The fact that only 5–10% of all cancer cases are due to genetic defects and that the remaining 90–95% are due to environment and lifestyle provides major opportunities for preventing cancer. Because tobacco, diet, infection, obesity, and other factors contribute approximately 25–30%, 30–35%, 15–20%, 10–20%, and 10–15%, respectively, to the incidence of all cancer deaths in the USA, it is clear how we can prevent cancer. Almost 90% of patients diagnosed with lung cancer are cigarette smokers; and cigarette smoking combined with alcohol intake can synergistically contribute to tumorigenesis. Similarly, smokeless tobacco is responsible for 400,000 cases (4% of all cancers) of oral cancer worldwide. Thus avoidance of tobacco products and minimization of alcohol consumption would likely have a major effect on cancer incidence.
Infection by various bacteria and viruses (Fig. 6) is another very prominent cause of various cancers. Vaccines for cervical cancer and HCC should help prevent some of these cancers, and a cleaner environment and modified lifestyle behavior would be even more helpful in preventing infection-caused cancers.
The first FDA approved chemopreventive agent was tamoxifen, for reducing the risk of breast cancer. This agent was found to reduce the breast cancer incidence by 50% in women at high risk. With tamoxifen, there is an increased risk of serious side effects such as uterine cancer, blood clots, ocular disturbances, hypercalcemia, and stroke (http://www.fda.gov/cder/foi/appletter/1998/17970s40.pdf). Recently it has been shown that a osteoporosis drug raloxifene is as effective as tamoxifen in preventing estrogen-receptor-positive, invasive breast cancer but had fewer side effects than tamoxifen. Though it is better than tamoxifen with respect to side effects, it can cause blood clots and stroke. Other potential side effects of raloxifene include hot flashes, leg cramps, swelling of the legs and feet, flu-like symptoms, joint pain, and sweating (http://www.fda.gov/bbs/topics/NEWS/2007/NEW01698.html).
The second chemopreventive agent to reach to clinic was finasteride, for prostate cancer, which was found to reduce incidence by 25% in men at high risk. The recognized side effects of this agent include erectile dysfunction, lowered sexual desire, impotence and gynecomastia (http://www.cancer.org/docroot/cri/content/cri_2_4_2x_can_prostate_cancer_be_prevented_36.asp). Celecoxib, a COX-2 inhibitor is another approved agent for prevention of familial adenomatous polyposis (FAP). However, the chemopreventive benefit of celecoxib is at the cost of its serious cardiovascular harm (http://www.fda.gov/cder/drug/infopage/cox2/NSAIDdecisionMemo.pdf). The serious side effects of the FDA approved chemopreventive drugs is an issue of particular concern when considering long-term administration of a drug to healthy people who may or may not develop cancer. This clearly indicates the need for agents, which are safe and efficacious in preventing cancer. Diet derived natural products will be potential candidates for this purpose.
Diet, obesity, and metabolic syndrome are very much linked to various cancers and may account for as much as 30–35% of cancer deaths, indicating that a reasonably good fraction of cancer deaths can be prevented by modifying the diet. Extensive research has revealed that a diet consisting of fruits, vegetables, spices, and grains has the potential to prevent cancer (Fig. 8). The specific substances in these dietary foods that are responsible for preventing cancer and the mechanisms by which they achieve this have also been examined extensively. Various phytochemicals have been identified in fruits, vegetables, spices, and grains that exhibit chemopreventive potential (Fig. 9), and numerous studies have shown that a proper diet can help protect against cancer (46, 67–69). Below is a description of selected dietary agents and diet-derived phytochemicals that have been studied extensively to determine their role in cancer prevention.
Fruits and Vegetables
The protective role of fruits and vegetables against cancers that occur in various anatomical sites is now well supported (46,69). In 1966, Wattenberg (70) proposed for the first time that the regular consumption of certain constituents in fruits and vegetables might provide protection from cancer. Doll and Peto (21) showed that 75–80% of cancer cases diagnosed in the USA in 1981 might have been prevented by lifestyle changes. According to a 1997 estimate, approximately 30–40% of cancer cases worldwide were preventable by feasible dietary means (http://www.dietandcancerreportorg/?p=ER). Several studies have addressed the cancer chemopreventive effects of the active components derived from fruits and vegetables.
