The Impact of Whole Grain Intake on Gastrointestinal Tumors: A Focus on Colorectal, Gastric, and Esophageal Cancers

Cereals are one of staple foods in human diet, mainly consumed as refined grains. Nonetheless, epidemiological data indicate that whole grain (WG) intake is inversely related to risk of type 2 diabetes, cardiovascular disease, and several cancer types, as well as to all-cause mortality. Particularly responsive to WG positive action is the gastrointestinal tract, daily exposed to bioactive food components. Herein, we shall provide an up-to-date overview on relationship between WG intake and prevention of gastrointestinal tumors, with a particular focus on colorectal, stomach, and esophagus cancers. Unlike refined counterparts, WG consumption is inversely associated with risk of these gastrointestinal cancers, most consistently with the risk of colorectal tumor. Some WG effects may be mediated by beneficial constituents (such as fiber and polyphenols) that are reduced/lost during milling process. Beside health-promoting action, WGs are still under-consumed in most countries; therefore, World Health Organization and other public/private stakeholders should cooperate to implement WG consumption in the whole population, in order to reach nutritionally effective intakes.

Rice, maize, and wheat are the most common farmed cereals with a production of 2646 million tons in 2018-2019 [4]. As a primary source of carbohydrates, cereals provide about 60% of food energy worldwide and are mostly consumed as refined grains (RGs). However, health effects of cereals are mainly attributed to whole grains (WGs), and therefore governmental authorities are increasingly encouraging WG consumption.
Scientific interest in health properties of WGs began in the late 1970s, when the surgeon Denis Parsons Burkitt, noting the difference in disease incidence in rural Africa and the UK, brought together data coming from several disciplines and launched the dietary fiber hypothesis; he and other researchers, indeed, noted that a diet highly refined and lacking WG foods might be involved in several diseases, including coronary heart disease, obesity, diabetes, dental caries, as well as in some cancer types, such as gastric and colon tumors [5]. Since then, both epidemiological and interventional studies have reported potential health effects of unrefined grains [6][7][8][9][10][11][12][13][14]. Nonetheless, most of the population does not consume WGs, much likely due to several factors, including lack of nutritional education programs, low variety and palatability of WG-containing 50%), proteins, starch (as "contamination" from endosperm), B vitamins, minerals, and several bioactive compounds, such as polyphenols ( Figure 1) [31][32][33]. During milling process of refined flours, bran and germ are removed (and used for food and non-food applications) [33,34]; as a result, RG products contain fewer nutrients than WG counterparts. For example, in refined wheat flour, pantotenic acid, folate, iron and copper content and fiber are reduced, while some vitamins, potassium, magnesium, and manganese are even lost [35]; however, this loss can be compensated by fortifications, such as mandatory folate addition in USA [36].

Whole Grains and Health
As above mentioned, compared to refined counterparts, WG are associated with benefits for human health [30]. Among mechanisms of action, effects on postprandial glycemia, appetite and ad libitum energy intake have been proposed. In a randomized crossover study enrolling twenty young subjects (10 females and 10 males; Body Mass Index (BMI) = 21.7 ± 2.2 kg/m 2 ), Kristensen and co-workers [37] reported that, with respect to refined counterparts, WG wheat bread ingestion led to increased satiety and reduced hunger, without modifying energy intake at the subsequent meals. The same group obtained similar results for WG pasta ingestion in overweight/obese (25 < BMI < 40 kg/m 2 ) subjects [38]. Accordingly, WG food intake appears to be closely associated with reduced risk of obesity: cross-sectional dietary data from NHANES 2001-12 (which included 15,280 children and 29,683 adults) documented that WG intake inversely related to BMI, waist circumference and percentage of overweight/obese individuals [20]. Besides its beneficial role in obesity, WG consumption is closely associated with reduced risk of other chronic diseases, including cardiovascular disease, type II diabetes, metabolic syndrome, and several cancer types [8,[10][11][12][13][14].
