlm.nih.gov/pubmed/4025206
(8) http://www.ncbi.nlm.nih.gov/pubmed/15488546
(9) http://www.ncbi.nlm.nih.gov/pubmed/16368148
(10) http://www.ncbi.nlm.nih.gov/pubmed/8239292
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