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). CARDIOVASCULAR DISEASE
Epidemiologic studies show an inverse correlation between garlic consumption and progression of CVD in general (Rahman & Lowe 2006).
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.
Hypertension 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 (Silagy & Neil 1994). 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 (Mulrow et al 2000). When significant reductions in blood pressure were observed, these were small. Garlic 490
Several newer trials have since been published further confirming that the effect on blood pressure is small and sometimes non-significant (Andrianova et al 2002, McMahon &Vargas 1993, Zhang etal 2001a).
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 1 52 volunteers to determine whether garlic powder supplements (Kwai 900 mg daily) directly alter plaque volumes in carotid and/or femoral arteries (Koscielny et al 1999). 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 (Siegel & Klussendorf 2000).
Hyperlipidaemia 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 (Stevinson et al 2001). 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 (Mulrow et al 2000a). 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/dL at 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 (Gardner et al 2001, Kannar et al 2001, Peleg etal 2003, Turner etal 2004, Zhang etal 2001 b, Ziaei et al 2001). According to one review, non-enteric coated tablets containing dehydrated garlic powder (standardised to 1.3% allicin) produce the most consistent results (Ulbricht & Basch 2005).
Comparative studies 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 (Holzgartner et al 1992) whereas clofibrate 500 mg was more effective than an essential oil extract of 50 g raw garlic (Arora & Arora 1981).
Commission E approves the use of garlic as an adjunct to dietary changes in the treatment of hyperlipidaemia (Blumenthal et al 2000).
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 (Ashraf et al 2005).
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 (Lawson et al 2001). 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 Antiplatelet effects of garlic are well recognised, but the dose at which this becomes significant remains uncertain. Results from a 2001 doubleblind study have identified a dose of 7.2 g/day of aged garlic extract as significantly inhibiting platelet aggregation and adhesion (Steiner et al 1996).
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