An important role in chronic diseases seems to be played by gut microbiota, whose composition is influenced not only by genetics and age, but also by diet [39]. A strong link between microbiota composition and food intake exists, as a consequence of long-term dietary habits [40]. In particular, high consumption of WG, vegetables and fruits is associated with greater microbial variety, while diet rich in RG and fats and low in fiber is associated with lower biodiversity [41,42]. An observational study has shown that high adherence to the Mediterranean diet (MD), a typical eating pattern of the Mediterranean basin character-ized by high consumption of cereals, fruits, vegetables, and legumes, was associated with increased levels of anti-inflammatory compounds (such as short chain fatty acids, SCFAs) in fecal samples and reduced atherogenic compounds (such as trimethylamine N-oxide) in urine samples [43]. As MD recommends daily consumption of cereals, preferably as unrefined grains [44][45][46], it is conceivable that WGs cooperate with fruits and vegetables to change microbiota composition. Accordingly, in a randomized controlled, six-week trial, high WG consumption displayed better positive effects than high RG consumption, in terms of gut microbiota and immune responses [47].
In this context, it should be underlined that, unlike RG eaters, WG consumers generally follow health and diet recommendations and ingest few, if any, non-recommended, indulgent foods. For this reason, studies on WG intake must take into account all confounders, in order to remove potential bias from data.

Dietary Fiber and Polyphenols as Functional Compounds in Whole Grains
Fiber and polyphenols (or phenolic compounds) are the main dietary bioactive compounds studied for prevention of chronic diseases; they have different chemical structures, physical and biological properties, and ability to activate distinct metabolic pathways [48,49].
The definition of dietary fiber is constantly evolving and, although AACC has proposed that it is: "the remnants of the edible part of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine" [50], the more recently accepted definition is that provided by Codex Alimentarius (CAC), i.e., "carbohydrate polymers with ten or more monomeric units, which are not hydrolyzed by the endogenous enzymes in the small intestine of humans" [51].
In WG, dietary fiber primarily derives from the outer portion of cereal kernel (although it can also be found in endosperm of some grains, like wheat and barley) and mainly consists of non-starch polysaccharides, distinguished by fermentability to SCFAs, solubility in water, viscosity, and cation exchange capacity [52,53]. Cellulose, galactomannans, xylans, xyloglucans, and lignin are part of insoluble dietary fiber, while arabinoxylans, arabinogalactans, β-glucans, and pectins of soluble dietary fiber [52]. Among cereals, wheat, triticale, and rye are rich in arabinoxylans, while oats and barley mainly contain βglucans [54] (Table 1). As it will be discussed, high dietary fiber intake improves intestinal health, increases satiety, and reduces risk of some chronic diseases, including cancer [55,56]. 1 All data are expressed as µg/g dry weight, except for dietary fiber, expressed as g/100 g of grain and total polyphenols, expressed as mg of Gallic Acid Equivalent/100 g dry weight. 2 Percentage of bound form. n.r.: not reported by authors.
Polyphenols, secondary metabolites found in plant tissues, are heterogeneous compounds, possessing one or more aromatic rings with one or more hydroxyl groups. Polyphenols can be subdivided into (i) phenolic acids, (ii) flavonoids, (iii) stilbenes, and (iv) lignans [59,64]; they can also be distinguished in soluble (free molecules in cell vacuoles) and insoluble (bound to cell wall elements, such as dietary fiber) compounds [32]. These phytochemicals are important for plant growth, defense, reproduction, and color; consequently, beyond genetics, also environmental factors significantly affect their levels, that vary greatly even between cultivars of the same species. Therefore, it is not possible to establish precise amounts of each compound in different plant-foods, and several polyphenols are still unidentified; therefore, literature data on polyphenol content in plant foods, including grains, is incomplete, difficult to compare and often contradictory. According to available data, WGs seem to contain polyphenol amounts similar to those found in fruits and vegetables, with some highly active phenolic compounds more represented (Table 1) [60][61][62]65,66]. Because of frequency of consumption [4], it has been estimated that WGs provide for about one-third of total polyphenol dietary intake [67].
Ferulic, p-cumaric, vanillic, siryngic, gallic, and caffeic acids are the most common phenolic acids of grains (Table 1) [60][61][62]65,66,68]. Some of them are present as esters or amides; this is the case of γ-oryzanol, a blend of ferulic acid esters and phytosterols, mostly found in rice [69] and avenanthramides, phenolic amides containing anthranilic acid and hydroxycinnamic acid moieties, exclusively found in oats [70]. Significant differences in phenolic acid amounts exist, depending on grain dimension and species, as well as on fiber type and content; ferulic acid, for example, is more abundant in smaller than in larger grains, and the higher the fiber content, the higher the ferulic acid content (Table 1) [61,71].
