Garlic (Allium
sativum) is a perennial plant
and is included as a member of the Liliaceae family. [1] Botanically,
the seeds
of the garlic plant are quite unfertile and is
practically considered an annual,
depending on humans for propagation. Due to garlic's culinary popularity,
familiarity with the bulb of garlic
is common. Garlic is divided into
two groups: softneck - sativum, or
cultivated garlic; and hardneck -
which is of the ophioscorodon
subspecies. [2]
Contrary to popular belief, garlic does not exist in the wild.
The majority of garlic bulbs
consumed in human nutrition are cultivated,
with the majority of product coming
from Central California. [3] There are many cultivars of garlic,
including Italian Purple and Silverskin. The bulb is the part of
the plant that is used medicinally
as well.
Extracts of garlic are
often described as tasting pungent
and 'warm.' [1]
There are a number of
active constituents in garlic. However, one particular category is
responsible for majority of its
credited medicinal activity. The sulfur-containing compounds, sulfoxides (alliin) and thiocyanates,
are the primary constituents of
interest. Volatile oils are are
also significant, making up 0.1 - 0.3% of
the bulb, and are composed of about
14 different components. Garlic also
contains; vitamin A, thiamine, nicotinaminde, vitamin C, protein,
high concentration of trace minerals
(i.e. selenium), glucosinolates, enzymes
(i.e. alliinase, peroxidase, myrosinase),
choline, iodine, and saponins. [4,
5]
The sulfur containing
compound, alliin, has been of
particular interest to researches
concerning its possible therapeutic
implications on human health.
Alliin is exposed
to the enzyme alliinase when the
bulb is
crushed, resulting in the creation
of allicin. Interestingly, alliin
itself is
odorless until it is crushed or
chewed. Only upon its conversion
to allicin does it obtain its
pungent smelling odor. [3] The entire process takes
approximately six seconds. Allicin is readily
absorbed into the bloodstream and
eliminated primarily via the lungs
and skin. It is unstable agent
that is immediately converted to numerous other organosulfur compounds, including ajoene, trisulfide and diallyl
disulfide. [6]
Supplements can be
purchased as dried garlic
powder, keeping a consistent level
of alliin and allinase; odorless, aged garlic
extract (which is rich in
modified sulfur components that also
have medicinal activity); and steam
distilled garlic oil containing diallyl sulfides. [7] There are also enteric coated
products which prevent digestion of
the capsule in the stomach where
hydrochloric acid inactivates the
enzyme allinase, which would
likely prevent its subsequent conversion to allicin. These
preparations favor the chemical activity
associated with allicin.
Medicinal actions
ascribed to garlic:
Garlic is an
herbal medicine that is used for a
panacea of disease conditions and
disorders. Nevertheless, much of the
research on garlic has focused on
specific cardiovascular and immune
applications.
A double-blind
randomized, placebo-controlled
intervention study was conducted in
46 hypercholesterolemic subjects who
had failed, or were not compliant, with drug therapy. [8]
Enteric-coated Australian
garlic powder tablets with 9.6
milligrams allicin-releasing potential or
placebo was administered to test
subjects over a
twelve week period. The results
showed that the subjects taking
garlic had a significant reduction
in total cholesterol and LDL-cholesterol,
with significant increases in HDL
cholesterol levels; while the placebo group
had a non-significant increase in
total cholesterol and LDL-C.
Additionally, no
significant difference in
triglycerides or in LDL/HDL ratio
was observed between groups. The
authors conclude that the garlic
preparation used in this study can
have value in mild to moderate hypercholesterolemic patients when
combined with a low fat diet. Furthermore, two meta-analyses
concluded that garlic has a
lipid-lowering effect of 9 - 12%. [9,
10]
In addition to its
beneficial effects on cholesterol,
garlic has also demonstrated
anti-platelet aggregation effects
which is important in thrombus
formation. A 10-month study
comparing the effect of aged
garlic extract (AGE) with
placebo on the lipid profiles of
moderately hypercholesterolemic men
was conducted. [11] Platelet functions and
susceptibility of lipoproteins to
oxidation in a subgroup of this
study population was also examined.
The results showed that subjects
supplemented with 7.2 AGE per day
showed a significant reduction of
epinephrine- and collagen-induced
platelet aggregation.
Platelet adhesion to
fibrinogen was reduced by
approximately 30% in the AGE group
when compared with placebo. A
trend toward decreased
susceptibility of lipoproteins to
oxidation was also observed. The
conclusion of the study was that the
beneficial effect of garlic
preparations on lipids and blood
pressure also extends to platelet
function, providing a wider
potential protection of the
cardiovascular system.