More than 25,000 different phytochemicals have been identified that may have potential against various cancers. These phytochemicals have advantages because they are safe and usually target multiple cell-signaling pathways (71). Major chemopreventive compounds identified from fruits and vegetables includes carotenoids, vitamins, resveratrol, quercetin, silymarin, sulphoraphane and indole-3-carbinol.
Various natural carotenoids present in fruits and vegetables were reported to have anti-inflammatory and anticarcinogenic activity. Lycopene is one of the main carotenoids in the regional Mediterranean diet and can account for 50% of the carotenoids in human serum. Lycopene is present in fruits, including watermelon, apricots, pink guava, grapefruit, rosehip, and tomatoes. A wide variety of processed tomato-based products account for more than 85% of dietary lycopene. The anticancer activity of lycopene has been demonstrated in both in vitro and in vivo tumor models as well as in humans. The proposed mechanisms for the anticancer effect of lycopene involve ROS scavenging, up-regulation of detoxification systems, interference with cell proliferation, induction of gap-junctional communication, inhibition of cell-cycle progression, and modulation of signal transduction pathways. Other carotenoids reported to have anticancer activity include beta-carotene, alpha-carotene, lutein, zeaxanthin, beta-cryptoxanthin, fucoxanthin, astaxanthin, capsanthin, crocetin, and phytoene (72).
The stilbene resveratrol has been found in fruits such as grapes, peanuts, and berries. Resveratrol exhibits anticancer properties against a wide variety of tumors, including lymphoid and myeloid cancers, multiple myeloma, and cancers of the breast, prostate, stomach, colon, and pancreas. The growth-inhibitory effects of resveratrol are mediated through cell-cycle arrest; induction of apoptosis via Fas/CD95, p53, ceramide activation, tubulin polymerization, mitochondrial and adenylyl cyclase pathways; up-regulation of p21 p53 and Bax; down-regulation of survivin, cyclin D1, cyclin E, Bcl-2, Bcl-xL, and cellular inhibitor of apoptosis proteins; activation of caspases; suppression of nitric oxide synthase; suppression of transcription factors such as NF-κB, AP-1, and early growth response-1; inhibition of cyclooxygenase-2 (COX-2) and lipoxygenase; suppression of adhesion molecules; and inhibition of angiogenesis, invasion, and metastasis. Limited data in humans have revealed that resveratrol is pharmacologically safe. As a nutraceutical, resveratrol is commercially available in the USA and Europe in 50 µg to 60 mg doses. Currently, structural analogues of resveratrol with improved bioavailability are being pursued as potential chemopreventive and therapeutic agents for cancer (73).
The flavone quercetin (3,3′,4′,5,7-pentahydroxyflavone), one of the major dietary flavonoids, is found in a broad range of fruits, vegetables, and beverages such as tea and wine, with a daily intake in Western countries of 25–30 mg. The antioxidant, anti-inflammatory, antiproliferative, and apoptotic effects of the molecule have been largely analyzed in cell culture models, and it is known to block NF-κB activation. In animal models, quercetin has been shown to inhibit inflammation and prevent colon and lung cancer. A phase 1 clinical trial indicated that the molecule can be safely administered and that its plasma levels are sufficient to inhibit lymphocyte tyrosine kinase activity. Consumption of quercetin in onions and apples was found to be inversely associated with lung cancer risk in Hawaii. The effect of onions was particularly strong against squamous cell carcinoma. In another study, an increased plasma level of quercetin after a meal of onions was accompanied by increased resistance to strand breakage in lymphocytic DNA and decreased levels of some oxidative metabolites in the urine (74).
