Journal of Medicinal Plants Research Vol. 5(13), pp. 2922-2928, 4 July, 2011
Available online at http://www.academicjournals.org/JMPR
ISSN 1996-0875 ©2011 Academic Journals
Full Length Research Paper
Effects of garlic on blood glucose levels and HbA1c in
patients with type 2 diabetes mellitus
Rizwan Ashraf M. Phil1*, Rafeeq Alam Khan1 and Imran Ashraf2
1Department of Pharmacology, Faculty of Pharmacy, University of Karachi, Pakistan.
2Department of Geriatric Medicine, King’s Hospital London, London, UK.
Accepted 8 April, 2011
The aim of the present study was to evaluate the effects of garlic on fasting blood sugar and HbA1c in
patients with type 2 diabetes mellitus. This was a 24 week, single-blind placebo controlled study. The
patients (n = 210) with fasting blood sugar above 126 mg/dl were recruited and divided into 7 groups (A,
B, C, D, E, F and G), each comprised of 30 patients. Group A, B, C, D, and E were given garlic tablets at
doses of 300, 600, 900, 1200, and 1500 mg per day respectively. Group F received metformin while group
G received placebo. FBS and HbA1c were measured at week 0, 12 and 24. Present study showed
significant decrease in fasting blood sugar and HbA1c in both dose and duration dependent manner. In
each garlic treated group, significant reduction in FBS (p <0.005) and HbA1c (p <0.005) were observed
when compared with placebo. Highly significant reduction in FBS and improvement in HbA1C were
observed at higher doses of garlic and with increase in the duration of study. Garlic is more effective
than placebo and comparable to metformin in reducing fasting blood glucose and may be a valuable
addition in the management of diabetic patients.
Key words: Garlic, diabetes mellitus, fasting blood sugar, HbA1C, complementary therapy.
Diabetes mellitus is a group of diseases characterized by
high blood glucose levels resulting from defects in insulin
secretion, insulin action, or both. Abnormalities in the
metabolism of carbohydrate, protein, and fat are also
present (Defronzo, 1997). The prevalence of type 2
diabetes is increasing globally (Zimmet et al., 2001;
Mokdad et al., 2001). It is anticipated that by the year
2025 over 75% of all people with diabetes will belong to
the developing countries. In Pakistan, no of patients
afflicted with diabetes are estimated to increase from 4.3
million in 1995 to 14.5 million in 2025 (Shera et al., 2007).
Despite the availability of medication for management
of diabetes, the interest in alternative traditional remedies
is increasing (Pari and Umamaheswari, 2000). The use of
*Corresponding author. E-mail: email@example.com Tel: +92-
21-6634848. Fax: +96672355059.
natural substances has become more extensive over the
past few years, motivated certainly by the faith that they
may have fewer side effects as compared to
pharmaceuticals and by their effortless accessibility to the
populace without prescriptions or visits to the health
providers (Ashraf et al., 2005). Many natural substances
have shown the potential to condense number of
cardiovascular risk factors including diabetes mellitus
(Marles and Farnsworth, 1995; Alarcon-Aguilara, 1998).
Garlic and various forms of extracts prepared from it
represent an example of such natural substances
(Steiner and Li, 2001).
Garlic is known to possess a number of biologically
active compounds having anticoagulant (Fukao et al.,
2007), antioxidant (Banerjee et al., 2003; Lee et al.,
2009), antihyperlipidemic (Gupta and Porter, 2001) and
antihypertensive effects (Verma et al., 2008).
Antidiabetic potential of garlic has been reported in
many previous trials conducted in animal models (Marles
and Farnsworth, 1995; Alarcon-Aguilara et al., 1998;
Banerjee and Maulik, 2002; Anwar and Meki, 2003; Liu et
al., 2005; Liu et al., 2006; Eidia et al., 2006; Jelodar et al.,
2005; Lee et al., 2009).