Garlic's
antimicrobial activity makes it a
prime candidate for the treatment of
infectious disease. A randomized
double-blind, placebo-controlled
trial of 146 volunteers was
conducted in the United Kingdom to
identify the effect of garlic
administration in the prevention of the
common cold. [12]
Over a 12-week period between
November and February, subjects
received an allicin-containing
garlic capsule or placebo. They
recorded any symptoms or incidence
of colds in a diary using a five
point assessment scale.
Analysis of the study showed that subjects
taking the garlic supplement
had significantly fewer colds than
the placebo group, and recovered
faster when infection occurred.
Antimycotic activity
has also been documented for garlic. A randomized trial of soldiers with
diagnosed dermatophytoses was
conducted. [13] Subjects were divided, randomly,
into two treatment groups: one that
received standard prescription
topical application of 0.1% terbinafine, the other topical 0.6%
ajeone - an organosulfur compound of
garlic. After 60 days of treatment,
the healing rate of 73% and 71% was
observed in the
groups treated with ajoene and
terbinafine, respectively. The
authors concluded that, together
with previous studies, ajoene is a
new agent for the topical treatment
of superficial mycoses; for the
first time showing the therapeutic
usefulness of an inhibitor of
phospholipids biosynthesis in
eukaryotes.
Garlic has
also been studied in the treatment
of various types of cancer. In a
study of 21 patients with basal cell
carcinoma, a topical preparation of ajoene, an organosulfur compound of
garlic, was applied to the tumor
sites. [14] In 17 of these patients, tumor size was
reduced. The
mechanism of action was enabled via
induction of the
mitochondria-dependent route of
apoptosis.
A meta-analysis was
also
conducted for all cancers mutually
and separately for colorectal and
stomach cancers; in relation to
consumption of exclusively raw
garlic, cooked garlic, or
both. [15]
The authors concluded that high
intake of raw and cooked garlic may
be associated with a protective
effect against stomach and
colorectal cancers.
In addition, garlic
may be beneficial as an oral rinse,
and potential treatment for hypertension, atherosclerosis,
HIV infection, intermittent claudication, recurrent vaginal
infections, diabetes mellitus, and
erectile dysfunction. [16-20]
Research has been
conducted on many of the preparation
types discussed in the introduction. When choosing a particular product,
close attention should be paid to
the type of preparation (i.e. aged
garlic or enteric coated garlic) used in
research trials. Taking note of the dosage range used
for particular conditions is equally
important. Generally, 4000 milligrams
(mg) of
fresh garlic = 10 mg alliin = 4000
μg allicin potential. [5] A common dose would
yield approximately 5000 μg allicin potential daily for the treatment of actual disease
conditions. [3] For preventative or
wellness purposes, half that dose is
likely sufficient.
|
Garlic Toxicities and
Contraindications |
Caution must be taken
in patients with blood clotting
disorders, including
hypercoagulation due to the
possibilty of embolic complications.
Garlic should not be used
within 10 days of surgery. [5]
Garlic can destroy
vitamin K producing bacteria in the
gut. A dose of 3 grams daily for 26
weeks can also reduce platelet
aggregation. Therefore, patients on
anticoagulant therapy should have PT
and INR's monitored to stabilize the
dosage of coumadin. [5]
Garlic may
interact with certain prescription
medications. The chemcial
constituents of garlic can modulate the
activity of drug-metabolizing
enzymes (notably cytochrome p450
isozymes) and/or the drug
transporter P-glycoprotein, impacting the
levels of many prescription drugs,
including anti-cancer agents. [21] For this reason, garlic use should
be disclosed to providers to ensure
that patients on drug therapies maintain
adequate blood levels of prescribed
drugs.
Garlic has
also been shown to produce
hypoglycemia when taken in
combination with chlorpropamide. [22]
Garlic is
generally well tolerated with few
side effects. However, adverse
effects may include irritations to the
gastric mucosa in sensitive
individuals and garlic breath; which
is a common complaint that passes
quickly. When garlic exits
the lungs and skin, the odor is
noticeable. Patients should be
aware of this aspect of garlic
therapy as well. Although
rare, there have also been case
reports citing allergic reactions.
The following table lists various
products that contain a safe and
reliable source of Garlic:
|
Manufacturer |
Ingredient |
Amount |
US RDA |
Learn More |
|
NSI |
Garlic |
600 mg |
-- |
Click Here
 |
|
GNC |
Garlic Powder |
810 mg |
-- |
Click Here
 |