The flavonoid silymarin (silybin, isosilybin, silychristin, silydianin, and taxifolin) is commonly found in the dried fruit of the milk thistle plant Silybum marianum. Although silymarin’s role as an antioxidant and hepatoprotective agent is well known, its role as an anticancer agent is just emerging. The anti-inflammatory effects of silymarin are mediated through suppression of NF-κB-regulated gene products, including COX-2, lipoxygenase (LOX), inducible NO synthase, TNF, and IL-1. Numerous studies have indicated that silymarin is a chemopreventive agent in vivo against various carcinogens/tumor promoters, including UV light, 7,12-dimethylbenz(a)anthracene (DMBA), phorbol 12-myristate 13-acetate, and others. Silymarin has also been shown to sensitize tumors to chemotherapeutic agents through down-regulation of the MDR protein and other mechanisms. It binds to both estrogen and androgen receptors and down-regulates prostate specific antigen. In addition to its chemopreventive effects, silymarin exhibits activity against tumors (e.g., prostate and ovary) in rodents. Various clinical trials have indicated that silymarin is bioavailable and pharmacologically safe. Studies are now in progress to demonstrate the clinical efficacy of silymarin against various cancers (75).
The flavonoid indole-3-carbinol (I3C) is present in vegetables such as cabbage, broccoli, brussels sprout, cauliflower, and daikon artichoke. The hydrolysis product of I3C metabolizes to a variety of products, including the dimer 3,3′-diindolylmethane. Both I3C and 3,3′-diindolylmethane exert a variety of biological and biochemical effects, most of which appear to occur because I3C modulates several nuclear transcription factors. I3C induces phase 1 and phase 2 enzymes that metabolize carcinogens, including estrogens. I3C has also been found to be effective in treating some cases of recurrent respiratory papillomatosis and may have other clinical uses (76).
Sulforaphane (SFN) is an isothiothiocyanate found in cruciferous vegetables such as broccoli. Its chemopreventive effects have been established in both in vitro and in vivo studies. The mechanisms of action of SFN include inhibition of phase 1 enzymes, induction of phase 2 enzymes to detoxify carcinogens, cell-cycle arrest, induction of apoptosis, inhibition of histone deacetylase, modulation of the MAPK pathway, inhibition of NF-κB, and production of ROS. Preclinical and clinical studies of this compound have suggested its chemopreventive effects at several stages of carcinogenesis. In a clinical trial, SFN was given to eight healthy women an hour before they underwent elective reduction mammoplasty. Induction in NAD(P)H/quinone oxidoreductase and heme oxygenase-1 was observed in the breast tissue of all patients, indicating the anticancer effect of SFN (77).
Teas and Spices
Spices are used all over the world to add flavor, taste, and nutritional value to food. A growing body of research has demonstrated that phytochemicals such as catechins (green tea), curcumin (turmeric), diallyldisulfide (garlic), thymoquinone (black cumin) capsaicin (red chili), gingerol (ginger), anethole (licorice), diosgenin (fenugreek) and eugenol (clove, cinnamon) possess therapeutic and preventive potential against cancers of various anatomical origins. Other phytochemicals with this potential include ellagic acid (clove), ferulic acid (fennel, mustard, sesame), apigenin (coriander, parsley), betulinic acid (rosemary), kaempferol (clove, fenugreek), sesamin (sesame), piperine (pepper), limonene (rosemary), and gambogic acid (kokum). Below is a description of some important phytochemicals associated with cancer.
More than 3,000 studies have shown that catechins derived from green and black teas have potential against various cancers. A limited amount of data are also available from green tea polyphenol chemoprevention trials. Phase 1 trials of healthy volunteers have defined the basic biodistribution patterns, pharmacokinetic parameters, and preliminary safety profiles for short-term oral administration of various green tea preparations. The consumption of green tea appears to be relatively safe. Among patients with established premalignant conditions, green tea derivatives have shown potential efficacy against cervical, prostate, and hepatic malignancies without inducing major toxic effects. One novel study determined that even persons with solid tumors could safely consume up to 1 g of green tea solids, the equivalent of approximately 900 ml of green tea, three times daily. This observation supports the use of green tea for both cancer prevention and treatment (78).