Garlic principle active agent appears to be allicin, a
sulfur-containing compound that with its breakdown
products gives garlic its characteristic odour (Elkayam et
al., 2003). Allicin is formed enzymatically from an
odourless precursor, alliin, when garlic cloves are
mechanically disrupted (Alpers, 2009). The probable
mechanism underlying garlic hypoglycemic effects most
likely is increased insulin secretion and sensitivity (Birdee
and Yeh, 2010). Despite that the antidiabetic potential of
garlic has been confirmed in animal studies, scientific
evidence from human studies is lacking (Liu et al., 2007).
Most of the clinical studies have observed the effects of
garlic on blood glucose in normal healthy individuals but
not in diabetic patients, leaving no doubt that the role of
garlic in the management of diabetic patients still needs
to be confirmed. However, bearing in mind that garlic has
been an essential part of our diet for centuries, it is taken
for granted that garlic is safe in a wide range of doses.
Few non-specific adverse effects were reported in clinical
studies using garlic and its preparations and frequently
consist of gastrointestinal discomfort and nausea. Allergic
contact dermatitis has been reported in people with
occupational exposure to garlic. There have also been
infrequent reports of allergic conjunctivitis, rhinitis,
bloating, headache, dizziness, profuse sweating and
bronchospasm occurring in response to garlic inhalation
or ingestion. Rarely ingestion of fresh garlic and garlic
powder was reported to have synergistic effects with
anticoagulants or platelet aggregation inhibitors
increasing risk of haemorrhage (Banerjee and Maulik,
2002). Thus comprehensive clinical studies in diabetic
patients are justified to confirm the efficacy and possible
role of garlic in the management of diabetic patients
(Thomson et al., 2007).
As previous clinical trials have cast doubt on the
proposed hypoglycemic effects of garlic and there is no
previous trial showing the effects or benefits of garlic at
different doses on blood glucose in patients with diabetes
and also keeping in mind the incidence of diabetes that is
quite high in Pakistan
and a wide spread belief among
general public that garlic has beneficial effects on various
cardiovascular risk factors, we designed a study to
evaluate the hypoglycemic effects of garlic in patients
with type 2 diabetes mellitus in comparison with the
standard antidiabetic agent metformin and placebo.
MATERIALS AND METHOD
This was a 24 week, single-blind study, conducted simultaneously
in different primary health care centers in Karachi, Pakistan in
cooperation with registered medical practioners. Research protocol
has been approved by Board of advanced studies and Research of
the University of Karachi.
Phil et al. 2923
Patients with previously diagnosed type 2 diabetes mellitus (n =
210) were enrolled after taking informed and written consent and
were divided randomly into 7 groups: A, B, C, D, E, F and G, each
comprised of 30 patients. Patients in group A, B, C, D and E were
given garlic tablets at the dose of 300, 600, 900, 1200 and 1500 mg
per day in divided doses respectively for 24 weeks. Group F was
given tablet metformin 500 mg twice daily for 24 weeks while
patients in Group G were given placebo.
Prior to enrollment we conducted a screening examination in
which inclusion and exclusion criteria were evaluated, informed and
written consent was taken from each study participant. All the
patients were seen at fortnightly interval. Fasting blood sugar and
HbA1c were measured at week 0, week 12 and week 24 of study.
At each visit, patients were queried about drug compliance and side
effects of drugs. Diet control consisted of asking subjects at
recruitment to keep to their usual diet throughout the study and
especially not to alter their garlic or onion intake. Patients were also
motivated to keep their nutritional plan, physical activity and general
life style as c onstant as possible t hroughout the study period. The
patients were advised to avoid taking any other medication during
the study period without informing the study physician.
Blood samples were drawn from each patient on the morning of
week 0, week 12 and week 24. Blood was drawn by venepuncture
with the help of a sterile plastic disposable syringe under aseptic
measures. Blood glucose was determined by the enzymatic
colorimetric method. Glycated hemoglobin ( HbAlc) was estimated
by Fast Ion-Exchange Resin Separation Method.
All patients in groups A, B, C, D, E, F and G were s elected
according to following criteria.
Patients with diagnosed type 2 diabetes mellitus of either s ex with
fasting blood sugar levels above 126 mg/dl. Patients aged between
25 to 70 years.
