- 1 Clinical Use
- 1.1 CARDIOVASCULAR DISEASE
- 1.2 PERIPHERAL ARTERIAL OCCLUSIVE DISEASE
- 1.3 INFECTION
- 1.4 PROTECTIVE EFFECTS AGAINST CANCER
- 2 Other Uses
Most studies have used a non-enteric coated dehydrated garlic powder preparation standardised to 1.3% alliin content (Kwai, Lichtwer Pharma) or an aged garlic extract (Kyolic, Wakunaga of America).
Epidemiologic studies show an inverse correlation between garlic consumption and progression of CVD in general.
This review will consider the evidence for garlic in the management of specific risk factors such as hypertension and hyperlipidaemia. Additionally, investigation into the effects of garlic directly on the atherosclerotic and arteriosclerotic processes is presented.
A meta-analysis of seven clinical trials using a garlic preparation, produced commercially as Kwai, found that three showed a significant reduction in SBP and four in DBP. Kwai was used in these studies in the dosage of 600-900 mg daily. Garlic treatment resulted in a mean reduction in SBP of 7.7 mmHg and 5.0 mmHg in DBP compared with placebo.
In 2000, the Agency for Health Care Research and Quality analysed results from 27 randomised, placebo-controlled trials and reported that results were mixed. When significant reductions in blood pressure were observed, these were small.
Several newer trials have since been published further confirming that the effect on blood pressure is small and sometimes non-significant.
Atherosclerosis and arteriosclerosis
Garlic indirectly affects atherosclerosis by reduction of hyperlipidaemia, hypertension, and prevention of thrombus formation.
Koscielny et al conducted a randomised, double-blind, placebo-controlled clinical trial involving 152 volunteers to determine whether garlic powder supplements (Kwai 900 mg daily) directly alter plaque volumes in carotid and/or femoral arteries. After 4 years’ treatment, garlic intake significantly reduced the expected increase in arteriosclerotic plaque volume by 5-18%, with a slight regression also observed. A subsequent re-evaluation of the results found that significant effects were limited to women only.
In 2000, a meta-analysis of 13 clinical trials concluded that garlic reduces total cholesterol levels significantly more than placebo; however, the effects can only be described as modest. The same year, a systematic review and meta-analysis were published by the Agency for Health Care Research and Quality, which analysed results from 44 studies with lipid outcomes. Most studies involved fewer than 100 volunteers and randomisation techniques were unclear in 82% of studies. Pooled data from the placebo-controlled trials reporting changes in total cholesterol levels found a significant average reduction in total cholesterol levels of 7.2 mg/dL after 4-6 weeks using any form of garlic and a reduction of 17.1 mg/dLat 8-12 weeks. Results at 20-24 weeks were not significant and thought to be due to low statistical power, fewer long-term studies or a time-dependant effect of garlic.
Since then several new studies have been published, with most showing no significant reduction to total cholesterol levels. According to one review, non-enteric coated tablets containing dehydrated garlic powder (standardised to 1.3% allicin) produce the most consistent results.
Two clinical studies have compared different garlic preparations with pharmaceutical cholesterol-lowering medicines. Garlic taken as 300 mg three times daily (Kwai) produced similar lipid-lowering effects to 200 mg bezafibrate (a hypolipidaemic fibrate) three times daily in subjects with primary hyperlipidaemia whereas clofibrate 500 mg was more effective than an essential oil extract of 50 g raw garlic.
Commission E approves the use of garlic as an adjunct to dietary changes in the treatment of hyperlipidaemia.
Diabetics with hyperlipidaemia
A 12-week placebo-controlled, single-blind, randomised study of 70 patients with type 2 diabetes and newly diagnosed dyslipidaemia found that treatment with a garlic tablet (Garlex-Bosch Pharmaceuticals: 300 mg, containing 1.3% allicin) twice daily, together with a diet and exercise plan, resulted in a significant reduction in total cholesterol of 28 mg/dL (12.03%) compared to placebo.
Clinical note — Not all garlic preparations are the same
One of the difficulties encountered when interpreting the research available for garlic is comparing the effects of different preparations, which often have not been tested for the presence of important constituents or allicin-releasing potential. It is known that fresh garlic and dried garlic powder contain alliin and the enzyme allinase required for biotransformation, but some other forms may only contain alliin, and not the necessary alliinase component, thus compromising allicin-releasing potential. An example of the manufacturing process affecting potency has been suggested for a commercial garlic product known as Kwai, which has often been used in cholesterol research. According to a 2001 experiment, substantial differences were found between tablets manufactured before 1993 (the years when all but one of the positive trials were conducted) and those manufactured after 1993 (the years when all of the negative trials were conducted). Kwai products manufactured after 1993 released only one-third as much allicin as older preparations. Those preparations from before 1993 disintegrated more slowly, protecting alliinase from acid exposure and inactivation.