As above mentioned, WGs also represent a source of flavonoids, among which there are the two flavones apigenin and luteolin and the two flavanones naringenin and eriodictyol [60,63]. Additionally, anthocyanins have been reported in pigmented varieties of some WGs, such as barley, rice, rye, and wheat [63,72]; finally, among lignans, secoisolariciresinol is present in buckwheat and pinoresinol in oats [73].
Phenolic compounds might play a role in chronic diseases and, due to their antioxidant properties and ability to modulate specific signaling pathways involved in cell survival and death, are particularly beneficial in cancer [74][75][76]. However, physiological effects of these WG components strictly depend on their bioavailability, in turn influenced by binding to dietary fiber [77][78][79]. In cereals, most polyphenols (95%) are indeed covalently linked to polysaccharide chains of dietary fiber, mainly arabinoxylans [64]. As a consequence, although dietary fiber properties are generally attributed to non-starch polysaccharides, the "dietary fiber concept" is changing towards the "antioxidant dietary fiber concept" [80]. When gut microbiota ferments fiber, phenolic compounds are released into the intestinal lumen and absorbed by enterocytes. Non-fermented and non-absorbable polyphenols counteract the pro-oxidant effects of ingested foods, by scavenging free radicals [48], and meanwhile they synergize with bacteria-derived SCFAs in modulating cell death and differentiation [64,81]. Furthermore, dietary fiber-polyphenol association can downregulate energy metabolism, nuclear receptor signaling and lipid biosynthesis (via tumor necrosis factor-α and peroxisome proliferator-activated receptor-α), pathways involved not only in obesity, but also in cancer (especially of the gastrointestinal tract) [64,82,83].

Whole Grains and Gastrointestinal Cancers: An Overview
According to global cancer statistics, 19.3 million new cancer cases and 10 million all cancer deaths occurred in 2020 worldwide; more than one-third of cancer victims suffered from gastrointestinal tumors [84]. Based on molecular phenotype and histological characteristics, these tumors include cancers affecting upper and lower gastrointestinal tract, as well as salivary gland, liver and bile ducts, gallbladder, and exocrine pancreas [85]. Although a geographic description of cancer-and sex-specific incidence and mortality patterns exists, overall more than 60% of gastrointestinal cancer cases and deaths occurred in Asia, followed by Europe and North America [84].
Clinical management of gastrointestinal cancers remains a major challenge for clinicians, especially because most cases are diagnosed in advanced stages, when treatment options are limited [86]. A variety of etiological factors have been identified; it has been estimated that genetic defects account only for 5-10%, while harmful environmental conditions and unhealthy lifestyle represent 90-95% of risk factors [87]. Consequently, primary and secondary prevention strategies, including promotion of healthy lifestyle aimed at deeply modifying some risk behaviors (e.g., tobacco use, physical inactivity, unhealthy diet, and alcohol abuse), are particularly relevant for reducing cancer risk and outcomes.
Consumption of WGs is strongly recommended for gastrointestinal health. A large body of literature data concerning WG effects on gastrointestinal cancers are available, although WG action is not equal (and even absent) in different gastrointestinal organs. To the best of our knowledge, no epidemiological studies about WG intake and risk of gallbladder and bile duct carcinomas have been published, while only one study demonstrated inverse association between WG (and possibly bran and cereal fiber) intake and risk of hepatocellular carcinoma, the predominant histological form of primary liver cancer [88]. As emerged by a meta-analysis of case-control and cohort studies, high intake of WGs was also associated with reduced risk of pancreatic cancer [89]; nonetheless, lack of more prospective cohort studies prevents to draw robust conclusions.
Similarly, literature data on association between unrefined grains and oral cavity and oropharynx cancers are scarce, not updated and just based on few case-control and cohort studies. Some investigations highlighted that WG intake was favorably related to risk of upper aerodigestive tract cancers [90][91][92][93][94][95]. Conversely, other studies reported no [96][97][98][99] or even positive associations [100][101][102]. Due to these controversial results, data on WGs and oropharyngeal cancer risk are less consistent than those for other plant-derived foods. Finally, except for a large US prospective cohort study showing a marginal inverse relationship between WG food consumption and small intestinal cancer [103], also data referred to small bowel tumors are sparse and difficult to interpret.