Curcumin is one of the most extensively studied compounds isolated from dietary sources for inhibition of inflammation and cancer chemoprevention, as indicated by almost 3000 published studies. Studies from our laboratory showed that curcumin inhibited NF-κB and NF-κB-regulated gene expression in various cancer cell lines. In vitro and in vivo studies showed that this phytochemical inhibited inflammation and carcinogenesis in animal models, including breast, esophageal, stomach, and colon cancer models. Other studies showed that curcumin inhibited ulcerative proctitis and Crohn’s disease, and one showed that curcumin inhibited ulcerative colitis in humans. Another study evaluated the effect of a combination of curcumin and piperine in patients with tropical pancreatitis. One study conducted in patients with familial adenomatous polyposis showed that curcumin has a potential role in inhibiting this condition. In that study, all five patients were treated with curcumin and quercetin for a mean of 6 months and had a decreased polyp number (60.4%) and size (50.9%) from baseline with minimal adverse effects and no laboratory-determined abnormalities.
The pharmacodynamic and pharmacokinetic effects of oral Curcuma extract in patients with colorectal cancer have also been studied. In a study of patients with advanced colorectal cancer refractory to standard chemotherapies, 15 patients received Curcuma extract daily for up to 4 months. Results showed that oral Curcuma extract was well tolerated, and dose-limiting toxic effects were not observed. Another study showed that in patients with advanced colorectal cancer, a daily dose of 3.6 g of curcumin engendered a 62% decrease in inducible prostaglandin E2 production on day 1 and a 57% decrease on day 29 in blood samples taken 1 h after dose administration.
An early clinical trial with 62 cancer patients with external cancerous lesions at various sites (breast, 37; vulva, 4; oral, 7; skin, 7; and others, 11) reported reductions in the sense of smell (90% of patients), itching (almost all patients), lesion size and pain (10% of patients), and exudates (70% of patients) after topical application of an ointment containing curcumin. In a phase 1 clinical trial, a daily dose of 8,000 mg of curcumin taken by mouth for 3 months resulted in histologic improvement of precancerous lesions in patients with uterine cervical intraepithelial neoplasm (one of four patients), intestinal metaplasia (one of six patients), bladder cancer (one of two patients), and oral leukoplakia (two of seven patients).
Results from another study conducted by our group showed that curcumin inhibited constitutive activation of NF-κB, COX-2, and STAT3 in peripheral blood mononuclear cells from the 29 multiple myeloma patients enrolled in this study. Curcumin was given in doses of 2, 4, 8, or 12 g/day orally. Treatment with curcumin was well tolerated with no adverse events. Of the 29 patients, 12 underwent treatment for 12 weeks and 5 completed 1 year of treatment with stable disease. Other studies from our group showed that curcumin inhibited pancreatic cancer. Curcumin down-regulated the expression of NF-κB, COX-2, and phosphorylated STAT3 in peripheral blood mononuclear cells from patients (most of whom had baseline levels considerably higher than those found in healthy volunteers). These studies showed that curcumin is a potent anti-inflammatory and chemopreventive agent. A detailed description of curcumin and its anticancer properties can be found in one of our recent reviews (79).
