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- Herb Extracts
- Phytochemicals
- Cognition and Mental Function
| Date:
07-29-2011 | HC#
041154-429
|
Re: A Review of Phytochemicals that Enhance Human Brain Function
Kennedy
DO, Wightman EL. Herbal extracts and phytochemicals: plant secondary
metabolites and the enhancement of human brain function. Adv Nutr.
January 2011;2(1):32-50.
There
is a limited number of drugs that can help people slow the cognitive decline
associated with aging, and these drugs have undesirable side effects. Many
people turn to herbal products to help preserve memory and cognitive abilities.
Plants have many chemicals that influence brain function, yet few have been
thoroughly tested in humans. The purpose of this article was to review current
knowledge about plant extracts and phytochemicals which have been tested in
humans and assess their effectiveness in improving brain function.
Many
secondary metabolites affect the central nervous system in insects and animals.
Secondary metabolites that have central nervous system effects and that have been
studied in humans include alkaloids, terpenes, and phenolic compounds.
Alkaloids
Humans
have used alkaloid-containing plants medicinally for thousands of years.
Medicinal and social alkaloids include atropine, ephedrine, cocaine, morphine,
nicotine, and caffeine.
Caffeine
is the most widely consumed psychoactive substance in the world. It is an
antagonist of the inhibitory adenosine A1 and A2 receptors, which is
responsible for the stimulation, and also causes constriction of blood vessels
in the brain. Caffeine also increases activity in dopamine neuronal systems. At
low doses, caffeine increases alertness and improves performance on tests of
attention. At high doses, caffeine can cause anxiety, insomnia, rapid heart
beat, and agitation.
Nicotine
is found in tobacco (Nicotiana tabacum).
Nicotine binds to acetylcholine receptors and increases the release of
acetylcholine, serotonin, and other neurotransmitters. It increases dopamine
activity, which is associated with its addictive effect. Clinical trials in
smokers and non-smokers show that nicotine improves performance on tests of
memory and attention.
Terpenes
Terpenes
are a diverse group of lipid-soluble compounds. Some terpenes are non-toxic and
are common in foods and spices; others are toxic.
Ginkgo
(Ginkgo biloba) leaves contain a
number of terpenes, including bilobalide and ginkgolides A, B, C, and J.
Effects of ginkgo leaf extracts in the central nervous system include
modulation of several neurotransmitter systems, enhanced blood flow in the
brain, and scavenging of free radicals. Ginkgo is one of the most popular
herbal products, and it is used to improve cognitive performance. More than 30
clinical trials have tested the effects of ginkgo on cognitive function in
people with dementia or age-related cognitive impairment. One meta-analysis
concluded that the evidence for cognitive improvement with ginkgo
supplementation is inconsistent, while another meta-analysis concluded that ginkgo
improves attention, executive function, and long-term memory.
Lemon
balm (Melissa officinalis) contains a
variety of monoterpenoids and sesquiterpenes. Effects of lemon balm on the
central nervous system include binding to specific cholinergic receptors,
increased activity of the neurotransmitter gamma-aminobutyric acid (GABA), and
antioxidant activity. Limited clinical trials have shown that lemon balm has
anti-anxiety effects, improves agitation and quality of life in people with
severe dementia, but has inconsistent effects on memory.
Asian
ginseng (Panax ginseng) roots contain
40 or more triterpene saponins known as ginsenosides. Ginseng root extracts have
neuroprotective effects, modulate the neuroendocrine system and the synthesis
of nitric oxide, which relaxes blood vessels and influences many cellular
activities. Clinical trials have shown that ginseng improves accuracy on memory
tests and improves the speed of performing attention tasks. The impact of
ginseng on mood has been inconsistent in clinical trials to date.
Sage
(Salvia officinalis) contains a range
of monoterpenes. Effects on the central nervous system include anti-inflammatory
activity and decreased breakdown of the acetylcholine neurotransmitter.
Clinical trials have demonstrated improved memory, attention, and alertness in
healthy people after single doses of sage extract and improved cognitive
function in people with Alzheimer's disease after 16 weeks of an alcoholic
tincture of sage.
Valerian
(Valeriana officinalis) root contains
a variety of terpenes, including the valepotriates and valerenic acid. Valerian
compounds modulate serotonin receptor subtypes, GABA, and adenosine receptors
and have anxiolytic activity. Some clinical trials suggest that valerian
improves sleep quality; others were not conclusive.
Phenolics
Phenolics
are ubiquitous in plants and they are important components in the human diet.
They range from simple, low molecular weight compounds to complex, large
structures such as flavonoids, tannins, and anthocyanins.
Curcumin
is a polyphenol from turmeric (Curcuma
longa). Curcumin prevented cognitive deficits and improved learning and
memory in mouse models of Alzheimer's disease. Curcumin also reversed amnesia
in rats. Many small pilot studies have been conducted in humans, but there are
few controlled clinical trials to support a benefit of curcumin in brain
function.
Epigallocatechin-3-gallate
(EGCG) and related polyphenols are present in tea (Camellia sinensis). In vitro and animal studies suggest that EGCG may
have protective effects in Alzheimer's disease and Parkinson's disease. EGCG
improved cognitive performance and increased antioxidant capacity in rats. In
epidemiological studies, greater consumption of green tea is associated with
reduced risk of cognitive impairment and neurodegenerative disorders.
St.
John's wort (SJW; Hypericum perforatum)
extract contains a variety of phenolic compounds that have an impact on brain
function, including phenolic acids, flavonoids, hyperforin, and hypericin. SJW
extracts have been reported to inhibit reuptake of serotonin, dopamine, and
GABA, but this finding is highly doubtful. Other reports include increased
neurotransmitter sensitivity and altered receptor binding. Clinical trials have
established that SJW is an effective treatment in people with mild to moderate
depression.
Resveratrol
is found in the skin of red grapes (Vitis
vinifera) and in some other edible plants. Resveratrol preserved behavior
and cognitive performance in older rats with brain injuries. In healthy humans,
single doses of resveratrol increased blood flow and oxygen uptake in the
frontal cortex of the brain, suggesting it may have benefits in Alzheimer's
disease and other neurological disorders.
Soy
(Glycine max) extract contains isoflavones
such as genistein, daidzein, and glycetin that have very weak estrogen-like
effects. Several clinical trials have shown that soy isoflavones modestly
improve neurocognitive function and mood in postmenopausal women, but other
clinical trials showed no improvement. Compared to a diet with no soy
isoflavones, a diet rich in soy isoflavones was associated with improved
short-term and long-term memory in men and women.
Conclusion
The
authors conclude that the literature describing the effectiveness of herbal
extracts for improving brain function is "somewhat equivocal." Research
on caffeine and nicotine has been hampered by the addictive nature and serious adverse
effects attributed to these alkaloids. Research is progressing among plants
containing terpenes, particularly ginkgo and valerian. However, the
methodological quality of some of the clinical trials has been poor, and
results have been inconsistent. Interest in the potential cognitive benefits of
curcumin, EGCG, resveratrol, and soy isoflavones is relatively recent, and
human clinical trials are still in the early stages.
The
authors describe challenges in developing plant-based products to prevent or
reverse age-related cognitive decline. Challenges include identifying the
active components, understanding the synergism among the active components,
defining the environmental stressors and growing conditions under which plants
produce increased amounts of secondary metabolites, and standardizing plant
products to provide beneficial amounts of active compounds.
—Heather S. Oliff, PhD
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