Garlic and Disease Prevention- Cancer
Many in vitro and in vivo studies have suggested possible cancer-preventive effects of garlic preparations and their respective constituents. Garlic has been found to contain a large number of potent bioactive compounds with anticancer properties, largely allylsulfide derivatives, allicin. Different garlic derivatives have been reported to modulate an increasing number of molecular mechanisms in carcinogenesis, such as DNA adduct formation, mutagenesis, scavenging of free radicals, cell proliferation and differentiation as well as angiogenesis.
A recent meta-analysis of 17 studies (mostly case-control studies) reported an inverse association between high versus low garlic consumption and the risk of gastric cancer. Nevertheless, this conclusion is hindered by a number of limitations, especially related to the retrospective design of most studies included in the analysis, as well as great variations in the amount and duration of garlic intakes. In a 2009 review of the literature, Kim et al. identified 20 human studies that examined garlic intake in relation to gastric cancer risk: three intervention studies, one case-cohort study, 13 case-control studies, and three cross-sectional/ecologic studies. Using the Food and Drug Administration (FDA)’s evidence-based criteria for the scientific evaluation of health claims, the authors excluded 16 studies for methodological flaws; only four studies (two case-control,one case-cohort,and one intervention ) received moderate-to-high quality ratings . Among these four studies, garlic intake during adolescence or 20 years prior to the interview was not found to be associated with the risk of gastric cancer in one of the case-control studies in Sweden (338 gastric cancer patients and 669 control subjects). Another case-control study in Korea failed to show an association between past garlic consumption and gastric cancer in 136 people diagnosed with gastric cancer and 136 cancer-free subjects . In addition, a prospective case-cohort study in the Netherlands found no association between the use of garlic supplements (unknown composition) and gastric cancer risk. Finally, a randomized, double-blind, placebo-controlled intervention study in 3,365 subjects from the Shandong province of China found that supplementation with aged garlic extract and steam-distilled garlic oil for 7.3 years did not reduce the prevalence of precancerous gastric lesions or the incidence of gastric cancer. An updated analysis of the data collected 7.3 years after garlic supplementation ended provided further confirmation for a lack of significant reduction in gastric cancer incidence or mortality with supplemental garlic.
Helicobacter pylori (H. pylori) infection and gastric cancer: Infection with some strains of H. pylori bacteriamarkedly increases the risk of gastric cancer. Although garlic preparations and organosulfur compounds could inhibit the growth of H. pylori in the laboratory, there is little evidence to suggest that high garlic intakes or garlic supplementation may help prevent or eradicate H. pylori infection in humans. Higher intakes of garlic were not associated with a significantly lower prevalence of H. pylori infection in China or Turkey.Moreover, clinical trials using garlic cloves, aged garlic extract, steam-distilled garlic oil, garlic oil macerate, or garlic powder have not found garlic supplementation to be effective in eradicating H. pyloriinfection in humans.
A 2014 meta-analysis of prospective cohort studies in 335,923 subjects (including 4,610 colorectal cancer [CRC] cases) found no association of consuming raw or cooked garlic (three studies, four cohorts) or supplemental garlic (four studies, five cohorts) with CRC. Another recent systematic review and meta-analysis that combined data from seven cohort and seven case-control studies also failed to find a statistically significant reduction in CRC riskwith garlic intake. Yet, these results are in contrast with previous pooled analyses of data from case-control studies or from both case-control and prospective studies that reported an approximate 30% lower CRC risk in individuals with the highest garlic intakes compared to those with the lowest intakes. Inclusion of case-control studies, which are more susceptible to bias, may explain these discrepancies among meta-analyses . For information regarding different types of epidemiological studies, see the Spring/Summer 2016 LPI Research Newsletter.
A small preliminary intervention trial in 37 patients with colorectal adenomas examined whether supplementation with aged garlic extract for 12 months affected adenoma size and recurrence. Both the number and size of adenomas were significantly reduced in patients given a high dose of aged garlic extract compared to those given a much lower dose (0.16 mL/day) Larger randomized controlled trials are needed to determine whether garlic or garlic extracts can substantially reduce adenoma progression to advanced cancer and recurrence.
Other types of cancer
In a small, placebo-controlled intervention study in 50 patients with cancer (42 with liver cancer, seven with pancreatic cancer, and one with colon cancer), supplementation with 500 mg/day of aged garlic extract for six months failed to prevent quality of life deterioration caused by disease progression and chemotherapy-associated adverse effects. Yet, the active treatment limited the decline in natural killer cell count and activity that accompanies digestive cancer progression and reduces patient survival.
At present, evidence from trials is limited and results from observational studies do not suggest a role of high intakes of garlic in the prevention of cancer in humans.
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