Diallyldisulfide, isolated from garlic, inhibits the growth and proliferation of a number of cancer cell lines including colon, breast, glioblastoma, melanoma, and neuroblastoma cell lines. Recent studies showed that this compound induces apoptosis in Colo 320 DM human colon cancer cells by inhibiting COX-2, NF-κB, and ERK-2. It has been shown to inhibit a number of cancers including dimethylhydrazine-induced colon cancer, benzo[a]pyrene-induced neoplasia, and glutathione S-transferase activity in mice; benzo[a]pyrene-induced skin carcinogenesis in mice; N-nitrosomethylbenzylamine-induced esophageal cancer in rats; N-nitrosodiethylamine-induced forestomach neoplasia in female A/J mice; aristolochic acid-induced forestomach carcinogenesis in rats; diethylnitrosamine-induced glutathione S-transferase positive foci in rat liver; 2-amino-3-methylimidazo[4,5-f]quinoline-induced hepatocarcinogenesis in rats; and diethylnitrosamine-induced liver foci and hepatocellular adenomas in C3H mice. Diallyldisulfide has also been shown to inhibit mutagenesis or tumorigenesis induced by vinyl carbamate and N-nitrosodimethylamine; aflatoxin B1-induced and N-nitrosodiethylamine-induced liver preneoplastic foci in rats; arylamine N-acetyltransferase activity and 2-aminofluorene-DNA adducts in human promyelocytic leukemia cells; DMBA-induced mouse skin tumors; N-nitrosomethylbenzylamine-induced mutation in rat esophagus; and diethylstilbesterol-induced DNA adducts in the breasts of female ACI rats.
Diallyldisulfide is believed to bring about an anticarcinogenic effect through a number of mechanisms, such as scavenging of radicals; increasing gluathione levels; increasing the activities of enzymes such as glutathione S-transferase and catalase; inhibiting cytochrome p4502E1 and DNA repair mechanisms; and preventing chromosomal damage (80).
The chemotherapeutic and chemoprotective agents from black cumin include thymoquinone (TQ), dithymoquinone (DTQ), and thymohydroquinone, which are present in the oil of this seed. TQ has antineoplastic activity against various tumor cells. DTQ also contributes to the chemotherapeutic effects of Nigella sativa. In vitro study results indicated that DTQ and TQ are equally cytotoxic to several parental cell lines and to their corresponding multidrug-resistant human tumor cell lines. TQ induces apoptosis by p53-dependent and p53-independent pathways in cancer cell lines. It also induces cell-cycle arrest and modulates the levels of inflammatory mediators. To date, the chemotherapeutic potential of TQ has not been tested, but numerous studies have shown its promising anticancer effects in animal models. TQ suppresses carcinogen-induced forestomach and skin tumor formation in mice and acts as a chemopreventive agent at the early stage of skin tumorigenesis. Moreover, the combination of TQ and clinically used anticancer drugs has been shown to improve the drug’s therapeutic index, prevents nontumor tissues from sustaining chemotherapy-induced damage, and enhances the antitumor activity of drugs such as cisplatin and ifosfamide. A very recent report from our own group established that TQ affects the NF-κB signaling pathway by suppressing NF-κB and NF-κB-regulated gene products (81).
The phenolic compound capsaicin (t8-methyl-N-vanillyl-6-nonenamide), a component of red chili, has been extensively studied. Although capsaicin has been suspected to be a carcinogen, a considerable amount of evidence suggests that it has chemopreventive effects. The antioxidant, anti-inflammatory, and antitumor properties of capsaicin have been established in both in vitro and in vivo systems. For example, showed that capsaicin can suppress the TPA-stimulated activation of NF-κB and AP-1 in cultured HL-60 cells. In addition, capsaicin inhibited the constitutive activation of NF-κB in malignant melanoma cells. Furthermore, capsaicin strongly suppressed the TPA-stimulated activation of NF-κB and the epidermal activation of AP-1 in mice. Another proposed mechanism of action of capsaicin is its interaction with xenobiotic metabolizing enzymes, involved in the activation and detoxification of various chemical carcinogens and mutagens. Metabolism of capsaicin by hepatic enzymes produces reactive phenoxy radical intermediates capable of binding to the active sites of enzymes and tissue macromolecules.