The patients were excluded from the study if they are, known to
have history of allergy to garlic, pregnant or lactating women,
patients with type 1 diabetes mellitus, patients having history of
myocardial infarction, coronary artery bypass grafting, established
coronary artery disease, unstable angina, clinically manifest heart
failure, patients with history of liver disease and impaired renal
function and if they are known to have any other concurrent medical
Results are presented as means ± SEM. The paired, two tailed
Student t test was performed t o compare measurements of FBS
and HbA1C at different durations of therapy. Statistical significance
was defined as p <0.05.
The demographic characteristics of the study population
are shown in Table 1. The patients were male (60%) and
females (40%). The mean age was 40 years in garlic
treated group and 50 years in metformin treated group
(range 25 to 70 years) (Table 1). Fasting blood glucose
and HbA1C were measured at week 0 and week 12 and
week 24 (Tables 2 and 3, Figure 1and 2).
A total of 210 patients were initially enrolled in the
study. 195 patients were associated throughout the study
2924 J. Med. Plant. Res.
Table 1. Demographic data of Garlic and standard group.
Placebo treated group Garlic treated group Metformin treated group
Men 15 90 17
Women 15 60 13
Age (years) 45 ± 4.58 40 ± 5.04 50 ± 5.80
Body weight (Kg) 69.1 ± 7.58 68.2 ± 10.45 65.4 ± 9.80
Height (cm) 166.4 ± 6.58 165.2 ± 8.81 167.60 ± 9.20
Average duration of type 2
diabetes mellitus 3 years 3 years 2.5 years
Co-morbid disease None None None
Table 2. Effect of different doses of Garlic in comparison with Placebo and Metformin on fasting blood sugar from week 0 to week
12 and week 24.
Group according to dose Fasting blood sugar (mg/dl)
Week 0 Week 12 Week 24
(Group A) Garlic 300 mg 127 ± 0.334 (n = 30) 126 ± 0.360** (n = 27) 125 ± 0.379** (n = 27)
(Group B) Garlic 600 mg 128 ± 0.311 (n = 30) 127 ± 0.369** (n = 27) 126 ± 0.446** (n = 27)
(Group C) Garlic 900 mg 128 ± 0.263 (n = 30) 126 ± 0.274** (n = 27) 124 ± 0.289** (n = 27)
(Group D) Garlic 1200 mg 128 ± 0.315 (n = 30) 125 ± 0.264** (n = 27) 123 ± 0.263** (n = 27)
(Group E) Garlic 1500 mg 129 ± 0.223 (n = 30) 126 ± 0.213** (n = 27) 123 ± 0.225** (n = 27)
(Group F) Metformin 500 mg 125 ± 1.246 (n = 30) 119 ± 1.243** (n = 27) 118 ± 1.045** (n = 27)
(Group G) Placebo 127 ± 0.192 (n = 30) 129 ± 0.241** (n = 27) 131 ± 0.363** (n = 27)
Data are presented as mean ± standard error. Asterisk denotes significant p values. * Significant p < 0.05 ** Highly signific ant p < 0.005.
Table 3. Changes in HbA1C with Garlic, Metformin and Placebo from week 0 to week 12 and week 24.
Group Blood levels of HbA1C (%)
Week 0 Week 12 Week 24 Mean difference
Group A (300 mg dose of garlic) 6.59 ± 0.044 6.26 ± 0.044** 6.02 ± 0.048** - 0.57
Group B (600 mg dose of garlic) 6.52 ± 0.055 6.33 ± 0.064 6.06 ± 0.054** - 0.46
Group C (900 mg dose of garlic) 6.60 ± 0.055 6.31 ± 0.053* 6.03 ± 0.054** - 0.57
Group D (1200 mg dose of garlic) 6.53 ± 0.056 6.26 ± 0.045 6 ± 0.053** - 0.53
Group E (1500 mg dose of garlic) 6. 73 ± 0.035 6.32 ± 0.041** 5.97 ± 0.032** - 0.76
Group F (Metformin 500 mg) 6.64 ± 0.044 6.26 ± 0.036** 6.21 ± 0.042** - 0.43
Placebo 6.31 ± 0.036 6.37 ± 0.035 6.40 ± 0.049 + 0.09
* Significant p < 0.05, ** Highly significant p < 0.005, (-) indicates decrease in mean HbA1C from week 0 to week 24. (+) indicates increase
in mean HbA1C from week 0 to week 24.