Antiplatelet effects of garlic are well recognised, but the dose at which this becomes significant remains uncertain. Results from a 2001 double-blind study have identified a dose of 7.2 g/day of aged garlic extract as significantly inhibiting platelet aggregation and adhesion.
PERIPHERAL ARTERIAL OCCLUSIVE DISEASE
In 2000, Mulrow et al reported on two double-blind, placebo-controlled trials in participants with atherosclerotic lower extremity disease (Mulrow et al 2000b). One study of 64 participants showed that pain-free walking increased by approximately 40 metres with standardised dehydrated garlic (Kwai 800 mg daily) compared with approximately 30 metres with placebo over 12 weeks. Cochrane reviewers report that the effect was not significant (Jepson et al 2000). The other study of 100 participants (Mulrow et al 2000b) showed that a combination treatment of garlic oil macerate/soya lecithin/hawthorn oil/wheat germ oil significantly increased the maximum walking distance (114%) compared to placebo (17%) (P< 0.05).
Garlic oil is effective against numerous bacteria, viruses and fungi including Staphylococcus aureus, MRSA, and several species of Candida, Aspergillus and Cryptococcusneoformans’m vitro. As such, it has been used both internally and externally to treat various infections and prevent wound infection.
Tinea pedis, tinea corporis, tinea cruris
A trial comparing the effects of three different strengths of ajoene cream (0.4%, 0.6% and 1 %) with 1 % terbinafine applied twice daily found the cure rate to be 72% for 0.6% ajoene, 100% for 1% ajoene, and 94% for 1% terbinafine after 60 days.
Taken internally as a ‘natural antibiotic‘ or applied topically in a cream base, garlic is used to treat vaginitis. The considerable antibacterial activity of garlic provides a theoretical basis for its use in this condition, but controlled studies are not available to determine effectiveness.
Common cold prevention
A 12-week, double-blind randomised study involving 146 people demonstrated that allicin-containing garlic preparations significantly reduce the incidence of colds and accelerate recovery compared with placebo. More specifically, the number of symptom days in the placebo group was 5.01 compared with 1.52 days in the garlic treated group. Additionally, garlic reduced the incidence of developing a second cold whereas placebo did not. Helicobacter pylori infection It has been suggested that gastrointestinal lesions, such as gastric ulcers, duodenal ulcers and gastric cancers, are strongly associated with Helicobacter pylori infection. Medical treatment consisting of ‘triple therapy’ has a high eradication rate, yet is associated with side-effects and has started to give rise to antibiotic resistance. Since garlic intake has been associated with a lowered incidence of stomach cancer, researchers have started investigating whether garlic has activity against Helicobacter pylori. Several in vitro and in vivo tests have shown garlic to be effective against Helicobacter pylori (see MAIN ACTIONS). However, to date only a few small clinical trials have been conducted with disappointing and controversial results.
A small pilot study of dyspeptic patients with confirmed Helicobacter pylori infection found that treatment with 4 mg garlic oil capsules taken four times daily with meals for 14 days did not alter symptoms or lead to Helicobacter pylori eradication. Another small study using garlic oil 275 mg three times a day (allicin 800 µg/capsule) either as stand-alone treatment or in combination with omeprazole (20 mg twice daily) found that both treatments produced similar results. These results were confirmed in another small clinical study.
PROTECTIVE EFFECTS AGAINST CANCER
A 2001 critical review of the epidemiological evidence suggests a preventive effect for garlic consumption in stomach and colorectal cancers, but not other cancers. In regard to gastric cancer protection, case-control studies suggested a protective effect for raw and/or cooked garlic when eaten at least once a week whereas protective effects against colorectal cancer seem to require at least two servings of garlic per week. A similar view was reported in a 2003 review by Ernst, which stated that the weight of evidence to support the use of allium vegetables, such as garlic, in cancer is clearly positive.
Intervention study in colorectal cancer A preliminary double-blind, randomised clinical trial in patients with colorectal adenomas-precancerous lesions of the large bowel produced promising results with the use of high-dose aged garlic extract (AGE 2.4 mL/day). The study of 51 patients measured the number and size of adenomas at baseline and at 6 and 12 months and found that AGE significantly suppressed both the size and the number of colon adenomas in patients after 1 year of treatment (P = 0.04). In comparison, the number of adenomas increased linearly in the control group from the beginning.
Some early research suggests that it may prevent the incidence of altitude sickness and reduce mosquito numbers. It has also been used to assist in heavy metal detoxification. Studies with experimental animal models provide some support for its use in this way.