Capsaicin can inhibit platelet aggregation and suppress calcium-ionophore–stimulated proinflammatory responses, such as the generation of superoxide anion, phospholipase A2 activity, and membrane lipid peroxidation in macrophages. It acts as an antioxidant in various organs of laboratory animals. Anti-inflammatory properties of capsaicin against carcinogen-induced inflammation have also been reported in rats and mice. Capsaicin has exerted protective effects against ethanol-induced gastric mucosal injury, hemorrhagic erosion, lipid peroxidation, and myeloperoxidase activity in rats that was associated with suppression of COX-2. While lacking intrinsic tumor-promoting activity, capsaicin inhibited TPA-promoted mouse skin papillomagenesis (82).
Gingerol, a phenolic substance mainly present in the spice ginger (Zingiber officinale Roscoe), has diverse pharmacologic effects including antioxidant, antiapoptotic, and anti-inflammatory effects. Gingerol has been shown to have anticancer and chemopreventive properties, and the proposed mechanisms of action include the inhibition of COX-2 expression by blocking of the p38 MAPK–NF-κB signaling pathway. A detailed report on the cancer-preventive ability of gingerol was presented in a recent review by Shukla and Singh (83).
Anethole, the principal active component of the spice fennel, has shown anticancer activity. In 1995, Al-Harbi et al. (84) studied the antitumor activity of anethole against Ehrlich ascites carcinoma induced in a tumor model in mice. The study revealed that anethole increased survival time, reduced tumor weight, and reduced the volume and body weight of the EAT-bearing mice. It also produced a significant cytotoxic effect in the EAT cells in the paw, reduced the levels of nucleic acids and MDA, and increased NP-SH concentrations.
The histopathological changes observed after treatment with anethole were comparable to those after treatment with the standard cytotoxic drug cyclophosphamide. The frequency of micronuclei occurrence and the ratio of polychromatic erythrocytes to normochromatic erythrocytes showed anethole to be mitodepressive and nonclastogenic in the femoral cells of mice. In 1996, Sen et al., (85) studied the NF-κB inhibitory activity of a derivative of anethole and anetholdithiolthione. Their study results showed that anethole inhibited H2O2, phorbol myristate acetate or TNF alpha induced NF-κB activation in human jurkat T-cells (86) studied the anticarcinogenic activity of anethole trithione against DMBA induced in a rat mammary cancer model. The study results showed that this phytochemical inhibited mammary tumor growth in a dose-dependent manner.
Nakagawa and Suzuki (87) studied the metabolism and mechanism of action of trans-anethole (anethole) and the estrogenlike activity of the compound and its metabolites in freshly isolated rat hepatocytes and cultured MCF-7 human breast cancer cells. The results suggested that the biotransformation of anethole induces a cytotoxic effect at higher concentrations in rat hepatocytes and an estrogenic effect at lower concentrations in MCF-7 cells on the basis of the concentrations of the hydroxylated intermediate, 4OHPB. Results from preclinical studies have suggested that the organosulfur compound anethole dithiolethione may be an effective chemopreventive agent against lung cancer. Lam et al, (88) conducted a phase 2b trial of anethole dithiolethione in smokers with bronchial dysplasia. The results of this clinical trial suggested that anethole dithiolethione is a potentially efficacious chemopreventive agent against lung cancer.
Diosgenin, a steroidal saponin present in fenugreek, has been shown to suppress inflammation, inhibit proliferation, and induce apoptosis in various tumor cells. Research during the past decade has shown that diosgenin suppresses proliferation and induces apoptosis in a wide variety of cancer cells lines. Antiproliferative effects of diosgenin are mediated through cell-cycle arrest, disruption of Ca2+ homeostasis, activation of p53, release of apoptosis-inducing factor, and modulation of caspase-3 activity. Diosgenin also inhibits azoxymethane-induced aberrant colon crypt foci, has been shown to inhibit intestinal inflammation, and modulates the activity of LOX and COX-2. Diosgenin has also been shown to bind to the chemokine receptor CXCR3, which mediates inflammatory responses. Results from our own laboratory have shown that diosgenin inhibits osteoclastogenesis, cell invasion, and cell proliferation through Akt down-regulation, IκB kinase activation, and NF-κB-regulated gene expression (89).