Based on this evidence, we focused on colorectal, gastric, and esophageal tumors, the most diagnosed and severe gastrointestinal cancers, for which investigations are more extensive and continuously updated.

Whole Grains and Colorectal Cancer
In 2020, 1.9 million of individuals were diagnosed for colorectal cancer, the second mostly incident cancer and the third leading cause of cancer death worldwide. It has slightly higher incidence in males (1,065,960 cases) than females (865,360 cases) [84]; although incidence (10% of all cancer cases) is decreasing in developed countries, cases are increasing among younger adults, especially in USA [104].
Depending on location (proximal colon, distal colon and rectum), colorectal cancer varies in terms of etiology and sensitivity to specific risk factors [49,105]. Only 1-2% of cases have been associated with ulcerative colitis, Crohn disease and inflammatory conditions [106], while modifiable lifestyle factors, typical of industrialized countries (tobacco smoking, physical inactivity, red/processed meat and alcohol consumption, low intake of fruits and vegetables), are long-established risk factors [107].
In this context, WGs represent protective factors, as high intakes have been associated with significant decrease of cancer risk ( Table 2). In a 14-year case-control study, conducted in Northern Italy and including 11,990 patients with several cancer types (among them, 955 colon and 625 rectum tumors), multivariate odd ratios (ORs) for the highest category of WG intake (>3 day per week) were 0.5 (95% CI 0.4-0.6) and 0.6 (95% CI 0.4-0.8) for colon and rectum cancers, respectively [108]. Intriguingly, Um and collaborators found sex-related differences in terms of WG association: the prospective CPS-II Nutrition Cohort study enrolling 50,118 men and 62,031 women (1742 incident colorectal cancer cases during the follow-up) found that the highest vs. lowest quintile of WG intake was associated with 23% and 43% lower risk of colorectal and rectal cancer, respectively, among men, but no association was found for women. Moreover, authors did not find any evidence of increased risk with consumption of RGs, grain-based sweets, or desserts [109]. Several meta-analyses have reinforced potential benefits of WGs against colorectal tumors [110][111][112][113]. For example, the World Cancer Research Fund International (WCRF) Continuous Update Project (CUP) has updated the systematic review and meta-analysis (until the end of May 2015) of prospective studies reporting 17% decreased risk for each 90 g/day WG increase [110].
[117] The American Institute for Cancer Research and the World Cancer Research Fund stated that eating at least 90 g/day WG reduces colorectal cancer risk, mainly due to its high fiber content [119]. Among mechanisms involved in WG protective effects, fibermediated reduction of fecal transit time, dilution, and removal of carcinogens (especially heterocyclic amines), maintenance of epithelial cell integrity and stimulation of bacterial fermentation (and, therefore, SCFA production that inhibits colon carcinogenesis) can be identified [120,121]. Accordingly, among all fiber containing foods, WGs are most consistently associated with incidence of colorectal cancer. Indeed, two large recent prospective US cohort studies did not find any association for total dietary fiber intake, but when different food sources were examined, lower risk for colorectal tumors was observed only in high cereal (especially unrefined) consumers. Moreover, such association was observed in men, but not in women; this sex-disparity might depend on lower fiber intake registered for women (mean fiber intake of 14 g/day for women and 20.0 g/day for men) [118]. Alegria-Lertxundi and co-workers [115] investigated the relationships between food groups, diet quality and colorectal cancer risk and reported no significant differences of intake between control and patient groups for the majority of food classes, except for lower WG intake (and higher egg consumption) in tumor cases; coherently, the observed protective effects of fiber-containing foods appeared to be mainly ascribed to WGs. A recent, large US cohort analysis (with more than 10,000 incident colorectal cases and more than 15 years of followup) further confirmed that fiber from grains, but not from other sources, was associated with lower incidence, especially for distal colon and rectal cancers [116]. High fiber and WG intake after diagnosis also leads to lower death rate, and this positive association again depends on fiber sources, with cereal fiber (especially from WG) showing the strongest link [117]. These data apparently disagree with the European Prospective Investigation into Cancer and Nutrition (EPIC) study that observed a significant lower risk of CRC in higher total fiber consumers [122,123]. Such a discrepancy may depend on less fiber in a typical American diet (with respect to the European one), as well as less proportional contribution of WG foods to total dietary fiber intake; indeed, about 39% of dietary fiber derives from grain foods containing no WGs, but RGs that have few amounts of fiber and are consumed in large quantities [124]. Therefore, further studies are necessary to evaluate dose-response relationship and influence of different fiber sources, taking into account that range of fiber intake widely varies depending on the examined population.