and completed the protocol till week 24. Out of the
dropped 15 patients, 3 patients were dropped from
metformin treated group, 2 patients due to vague
abdominal discomforts and refused to continue the
protocol while 1 patient complained of weight loss and did
not come for follow up after 12 weeks. The remaining
patients were dropped from different garlic treated
groups, 8 patients did not come for follow-up due to
unknown reasons, 4 patients refused to give the blood
samples for biochemical analysis so were forbidden to
continue the study. 3 patients were dropped in the garlic
treated group with 1500 mg, who reported heart burn in
the first week of study and refused to take any further
treatment. During this study, every possible attempt has
been made to keep the number of patients as much as to
provide a valid statistical data.
Phil et al. 2925
Placebo G 300 mg G 600 mg G 900 mg G1200 m g G 1500 mg METFORMIN
Figure 1. Changes in mean FBS at different doses of Garlic in comparison with Placebo and
Metformin from week 0 to week 12 and week 24.
Figure 2. Percentage changes in FBS from week 0 to week 12 and from week 0 to
week 24. Gr A = Garlic 300 mg, Gr B = Garlic 600 mg, Gr C = Garlic 900 mg, Gr D =
Garlic 1200 mg, Gr E = Garlic 1500 mg, Gr F = Metformin, Gr G = Placebo. The (-) sign
denote decrease in perc entage.
The result in the present study showed significant
decrease in fasting blood sugar and HbA1C in both dose
dependent and duration dependent manner when
compared with placebo and standard antidiabetic agent
metformin. The fasting blood sugar (FBS) was reduced
from 127.4 ± 0.334 at baseline from week 0 to 126.0 ±
0.360 (p <0.05) at week 12 and to 124.8 ± 0.378 (p
<0.001) at week 24 in group. A patient who have been
given garlic at a dose of 300 mg daily. In group B, at a
dose of 600 mg the reduction in FBS was found to be
almost equivalent with that in group A with a decrease
from baseline value of 128.3 ± 0.311 at week 0 to 126.9 ±
0.369 (p <0.05) at week 12 and 125.7 ± 0.446 (p <0.001).