Eugenol is one of the active components of cloves. Studies conducted by Ghosh et al. (90) showed that eugenol suppressed the proliferation of melanoma cells. In a B16 xenograft study, eugenol treatment produced a significant tumor growth delay, an almost 40% decrease in tumor size, and a 19% increase in the median time to end point. Of more importance, 50% of the animals in the control group died of metastatic growth, whereas none in the eugenol treatment group showed any signs of cell invasion or metastasis. In 1994, Sukumaran et al. (91) showed that eugenol DMBA induced skin tumors in mice. The same study showed that eugenol inhibited superoxide formation and lipid peroxidation and the radical scavenging activity that may be responsible for its chemopreventive action. Studies conducted by Imaida et al. (92) showed that eugenol enhanced the development of 1,2-dimethylhydrazine-induced hyperplasia and papillomas in the forestomach but decreased the incidence of 1-methyl-1-nitrosourea-induced kidney nephroblastomas in F344 male rats.
Another study conducted by Pisano et al. (93) demonstrated that eugenol and related biphenyl (S)-6,6′-dibromo-dehydrodieugenol elicit specific antiproliferative activity on neuroectodermal tumor cells, partially triggering apoptosis. In 2003, Kim et al. (94) showed that eugenol suppresses COX-2 mRNA expression (one of the main genes implicated in the processes of inflammation and carcinogenesis) in HT-29 cells and lipopolysaccharide-stimulated mouse macrophage RAW264.7 cells. Another study by Deigner et al. (95) showed that 1′-hydroxyeugenol is a good inhibitor of 5-lipoxygenase and Cu(2+)-mediated low-density lipoprotein oxidation. The studies by Rompelberg et al. (96) showed that in vivo treatment of rats with eugenol reduced the mutagenicity of benzopyrene in the Salmonella typhimurium mutagenicity assay, whereas in vitro treatment of cultured cells with eugenol increased the genotoxicity of benzopyrene.
The major wholegrain foods are wheat, rice, and maize; the minor ones are barley, sorghum, millet, rye, and oats. Grains form the dietary staple for most cultures, but most are eaten as refined-grain products in Westernized countries (97). Whole grains contain chemopreventive antioxidants such as vitamin E, tocotrienols, phenolic acids, lignans, and phytic acid. The antioxidant content of whole grains is less than that of some berries but is greater than that of common fruits or vegetables (98). The refining process concentrates the carbohydrate and reduces the amount of other macronutrients, vitamins, and minerals because the outer layers are removed. In fact, all nutrients with potential preventive actions against cancer are reduced. For example, vitamin E is reduced by as much as 92% (99).
Wholegrain intake was found to reduce the risk of several cancers including those of the oral cavity, pharynx, esophagus, gallbladder, larynx, bowel, colorectum, upper digestive tract, breasts, liver, endometrium, ovaries, prostate gland, bladder, kidneys, and thyroid gland, as well as lymphomas, leukemias, and myeloma (100,101). Intake of wholegrain foods in these studies reduced the risk of cancers by 30–70% (102).
How do whole grains reduce the risk of cancer? Several potential mechanisms have been described. For instance, insoluble fibers, a major constituent of whole grains, can reduce the risk of bowel cancer (103). Additionally, insoluble fiber undergoes fermentation, thus producing short-chain fatty acids such as butyrate, which is an important suppressor of tumor formation (104). Whole grains also mediate favorable glucose response, which is protective against breast and colon cancers (105). Also, several phytochemicals from grains and pulses were reported to have chemopreventive action against a wide variety of cancers. For example, isoflavones (including daidzein, genistein, and equol) are nonsteroidal diphenolic compounds that are found in leguminous plants and have antiproliferative activities. Findings from several, but not all, studies have shown significant correlations between an isoflavone-rich soy-based diet and reduced incidence of cancer or mortality from cancer in humans. Our laboratory has shown that tocotrienols, but not tocopherols, can suppress NF-κB activation induced by most carcinogens, thus leading to suppression of various genes linked with proliferation, survival, invasion, and angiogenesis of tumors (106).
