Garlic: Blood pressure lowering effect
A general definition of hypertension is a systolic blood pressure (SBP) of 140 mm Hg or higher or a diastolic blood pressure (DBP) of 90 mm Hg or higher or both. Prevention and proper management of hypertension decreases the incidence of related morbidity and mortality. A downward shift of 3 mm Hg in SBP decreases the mortality from stroke by 8% and from ischemic heart disease by 5% (Joint National Committee, 1993). Life style modification are definitive therapy for some and adjunctive therapy for all persons with hypertension (Joint National Committee, 1997). Diets that are high in fruits, vegetables and low-fat dairy products; have been shown to reduce hypertension. Increased consumption of garlic is associated with lower incidence of hypertension in population. Based on current information, garlic powder preparations are considered for recommendation as adjuncts in the treatment of hypertensive patients [90].
Animal Studies
In experimental animals, intravenous injection of garlic extracts produced slight reductions in both systolic and diastolic pressures [91,92]. Oral administration of garlic reduced experimentally induced hypertension, bringing blood pressure back to the normal range. For example 2.5 to 25 mg per kg of alcoholic garlic extract reduced blood pressure by 10 to 50 mm Hg [93]. Blood pressure in dogs has been significantly reduced for several hours following intragastric administration of a small dose of garlic powder (as low as 2.5 mg/kg b.wt) [94]. Other animal experiments on rats and dogs also indicate a 'normalizing' effect of garlic on elevated blood pressure [93,95-98]. The antihypertensive effect of garlic in these studies has been repeatedly confirmed.
Allicin, a major constituent of garlic, was also evaluated for its antihypertensive effects. Chronic oral administration of allicin lowered blood pressure in hypertensive rats [99,100]. Allicin also caused pulmonary vasodilatation in isolated lung of rat [101]. Single as well as multiple doses of aqueous garlic extract reduced thromboxane B2 and prostaglandin E2 level and thereby reduced hypertension in '2 kidney 1-clip' model of hypertension in rat [102]. Garlic also inhibited endothelin-1 induced contraction in a dose-dependent manner in isolated rat pulmonary arteries [103].
Garlic (100 mg/kg) administration for 5 days resulted in a complete inhibition of acute hypoxic pulmonary vasoconstriction in rat [104]. There was a marked decrease in systolic blood pressure in spontaneously hypertensive rats after oral administration of single dose of garlic [97]. Prolongation of life span was also found in hypertensive rats by dietary supplementation with garlic [105].
Human study
Blood pressure lowering effect of garlic on human is given in Table 4. Leoper and DeBray recognized the hypotensive effect of garlic in 1921 [106]. Damrau (1941) has reviewed the earlier literature, including his own investigations on 26 patients [107]. Blood pressure reduction was observed in 85% of the patients, the average decline being 12.3 mm Hg systolic (SBP) and 6.5 mm Hg diastolic (DBP) blood pressure, over one-quarter of the subjects experienced a decline in SBP of 20 mm Hg or more.
Piotrowski (1948) has reviewed some of the early clinical studies in which garlic was administered under controlled conditions to hypertensive patients [108]. Two-fifths of 100 patients exhibited a 20 mm Hg or greater decline in SBP generally within 1 week after initiation of treatment with 0.6 to 1.2 g daily of a dialyzed, alcoholic garlic extract.
Studies with a dried garlic powder (Kwai tablets) showed an average decrease in blood pressure of about 9% with 0.6 g garlic powder per day [77,109] and in a randomized double blind trial, a beneficial effect of garlic on blood pressure and blood lipids in mildly hypertensive subjects was demonstrated [110]. Those reports point in the same direction, that garlic can be useful in the control of mild hypertension in many if not all cases.
Pektov (1979) has also cited several studies, mostly from the Soviet Union and Bulgaria, which indicate that garlic and its extracts exhibit antihypertensive activity [111]. Besides subjective improvement, the results of these studies indicated a moderate hypotensive effect involving a drop in SBP of 20–30 mm Hg and in DBP of 10–20 mm Hg. Another study in China (1986) on 70 hypertensive patients who were given garlic oil equivalent to 50 gm of raw garlic/day, 47 patients showed moderate to marked reduction in blood pressure [112].
There is only one meta-analysis done by Silagy and Neil (1994) [113]. Eight trials were identified all using the same dried garlic powder preparation (Kwai). Data from 415 subjects were included in the analysis. Only three trials were specifically conducted in hypertensive subjects. Of the seven trials that compared the effect of garlic with that of placebo, three showed a significant reduction in systolic blood pressure (SBP) and four in diastolic blood pressure (DBP). The overall pooled mean difference in the absolute change (from baseline to final measurement) of SBP was greater in the subjects who were treated with garlic than in those treated with placebo. For DBP the corresponding reduction in the garlic-treated subjects was slightly smaller. This meta-analysis suggest that this "garlic powder preparation may be of some clinical use in subjects with mild hypertension". However, there is still insufficient evidence to recommend it as a routine clinical therapy for the treatment of hypertensive subjects. More rigorously designed and analyzed trials are needed for firm conclusion.
Possible mechanism/s
Rashid and Khan (1985) have postulated that mechanism of antihypertensive action of garlic is due to its prostaglandin like effects, which decreases peripheral vascular resistance [92]. The gamma-glutamylcysteines are the compounds in garlic that may lower blood pressure, as indicated by their ability to inhibit angiotensin-converting enzyme in in vitro [114]. Garlic modulates the production and function of both endothelium derived relaxing and constricting factors and this may contribute to its protective effect against hypoxic pulmonary vasoconstriction [103]. Garlic elicits nitric-oxide-dependent relaxation in pulmonary arteries. This hypothesis was explained by the fact that NG-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor) abolished the vasodilatory effect of garlic [103,104]. But another study reported that pulmonary vasodilatory effect of allicin are independent of the synthesis of NO, ATP-sensitive (K+) channel, activation of cyclooxygenase enzyme [101].