Concerning phenolic compounds, these phytochemicals exert anti-cancer activities in colon-cancer cells, mainly by inducing cell-cycle arrest and apoptosis. Just an example, ferulic and p-coumaric acids modulate S and G2/M phase transitions, respectively [125]; the two compounds also inhibit cancer cell proliferation, by inhibiting expression of epidermal growth factor receptor, one of the most relevant biomarkers in colorectal cancer [126], and related mitogenic signaling pathways [127,128]. Likewise, in human colon cancer cells, secoisolariciresinol diglycoside and its metabolites (enterolactone and enterodiol) induce S-phase cell cycle arrest, by modulating key regulatory proteins (cyclin A and cyclindependent kinase 4) [129][130][131]. By possessing estrogenic activity, some flavonoids (such as apigenin, naringenin, luteolin, and eriodictyol) contribute to colon cancer prevention, through activation of estrogen receptor-β in colonocytes [132][133][134][135]. Finally, some miRNAs involved in colorectal cancer are sensitive to phenolic compounds: for example, miRNA384 is up-regulated by luteolin, thus resulting in decreased expression levels of pleiotrophin, a cytokine upregulated in colorectal tumors [136][137][138].
In order to overcome challenges in polyphenol delivery to target tissues, recent studies have attempted to find novel strategies for improving bioavailability and anti-tumor efficacy of these phytochemicals. For example, the novel stable ferulic derivative tributyltin(IV) ferulate has been designed and found to potently exert anti-tumor activity; this synthetic compound, indeed, triggers autophagic cell death through generation of reactive oxygen species and endoplasmic reticulum stress in colon cancer cells [139]. Similarly, a novel nanoparticle system, consisting of encapsulated gallic acid and gum arabic as coating material, has shown promising anti-cancer properties: the formulated nanoparticles, indeed, were selectively internalized by cancer cells, thus exerting potent anti-oxidant and anti-neoplastic effects, as assessed by cytotoxic, migration, and apoptosis assays [140].

Whole Grains and Gastric Cancer
Gastric cancer is the fourth cause of tumor-related deaths. Incidence (5.6% of all cancer cases) is higher in males (719,523 cases) than females (369,580 cases); 75.3% of cases occur in Asia, followed by Europe (12.5%) and Latin America and Caribbean (6.2%) [84].
Among factors affecting cancer onset, dietary habits play an important role [150][151][152]: salt-preserved foods and smoked meats potentiate carcinogenic effects of H. pylori infection [153], whereas fruits, vegetables, and WGs are protective factors [154,155]. A prospective population-based case-control study reported in men, but not in women, a modestly lower risk of stomach cancer with diet patterns high in WGs, only when combined with citrus fruit and vegetables [156]. Several meta-analyses have been published on WG/gastric cancer association, relying primarily or entirely on case-control studies and without dose-response analyses. However, all studies reported that increasing WG consumption was notable in showing a negative association with stomach cancer risk (ranging from 13 to 50% lower risk for highest WG consumers) and/or RG intake generally appeared to be a dose-dependent risk factor (63-65% increment of the risk) (Table 3) [92,[157][158][159][160][161][162][163]. However, it should be recalled that RG-rich diet is usually poor in WGs (and other dietary fiber sources) and associated with unfavorable lifestyles. Therefore, for gastric cancer, nutritional and lifestyle combination, rather than RG alone, may account for direct associations observed in the studies.  To date, no conclusions on the role of fiber in WG/gastric cancer association can be drawn since available investigations are somehow misleading and difficult to interpret. Except for the cohort Iowa Women's Health Study (demonstrating strong protective effects of WG fiber against stomach cancer) [95], almost all studies consider only total dietary fiber intake and/or report no association at all for fiber from grains [55,161,162].

