5.3.1. Antibacterial Activity
Garlic has in vitro activity against many Gram-negative and Gram-positive bacteria, including species of Escherichia, Salmonella, Staphylococcus, Streptococcus, Klebsiella, Proteus, Bacillus, Clostridium, and Mycobacterium tuberculosis. Even some bacteria resistant to antibiotics, including methicil-lin-resistant Staphylococcus aureus, multidrug-resistant strains of Escherichia coli, Enterococcus spp., and Shigella spp. were sensitive to garlic (40). Activity against H. pylori is discussed in the GI effects section (see Subheading 4.2). A study in 30 subjects was done to determine activity of garlic against oral microorganisms. After using both garlic and chlorhexidine, antimicrobial activity from the subject's saliva was shown against Streptococcus mutans and no other oral microorganisms, but adverse effects were significantly higher for garlic (41). Antibacterial activity is thought to be caused by the allicin component of garlic. A characteristic unique to allicin is the low likelihood of most bacteria to develop resistance to it (40). However, more investigation should be done regarding this issue. Data is insufficient for the use of garlic to treat bacterial infections. In vitro data does not always correlate with in vivo clinical data, and such studies are not currently available.
Garlic has in vitro antifungal effects against Cryptococcus neoformans, Candida spp., Trichophyton, Epidermophyton, Microsporum, Aspergillus spp., and Mucorpusillus (40). When five volunteers consumed 10-25 mL of fresh garlic extract, urine samples had antifungal activity, but susceptibility from serum samples dropped significantly (42).
Data is also available suggesting efficacy of topical garlic on fungal infections. For tinea pedis, 1-week topical treatment with ajoene 1% twice daily resulted in mycological cure 60 days later in 100% of patients, compared to 94% for 1% topical terbinafine and 72% for 0.6% topical ajoene (43). Another study showed that 0.6% topical ajoene was as effective as 1% terbinafine cream, both applied twice daily for 1 week, for the treatment of tinea cruris and corposis. After 60 days, effectiveness (clinical plus myco-logical cure) was 73 vs 71%, respectively (44). In addition, a 0.4% cream was also shown to be effective (45). Although a topical preparation is not available commercially, it could likely be compounded.
Garlic has been shown in in vitro studies to have antiviral activity against several viruses including cytomegalovirus, influenza B, Herpes simplex virus types 1 and 2, parainfluenza virus type 3, and human rhinovirus type 2 (40). Antiviral activity is thought to be caused more by the ajoene component than the allicin component of garlic (46).
Garlic has in vitro activity against Entamoeba histolytica, Giardia lamblia, Leishmania major, Leptomonas colsoma, and Crithidia fasciculate (40,47). In vivo and clinical data is needed before garlic can be used for treatment of infections with these organisms.
In vitro and animal studies show that the organosulfur components of garlic suppress tumor incidence in breast, blood, bladder, colon, skin, uterine, esophagus, and lung cancers. Potential mechanisms include decreasing nitrosamine formation, decreased bioactivation of carcinogens, improved DNA
repair, immune stimulation, and antiproliferative effects (regulation of cell cycle progression, modification of pathways of signal transduction, and induction of apoptosis) (47,48). Other factors that may play a role in cancer prevention are cytochrome P450 enzyme stimulation, sulfur compound binding, or antioxidant activity (49). The chemical components of garlic that have shown these effects are ajoene, allicin, diallyl sulfide, diallyl disulfide, diallyl trisul-fide, SAC, and S-allylmercaptocysteine (48). Heating garlic (microwave or oven) destroys the active allyl sulfur compound formation; however, if crushed garlic is allowed to stand for 10 minutes before heating, the total loss of anticancer activity is prevented (50). Although much of the anticancer data is from in vitro and animal studies, epidemiological studies are available (51).
Several case-control studies and cohort studies were done evaluating dietary raw and cooked garlic consumption and association with colorectal cancer (52-56). The results were mixed but generally positive. One study showed an association of garlic with a reduction in the incidence of colon cancer (52). Another study showed an inverse relationship with rectal cancer in women for garlic consumers but no association in men (53). The third case-control study showed weak evidence of garlic consumption associated with a lower risk of colon cancer for men, although this was not significant, and no effect for women was shown (54). Two cohort studies (55,56) showed a nonsignificant inverse association with colon cancer, although one showed a significant inverse association when limited to just the distal colon (relative risk [RR] 0.52 [95% CI 0.3-0.93]) (55). These studies evaluating dietary garlic consumption cannot be extrapolated to garlic supplements. Only one study evaluated the effects of garlic supplements on colon and rectal cancer (57). This cohort study did not show an association between garlic supplement use and colon and rectal cancers. A meta-analysis showed that issues with the studies, including publication bias, heterogeneity of effect estimates, differences in doses, and confounding factors such as total vegetable consumption may not allow for definite conclusions (58). Overall, dietary garlic may have some efficacy in prevention of colorectal cancer, but there is not enough evidence for garlic supplements.
Two epidemiological studies show an inverse association between dietary raw and cooked garlic and gastric cancer (59,60) and one showed a slight protective effect (61). These results cannot be extrapolated to garlic supplements. One cohort study was done to examine the effects of garlic supplement use on gastric cancer. This study did not show a protective effect of garlic supplements on gastric cancer (62). In fact, there was a small, nonsignificant increase in risk. A meta-analysis showed that issues with the studies, including publication bias, heterogeneity of effect estimates, differences in doses, and confounding factors such as total vegetable consumption may not allow for definite conclusions (58). Dietary garlic may have some efficacy in the prevention of gastric cancer, but insufficient evidence exists for garlic supplements.
High dietary intake of garlic (approximately 1 clove per day) is associated with a 50% reduction in the risk of developing prostate cancer (63). Another study showed that a reduced risk of prostate cancer was associated with both dietary garlic (odds ratio [OR] 0.64; 95% CI 0.38-1.09) and garlic supplements (OR 0.68; 95% CI 0.41-1.1), although both just fell short of statistical significance (64).
Garlic supplements have been associated with an increased risk of lung carcinoma (RR 1.78; 95% CI 1.08-2.92) in a cohort study. However, this was not seen in those using garlic together with any other supplement (RR 0.93; 95% CI 0.46-1.86) (65). A case-control study showed an inverse relationship between dietary garlic consumption and the development of breast cancer (66); however, a cohort study evaluating the effects of garlic supplements did not show a protective effect (67). Insufficient epidemiological evidence exists for the effects of dietary garlic on head and neck cancers (51).
In summary, although the data is encouraging, more studies are needed before definitive conclusions can be made about the effect of garlic on the prevention or the cause (in the case of lung cancer) of cancer, especially with garlic supplements.
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