In group C, patients were given garlic at a dose of 900
mg and the FBS was found to be reduced from 128.4 ±
0.263 at the baseline from week 0 to 126.0 ± 0.274 (p
<0.001) at week 12 and 124.5 ± 0.289 at week 24 (p
<0.001). In group D, the dose of garlic given was 1200
mg/day. The reduction in FBS from baseline value of
128.2 ± 0.315 at week 0 to 125.3 ± 0.264 (p <0.001) at
week 12 and to 122.6 ± 0.263 (p <0.001) was found to be
highly significant when compared to group A and B at
week 12. Group E patients were given the highest dose
of garlic in the present study. The FBS decrease in this
group was highly significant from 128.7 ± 0.223 at week 0
125.8 ± 0.213 (p <0.001) and to 122.7 ± 0.225 at week 24
(p <0.001). Group F patients were given metformin at a
dose of 500 mg twice daily. In this group the FBS at week
0 was 125.1 ± 1.157, which was reduced to 119.5 ± 1.243
at week 12 (p <0.05) and to 118.0 ± 1.045 at week 24 (p
<0.001). In comparison significant increase change in the
FBS was observed in patients with placebo with the
mean increase of 4 mg/dl (P <0.005) Table 2, Figure 1
and 2. The HbA1C percentage was also found to be
improved in all the groups treated either by garlic or
metformin as compared to placebo. In group A, HbA1C at
2926 J. Med. Plant. Res.
week 0 was 6.585 ± 0.044. It was found to reduced to
6.256 ± 0.044 (p <0.001) at week 12 and to 6.019 ± 0.048
(p <0.001) at week 24. In group B, HbA1c was reduced
from 6.519 ± 0.055 at week 0 to 6.330 ± 0.064 at week 12
and 6.063 ± 0.054 (p <0.001) at week 24. This decrease
was statistically insignificant when compared from week 0
to week 12 but was found to be statistically significant
when compared from week to week 24. In group C
reduction in HbA1C was significant from baseline value of
6.596 ± 0.055 at week 0 to 6.307 ± 0.053 (p <0.05) at
week 12 and highly significant to 6.033 ± 0.054 (p
<0.001) at week 24. In group D, the HbA1C was reduced
from baseline value of 6.530 ± 0.056 to 6.256 ± 0.045 (p
<0.05) and 5.996 ± 0.052 (p <0.001) at week 12 and 24
respectively. Group E patients showed highest
improvement in HbA1C with a reduction from baseline
value of 6.726 ± 0.035 at week 0 to 6.332 ± 0.041 at
week 12 (p <0.001) and 5.974 ± 0.032 (p <0.001) at week
24. Metformin treated group F also showed similar
reduction like group E with a decrease in HBA1C value of
6.637 ± 0.044 at baseline to 6.259 ± 0.036 (p <0.001) at
week 12 and to 6.207 ± 0.042 (p <0.001) at week 24. In
comparison no significant change in HbA1C was
observed in patients who were given placebo.
The present study monitored effects of garlic in
comparison with placebo and a standard antidiabetic
agent ‘metformin’ on fasting blood sugar and HbA1c in
patients with type 2 diabetes mellitus with respect to its
ability to provoke a decrease in fasting blood sugar at
different doses and to find out the time taken for this
decrease. FBS and HbA1C were measured at week 0,
week 12 and week 24. The results observed in the
present study demonstrated significant decrease in
fasting blood glucose and improvement in serum HBA1C
when compared from week 0 to week 12 and week 24.
Garlic was found to be effective in improving the glycemic
control at all doses. Exceedingly significant changes
observed at the higher doses of 900, 1200 and 1500 mg
as compared to when garlic was administered at a dose
of 300 or 600 mg. The improvement in glycemic control
was found to be more when compared with placebo and
equivalent when compared with standard antidiabetic
agent metformin. Highly significant reduction in both
fasting blood sugar and HbA1C is observed at week 24 in
all the groups treated with garlic and metformin with
comparable efficacy as compared to placebo which
showed no significant change in FBS and HbA1C.
The present study is unique from previous clinical trials
as this is the first time that effects of garlic were observed
in a dose dependent and duration dependent manner in
patients with type 2 diabetes mellitus. The changes in
fasting blood sugar and HbA1C observed in the present
study are consistent with the observations from previous
preclinical and clinical trials. Many of the trials evaluating
the hypoglycemic effects of garlic were done in animal
models. Garlic was reported to be effective in reducing
blood glucose in streptozocin-induced as well as alloxan-
induced diabetic rats and mice (Mathew and Augusti,
1973; Swanston-Flatt et al., 1990; Kasuga et al., 1999;
Ohaeri, 2001). It was also reported that ingestion of garlic
juice and aqueous garlic extracts resulted in better
utilization of glucose in glucose tolerance test performed
in diabetic animal models (Jain and Vyas, 1973; Jain and
Vyas, 1975; Jalal et al., 2007). Two of the previous
studies reported that allicin, a sulfur containing amino
acid in garlic has a potential to reduce diabetic condition
in rat almost to the same extent as did glibenclamide and
insulin (Sheela and Augusti, 1992; Sheela et al., 1995).
Although many of the previous trials in animal models
showed significant effects of garlic on glycemic control,
hypoglycemic effect of garlic in human is not well studied.
