Garlic is the fresh bulb of Allium sativum, which is related to the lily family (Liliaceae).
The major constituents of garlic include alliin, allicin, diallyl disulphide and ajoene, but these compounds form only a small proportion of the compounds that have been isolated from crushed, cooked and dried garlic.
Alliin, present in fresh garlic, is converted by the enzyme allinase into allicin when the garlic bulb is crushed. Allicin can be converted (by heat) into diallyldisulphide, which in turn is converted into various sulphide-containing substances that cause the typical smell of garlic. Allicin and diallyldisulphide combine to form ajoene.
Garlic has been cultivated for thousands of years for medicinal purposes, such as bites, tumours, wounds, headache, cancer and heart disease, and but has also been used as a pungent flavouring agent for cooking.
Evidence for the beneficial effects of garlic is accumulating but is still incomplete. Interpretation of trials is made difficult by the fact that different forms of garlic are used and that active ingredients may be lost in processing.1 The use of standardised dried garlic preparations or fresh garlic appears to provide the most beneficial effects. Extracts or oils prepared by steam distillation or organic solvents, or ‘odourless’ garlic preparations, may have little activity.2 Any preparation that produces no odour whatsoever may be clinically ineffective, because release of the biologically active allicin has not occurred. However, allicin is rapidly destroyed even by crushing the fresh bulb, and some suggest that although allicin may be important for cholesterol lowering, it may not be important for protecting against cancer.
Many studies have looked at the potential effects of garlic on serum lipid levels. In a placebo-controlled, double-blind study of 40 patients with hypercholesterolaemia,3 total cholesterol, triglycerides and blood pressure decreased significantly in the group receiving garlic. Daily doses of 900 mg of a garlic powder preparation (equivalent to 2700 mg fresh garlic) were administered over 16 weeks, and differences were significant after 4 weeks of treatment.
In a randomised, double-blind, placebo-controlled study of 42 healthy adults, 300 mg of a standardised garlic preparation three times daily showed a significantly greater reduction in total and LDL cholesterol than placebo.4 In a study of eight healthy males, ingestion of a garlic clove (approximately 3 g a day for 16 weeks) resulted in an approximately 20% reduction in serum cholesterol.5
In another study, serum levels of total cholesterol, LDL and triglycerides decreased significantly after administration of 400 mg garlic three times a day for a month.5 In a study involving 56 men, administration of an aged garlic extract (7.2 g daily) resulted in a 7% reduction in total cholesterol compared with baseline and a 6% reduction compared with placebo.6 In a multicentre, placebo-controlled, double-blind study carried out in Germany7 using standardised dried garlic tablets, total cholesterol fell by 11%.
In 35 renal transplant patients, garlic 680 mg twice daily (equivalent to 4080 µg allicin) or placebo was administered over 12 weeks.8 After 6 weeks, total and LDL cholesterol had fallen, changes that were maintained at 12 weeks. Garlic had no effect on triglyceride or HDL levels. Yet other studies using garlic 700 mg daily for 8 weeks,9 200 mg three times a day for 12 weeks,10 or 1000 mg a day for 24 weeks11 have resulted in 12–14% reductions in serum cholesterol. Other studies12–15 demonstrated no effect of garlic on hyperlipidaemia, however.
A meta-analysis of studies16 evaluating the effect of garlic on serum cholesterol included five of the above studies.3,7,9–11 The garlic dose was 600–1000 mg daily for 8–24 weeks. The pooled results of the meta-analysis indi-cated that patients treated with garlic achieved mean total serum cholesterol concentrations of 230–290 mg/L lower than patients in placebo groups. Since the investigators used a range of dose regimens, the optimum dose for garlic could not be identified.
More recently another meta-analysis,17 which included 13 trials, found that garlic reduced total cholesterol level from baseline significantly more than placebo. The weighted mean difference was 0.41 mmol/L (157 mg/L). However, when the trials with the highest scores for methodological quality were analysed alone, the differences in cholesterol levels between garlic and placebo were non-significant. The authors concluded that garlic is superior to placebo in reducing cholesterol levels, but the robustness of the data is questionable and any effect likely to be small. The use of garlic for hypercholesterolaemia was therefore debatable.