Observational studies have suggested that a diet rich in soy isoflavones (such as the typical Asian diet) is one of the most significant contributing factors for the lower observed incidence and mortality of prostate cancers in Asia. On the basis of findings about diet and of urinary excretion levels associated with daidzein, genistein, and equol in Japanese subjects compared with findings in American or European subjects, the isoflavonoids in soy products were proposed to be the agents responsible for reduced cancer risk. In addition to its effect on breast cancer, genistein and related isoflavones also inhibit cell growth or the development of chemically induced cancers in the stomach, bladder, lung, prostate, and blood (107).
Although controversial, the role of vitamins in cancer chemoprevention is being evaluated increasingly. Fruits and vegetables are the primary dietary sources of vitamins except for vitamin D. Vitamins, especially vitamins C, D, and E, are reported to have cancer chemopreventive activity without apparent toxicity.
Epidemiologic study findings suggest that the anticancer/chemopreventive effects of vitamin C against various types of cancers correlate with its antioxidant activities and with the inhibition of inflammation and gap junction intercellular communication. Findings from a recent epidemiologic study showed that a high vitamin C concentration in plasma had an inverse relationship with cancer-related mortality. In 1997, expert panels at the World Cancer Research Fund and the American Institute for Cancer Research estimated that vitamin C can reduce the risk of cancers of the stomach, mouth, pharynx, esophagus, lung, pancreas, and cervix (108).
The protective effects of vitamin D result from its role as a nuclear transcription factor that regulates cell growth, differentiation, apoptosis, and a wide range of cellular mechanisms central to the development of cancer (109).
There is extensive evidence suggesting that regular physical exercise may reduce the incidence of various cancers. A sedentary lifestyle has been associated with most chronic illnesses. Physical inactivity has been linked with increased risk of cancer of the breast, colon, prostate, and pancreas and of melanoma (110). The increased risk of breast cancer among sedentary women that has been shown to be due to lack of exercise has been associated with a higher serum concentration of estradiol, lower concentration of hormone-binding globulin, larger fat masses, and higher serum insulin levels. Physical inactivity can also increase the risk of colon cancer (most likely because of an increase in GI transit time, thereby increasing the duration of contact with potential carcinogens), increase the circulating levels of insulin (promote proliferation of colonic epithelial cells), alter prostaglandin levels, depress the immune function, and modify bile acid metabolism. Additionally, men with a low level of physical activity and women with a larger body mass index were more likely to have a Ki-ras mutation in their tumors, which occurs in 30–50% of colon cancers. A reduction of almost 50% in the incidence of colon cancer was observed among those with the highest levels of physical activity (111). Similarly, higher blood testosterone and IGF-1 levels and suppressed immunity due to lack of exercise may increase the incidence of prostate cancer. One study indicated that sedentary men had a 56% and women a 72% higher incidence of melanoma than did those exercising 5–7 days per week (112).
Fasting is a type of caloric restriction (CR) that is prescribed in most cultures. Perhaps one of the first reports that CR can influence cancer incidence was published in 1940 on the formation of skin tumors and hepatoma in mice (113, 114). Since then, several reports on this subject have been published (115, 116). Dietary restriction, especially CR, is a major modifier in experimental carcinogenesis and is known to significantly decrease the incidence of neoplasms. Gross and Dreyfuss reported that a 36% restriction in caloric intake dramatically decreased radiation-induced solid tumors and/or leukemias (117, 118). Yoshida et al. (119) also showed that CR reduces the incidence of myeloid leukemia induced by a single treatment with whole-body irradiation in mice.
How CR reduces the incidence of cancer is not fully understood. CR in rodents decreases the levels of plasma glucose and IGF-1 and postpones or attenuates cancer and inflammation without irreversible adverse effects (120). Most of the studies done on the effect of CR in rodents are long-term; however, that is not possible in humans, who routinely practice transient CR. The effect that transient CR has on cancer in humans is unclear.