A prospective cohort study on the relationship between onion and leek consumption, garlic supplement use and the risk of colorectal carcinoma in The Netherlands

Garlic and cancer: a critical review of the epidemiologic literature

Animal and in vitro studies provide evidence of an anticarcinogenic effect of active ingredients in garlic. This review of the epidemiologic literature on garlic consumption addresses cancers of the stomach, colon, head and neck, lung, breast and prostate. Nineteen studies reported relative risk estimates for garlic consumption and cancer incidence. Site-specific case-control studies of stomach and colorectal cancer, in which multiple reports were available, suggest a protective effect of high intake of raw and/or cooked garlic. Cohort studies confirm this inverse association for colorectal cancer. Few cohort and case-control studies for other sites of cancer exist. Garlic supplements, as analyzed in four cohort studies and one case-control report, from two distinct populations, do not appear to be related to risk. Low study power, lack of variability in garlic consumption categorization within studies and poor adjustment for potential cofounders may limit the reliability of any conclusions regarding garlic supplements. However, an indication of publication bias was also found by visual inspection of a funnel plot and in a log-rank test (P = 0.004). Evidence from available studies nevertheless suggests a preventive effect of garlic consumption in stomach and colorectal cancers. The study limitations indicate the need for more definitive research and improved nutritional epidemiologic analyses of dietary data.

Vegetable and fruit consumption and risks of colon and rectal cancer in a prospective cohort study: The Netherlands Cohort Study on Diet and Cancer

The relation between vegetable and fruit consumption and colorectal cancer risk was comprehensively assessed in the Netherlands Cohort Study on Diet and Cancer using a validated 150-item food frequency questionnaire. After 6.3 years of follow-up (1986-1992), over 1,000 incident cases of colorectal cancer were registered. Using case-cohort analysis, the authors calculated rate ratios and 95% confidence intervals adjusted for age, alcohol intake, and family history of colorectal cancer. For colon cancer, no statistically significant associations with total vegetable intake or total fruit intake were found. However, among women, an inverse association was observed with vegetables and fruits combined (for the highest quintile vs. the lowest, the rate ratio was 0.66 (95% confidence interval: 0.44, 1.01)). Brassica vegetables and cooked leafy vegetables showed inverse associations for both men and women. Among women and, to a lesser extent, among men, inverse associations were stronger for distal colonic tumors than for proximal colonic tumors. For rectal cancer, no statistically significant associations were found for vegetable consumption or fruit consumption or for specific groups of vegetables and fruits; only Brassica vegetables showed a positive association in women. As in other cohort studies, the observed inverse relation between vegetable and fruit consumption and occurrence of colorectal cancer was less strong than relations reported in case-control studies.

Garlic consumption and cancer prevention: meta-analyses of colorectal and stomach cancers

BACKGROUND

Animal and in vitro studies have provided evidence of an anticarcinogenic effect of active ingredients in garlic.

OBJECTIVE

The objective was to conduct meta-analyses of the epidemiologic literature on the association between garlic consumption and risk of stomach, colon, head and neck, lung, breast, and prostate cancers.

DESIGN

Meta-analyses were conducted for all cancers mutually and separately for colorectal and stomach cancers in relation to consumption of exclusively raw garlic, cooked garlic, or both (RC garlic). Eighteen studies reported a relative risk estimate for RC garlic consumption and cancer risk.

RESULTS

In the meta-analyses of colorectal and stomach cancer, the reference categories ranged from no consumption to consumption of 3.5 g/wk, whereas the highest categories ranged from any consumption to >28.8 g/wk. The average difference between the highest and lowest categories was 16 g/wk. The random-effects relative risk (RR) estimate of colorectal cancer and RC garlic consumption, excluding garlic supplements, was 0.69 (95% CI: 0.55, 0.89). For stomach cancer, the random-effects RR estimate was 0.53 (95% CI: 0.31, 0.92). The heterogeneity among studies for the latter outcome (P: = 0.0002) indicates the questionableness of the generalizability of this summary estimate. An indication of publication bias for all cancers combined is evident from a funnel plot of RC garlic consumption and cancer risk and from the results of the Begg and Mazumdar test (P: = 0.049).

CONCLUSIONS

High intake of RC garlic may be associated with a protective effect against stomach and colorectal cancers. Heterogeneity of effect estimates, differences in dose estimation, publication bias, and possible alternative hypotheses (eg, confounding by total vegetable consumption) preclude sole reliance on summary effect estimates.

Counseling cancer patients about herbal medicine

More than half of all cancer patients now use some form of complementary/alternative medicine, yet the majority of these patients do not disclose this use to their physicians. Health care practitioners need to educate themselves about the complementary/alternative medicine products their patients are using. Eight herbal products (astragalus, essiac, Asian ginseng, Siberian ginseng, green tea, garlic, Hoxsey formula and iscador) commonly used by cancer patients are reviewed here and a list of recommended reference texts is provided. In addition, health care providers are encouraged to initiate discussions about complementary/alternative products and therapies with their patients so that they may help them make safe and informed decisions about these products. Not knowing what patients are taking is definitely a less desirable option.