Garlic has been evaluated for an effect in various psychopathological parameters in patients with hypercholesterolaemia. In a 16-week prospective, double-blind, placebo-controlled trial, 33 patients with hypercholes-terolaemia and no evidence of CVD were randomly assigned to receive garlic or placebo. Garlic in the form of alliin 22.4 mg daily was given to 13 patients and placebo to 20. Both groups received dietary counselling. No significant changes were observed in levels of total cholesterol, LDL, HDL and triglycerides, or in the psychopathologic parameters evaluated.18
An RCT in 15 men with angiographically proven coronary artery disease (CAD) investigated the role of aged garlic in endothelial function. Aged garlic was used because it contains antioxidant compounds that increase nitric oxide production and decrease the output of inflammatory cytokines from cultured cells. Oxidative stress and increased systemic inflammation may contribute to endothelial dysfunction. During supplementation, flow-mediated endothelium-dependent dilatation (FMD) increased significantly from the baseline and mainly in those men with lower baseline FMD. Markers of oxidant stress (plasma-oxidised LDL and peroxides), systemic inflammation (plasma C-reactive protein and interleukin-6) and endothelial activation did not change significantly during the study. The authors concluded that short-term treatment with aged garlic extract may improve endothelial function in men with CAD treated with aspirin and a statin.19
A systematic review of garlic’s effects on cardiovascular risk concluded that there are insufficient data to draw conclusions regarding garlic’s effects on clinical cardiovascular outcomes such as claudication and myocardial infarction. Garlic preparations may have a small positive short-term effect on lipids. Whether effects are sustainable beyond 3 months is unclear.20
Several studies have evaluated the efficacy of garlic in hypertension. In a multicentre, randomised, placebo-controlled, double-blind study of 47 patients with mild hypertension, garlic powder tablets, 200 mg three times a day for 12 weeks, produced significant reductions in blood pressure as well as total cholesterol and triglycerides.10 In an acute pilot study of nine patients with severe hypertension, single doses of 2400 mg of a garlic powder preparation (standardised to release 0.6% allicin) produced a significant reduction in diastolic blood pressure 5–14 h after administration.21
A meta-analysis of studies22 evaluated the efficacy of garlic on blood pressure. Only prospective, randomised studies with two or more treatment group comparisons and a duration of at least 4 weeks were included, and eight studies met the defined criteria. Six of the studies were placebo-controlled, one compared garlic with a diuretic and reserpine, and another compared garlic with bezafibrate. All but one of the studies were supposedly double-blind, but with the odour of garlic being difficult to mask, it is not clear whether the studies really were blinded. All eight studies used the same dried garlic powder in doses of 600–900 mg daily (equivalent to 1.8–2.7 g of fresh garlic daily) for 1–12 months. The pooled mean reduction in systolic blood pressure was 7.7 mmHg and the pooled mean diastolic pressure 5.0 mmHg more with garlic. However, there have not been enough trials with different garlic doses for the optimum dose to be defined.
In a systematic review,20 consistent reductions in blood pressure with garlic were not found and no effects on glucose or insulin sensitivity were found.
Peripheral artery disease
A Cochrane review assessed the effects of garlic for the treatment of peripheral arterial occlusive disease. One eligible trial (small, of short duration) found no statistically significant effect of garlic on walking distance.23
Preliminary data from in vitro animal and epidemiological studies suggest that garlic may have a protective effect in cancer development and progression. Results of epidemiological case-control studies in China24 and Italy25 suggest that garlic may reduce the risk of gastric cancer. Epidemiological studies cannot by themselves establish causal relationships and prospective data on this possible effect of garlic on cancer risk are required.
Garlic consumption has been associated with a reduced risk of colon cancer in some studies,26,27 but not others.28 However, in another study, garlic consumption was not associated with reduced risk of stomach cancer,29 breast cancer30 or prostate cancer.31 A meta-analysis of the relation between cooked garlic, raw garlic or both raw and cooked garlic on the risk of colorectal and stomach cancers showed that garlic may be associated with a protective effect against both types of cancer.32
A systematic review20 found that garlic supplementation for less than 3–5 years was not associated with decreased risk of breast, lung, gastric, colon or rectal cancer. Some case-control studies suggest that high dietary garlic consumption may be associated with decreased risks of laryngeal, gastric, colorectal and endometrial cancers and adenomatous colorectal polyps.
Garlic is being investigated for antibacterial, antifungal and antiviral activity, but current evidence is too limited to recommend garlic for the prevention of infections.
Hypersensitivity to garlic.
Pregnancy and breast-feeding
No problems have been reported. However, there have not been sufficient studies to guarantee the safety of garlic supplements in pregnancy and breast-feeding.
Unpleasant breath odour; indigestion; hypersen-sitivity reactions including contact dermatitis and asthma have been reported occasionally. A spinal haematoma (isolated report) has been attributed to the antiplatelet effects of garlic.
None reported. Theoretically, garlic could increase bleeding with anticoagulants, aspirin and antiplatelet drugs.
Garlic supplements are available in the form of tablets and capsules.
The dose is not established, but 400–1000 mg (equivalent to 2–5 g fresh garlic or one to two cloves) daily of a standardised garlic product has been used in several studies. Dietary supplements provide 400–1000 mg dried garlic daily. Standardised products may be standardised for allicin potential. However, allicin is now known not to be the only important active ingredient in garlic. One clove of fresh garlic is equivalent to 4000 µg allicin potential.
There is evidence from meta-analyses that garlic supplements reduce blood cholesterol and blood pressure. However, authors of meta-analyses have criticised studies for poor methodology, small numbers of subjects and short duration. There is preliminary evidence that garlic reduces platelet aggregation and may reduce the risk of cancer, but controlled clinical trials are needed to confirm this. Garlic has been used for centuries for antibacterial and antiviral effects. Although such effects have been demonstrated in vitro, controlled clinical studies are needed to confirm these findings.
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