All human studies (Kiesewetter et al., 1991; Jain et al.,
1993; Ali and Thomson, 1995; Bordia et al., 1998; Zhang
et al., 2001) apart from two (Afkhami-Ardekani et al.,
2006; Sobenin et al., 2008), has showed the effect of
garlic on blood glucose level in normal healthy individuals
but not in diabetic patients. The present study confirmed
the hypoglycemic effects of garlic observed in the two
previous clinical trials in patients with type 2 diabetes
mellitus. The effects of garlic tablets preparation used in
the present study were found to produce the similar
changes in the fasting blood glucose. In addition HbA1c
was also found to be significantly improved as compared
to baseline readings. The present study contradicts with a
previous clinical trial of Bordia et al., 1998, that evaluated
the effects of garlic on serum glucose and lipids in
patients with coronary artery disease and reported no
significant change in fasting and 2 h postprandial blood
glucose levels. The changes in FBS observed in the our
study are in accordance with the previous clinical trial by
Li et al., 2000 that reported the marked decrease in blood
sugar levels in cases with hyperglycemia.
The precise mechanism of garlic as antidiabetic agent
is not unambiguous but in-vivo, (Mathew and Augusti,
1973; Jain, 1973) as well as in vitro (Augusti and Sheela,
1996) studies showed that garlic acts as an insulin
secretagogue in diabetic rats. It has also been proposed
that antioxidant effect of S-allyl cysteine sulfoxide
(isolated product from garlic) may also contribute for its
beneficial effect in diabetes (Banerjee et al., 2003). Garlic
has been reported to spare insulin from sulphydryl group
which is known to inactivate insulin. Garlic allicin can
effectively combine with compounds like cysteine and
enhance serum insulin (Mathew and Augusti, 1973). It
has also been projected that garlic can act as an
antidiabetic agent by increasing either the pancreatic
secretion of insulin from the beta cells or its release from
bound insulin (Jain and Vyas, 1975). In addition to the
aforementioned proposed mechanisms garlic is also
known to prevent the long term complications of diabetes
by inhibiting the formation of advanced glycation end
products (AGEs). Advanced glycation end-products are
recognized contributors to the pathophysiology of aging
and diabetic chronic complications. People with diabetes
have higher levels of AGEs than nondiabetic subjects
because hyperglycemia and oxidative stress both
contribute to their accumulation (Huebschmann et al.,
2006). The formation of AGEs occurs at an accelerated
rate in patients with diabetes mellitus. AGEs are not only
markers but also important causative factors for the
pathogenesis of diabetes, cataracts, atherosclerosis, and
diabetic nephropathy (Gutiérrez et al., 2010). Compounds
having both antiglycation and antioxidant properties are
recommended therapeutic preference. It has been
suggested that aged garlic extract (AGE) inhibits both the
formation of AGEs and glycation-derived free radicals. A
key constituent of aged garlic, ‘S-Allylcysteine’, is an
effective antioxidant and can inhibit AGE formation
(Ahmad and Ahmed, 2006; Ahmad et al., 2007).
Organosulfur compounds derived from garlic, diallyl
sulfide, S-ethylcysteine, S-allylcysteine, and N-
acetylcysteine, are known to protect LDL against
oxidation and glycation and this might be a possible
mechanism how garlic protects against cardiovascular
disease (Ou et al., 2003). Although garlic is generally
considered safe herbal remedy and has been used for
many common ailments since ancient times, too much
utilization of garlic may cause problems most commonly
related to gastrointestinal tract like nausea and diarrhea.
Garlic breath and halitosis or bad body odour are also
one of the main garlic side effects reported especially
when fresh garlic is ingested. Infrequent allergic response
is also a known predictable adverse effect (Ackerman et
al., 2001). In the present study, garlic did not produce any
considerable problem in patients with type 2 diabetes
mellitus and only one patient has complained of gastric
discomfort, the possible reasons for this good tolerance is
most likely to the enteric coated, odourless garlic tablet
preparation used in this study.
Garlic was found to reduce fasting blood glucose and
HbA1C significantly as compared to metformin and
placebo and may be an effective addition in the family of
antidiabetic agents. Clinical trials using garlic in
combination with different antidiabetic agents are
warranted to further explore the benefits of garlic in
Author(s) are thankful for the help of registered medical
practioners from different primary health centers in
Karachi, Pakistan for their help in conduction of study by
Phil et al. 2927
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