Depressed p34cdc2 kinase activity and G2/M phase arrest induced by diallyl disulfide in HCT-15 cells

The present studies reveal the antiproliferative property of diallyl disulfide (DADS) in cultured human colon tumor cells (HCT-15) relative to its ability to decrease the proportion of cells in the G1 phase and increase the proportion of cells in the G2/M phase. The shift in the proportion of cells blocked in the G2/M phase increased as the concentration and duration of DADS exposure increased. Refeeding DADS-treated cells (50 microM) with complete medium without DADS resulted in a return to normal proliferation rates. Consistent with the G2/M phase arrest, DADS exposure inhibited p34cdc2 kinase activity within four hours of treatment. The maximum depression in p34cdc2 kinase activity (53%) occurred when 25 microM DADS was added to the medium. The present studies suggest that depressed p34cdc2 kinase activity is likely one of the early cellular events that may account for the antiproliferative property of DADS.

Ajoene, a compound of garlic, induces apoptosis in human promyeloleukemic cells, accompanied by generation of reactive oxygen species and activation of nuclear factor kappaB

The pharmacological role of garlic in prevention and treatment of cancer has received increasing attention, but thorough investigations into the molecular mechanisms of action of garlic compounds are rare. The present study demonstrates that ajoene, a major compound of garlic induces apoptosis in human leukemic cells, but not in peripheral mononuclear blood cells of healthy donors. The effect was dose and time dependent. Apoptosis was judged by three criteria, morphology of cells, quantification of subdiploid DNA content by flow cytometry, and detection of DNA fragmentation by gel electrophoresis. Ajoene increased the production of intracellular peroxide in a dose- and time-dependent fashion, which could be partially blocked by preincubation of the human leukemic cells with the antioxidant N-acetylcysteine. Interestingly, N-acetylcysteine-treated cells showed a 50% loss of ajoene-induced apoptosis. Moreover, ajoene was demonstrated to activate nuclear translocation of the transcription factor nuclear factor kappaB, an effect that was abrogated in N-acetylcysteine-loaded cells. These results suggested that ajoene might induce apoptosis in human leukemic cells via stimulation of peroxide production and activation of nuclear factor kappaB. This is a novel aspect in the biological profile of this garlic compound and an important step in elucidating the underlying molecular mechanisms of its antitumor action.

Micronutrients, dietary questionnaires and cancer

There is increasing evidence that many chemicals, although present in the diet at only low levels, play an important role in protection against cancer. Micronutrients are defined as nutrients present in the body in amounts less than 0.005% of body weight. Some micronutrients suggested to play a protective role in cancer are beta-carotene, vitamin E and vitamin C. In addition to those chemicals with an established role in nutrition, there is also a less well-defined group of chemicals, often referred to as phytochemicals, which may prove even more important. Examples here are a group of sulphur-containing chemicals present in brassicaceous vegetables, such as broccoli and cabbage, that appear to be very effective anticarcinogens. Epidemiology will be essential in accurately defining the role of phytochemicals and micronutrients in cancer. However, the large prospective studies that would be most desirable increasingly utilize food frequency questionnaires containing a limited number of questions. Such an approach has been well validated for macronutrients. However, there is often less accurate information available on micronutrients and phytochemicals, and the food combinations necessary (and acceptable) for macronutrients may be inappropriate for these other factors. It would be most desirable that fruits and vegetables are individually itemized or grouped according to plant families rather than macronutrient status. This may be the most appropriate way of identifying potentially novel protective factors in the diets of countries such as Morocco or New Zealand.

S-allyl cysteine inhibits nitrosomorpholine formation and bioactivation

Water extracts of garlic, deodorized garlic powder, and onions, but not leeks, were found to significantly (p < 0.05) reduce the in vitro formation of N-nitrosomorpholine (NMOR), a known liver carcinogen. Addition of increasing quantities (20, 40, and 80 mM) of S-allyl cysteine (SAC), a water-soluble compound in processed garlic, depressed NMOR formation by 16%, 27%, and 43%, respectively (p < 0.05). The ability of SAC to block NMOR formation decreased as the NaNO7 and morpholine concentrations increased. SAC and its non-allyl analog S-propyl cysteine effectively blocked NMOR formation. SAC and S-propyl cysteine were less effective than isomolar cysteine in reducing NMOR formation (p < 0.05). The oil-soluble sulfur compounds diallyl disulfide (DADS), dipropyl disulfide, and diallyl sulfide were ineffective inhibitors of NMOR generation (p > 0.05). SAC and DADS reduced the mutagenicity of NMOR in Salmonella typhimurium TA100 (p < 0.05). SAC at 70 mumol/plate reduced the number of histidine revertants per plate by 51% (p < 0.05), whereas DADS at 0.12 mumol/plate reduced mutant colony number by 76% (p < 0.05). SAC and DADS were more effective than isomolar cysteine in reducing NMOR mutagenicity (p < 0.05). The ability of sulfur compounds in garlic and onions to depress nitrosamine formation and bioactivation in these studies is consistent with epidemiologic evidence that higher intake of allium plants is associated with a reduction in the risks of some cancers.