Whole Grains in Esophageal Cancer
According to Globocan 2020, 508,585 cancer victims (5.3% of all cancer cases) were affected by esophageal cancer, the sixth cause of cancer deaths [84]. Incidence of esophageal cancer (3.1% of all cases) is higher in males than females; the highest mortality (78.2%) is registered in Asian continent [84].
Esophageal tumors are distinguished into esophageal squamous cell carcinomas, affecting upper layer cells lining esophagus, and adenocarcinomas, arising in glandular cells located between the esophagus-stomach junction [173,174]; esophageal squamous cell carcinomas are more frequent in developing countries, while esophageal adenocarcinomas predominate in eastern Asia and Africa [175]. Distinct risk factor profiles have been identified: tobacco smoking and alcohol abuse are main risk factors for esophageal squamous cell carcinoma, while obesity and gastro-esophageal reflux disease are key risk factors for adenocarcinoma. Specific dietary items and nutrients impact risk of both types of cancer [176]: for example, red, pork and processed meat, moldy food and pickled vegetable consumption are risks attributable to the entire population, while more varied diet, raw and cooked vegetables, vitamins, fiber, and carbohydrates are included among protective dietary factors [157,[177][178][179][180].
Higher frequency of WG food consumption may be accounted among indicators of reduced risk of esophageal cancer (Table 4). In a small case-control study, for example, Levi and co-workers reported significant decrease in cancer risk in individuals consuming high amounts of WG foods (whole wheat bread and cereals), while cancer onset was directly related to consumption of RG items (white bread and biscuits, pizza, pasta, and rice) [91]. Decreased risk for high WG intake has been reported by retrospective and prospective studies, although with different ratios: for example, the above mentioned Italian casecontrol study from La Vecchia's group [108] reported 60% decreased risk for the highest WG intake, while the recent HELGA cohort study from Skeie and co-workers showed 35-45% reduction [181]. Noticeably, authors observed that such association varied with cereals and food products, with WG wheat and bread being associated with lower risk. This finding can be explained considering that cereals and cereal-based foods have different composition and concentration of nutrients and bioactive compounds, which cooperate to exert positive effects [182,183]. In this context, dietary fiber may play a crucial role, as inverse correlation exists between dietary fiber intake and risk of both Barrett's esophagus, an intermediate pre-neoplastic lesion, and esophageal cancer [180,184]. Potential mechanisms of protective action include modification of gastroesophageal reflux and/or weight control, neutralization of carcinogens contained in food, amelioration of cancer-associated esophageal dysbiosis, and direct action on cancer cells [180,[184][185][186][187][188]. The prospective 14-year Iowa Women's Health Study, enrolling a cohort of 34,651 post-menopausal, initially cancer-free women, reported that malignancy incidence was inversely associated with WG intake, as well as with total fiber intake. In this context, some interesting data emerged: (i) none of inverse associations observed for fruit fiber, vegetable fiber, and total grain fiber was statistically significant; (ii) no protective effect was found for fiber from RGs (according to the evidence that milling process lowers content of fiber and bioactive compounds); (iii) the relationship with dietary fiber was driven by strong inverse association for WG fiber [95]. In the light of these findings, it should be advised to distinguish WGs or RGs as source of fiber, in order to avoid biased data [55,180].  Additionally, polyphenols could be beneficial in esophageal cancer, thanks to their antioxidant activity, ability to improve esophageal reflux-related inflammation, and modulation of cell proliferation and survival [189,190]. Gallic acid, for example, induces cell death in human squamous esophagus carcinoma cells, much likely by activating both extrinsic and intrinsic apoptotic pathways, as well as by downregulating the Akt/mTOR survival signaling cascade [191]. Recently, protective roles of apigenin have been confirmed in esophageal tumors: in in vitro and in vivo experimental models, this flavonoid has been reported to (i) induce apoptosis of tumor cells, (ii) inhibit tumor-dependent angiogenesis, and (iii) attenuate inflammatory responses, by inhibiting gene expression of the pro-inflammatory cytokine interleukin-6, whose levels are elevated in tumor tissues [192,193].

Conclusions and Future Perspectives
Cancer onset, progression, and outcome are strictly dependent on interaction among genetic, metabolic, and environmental factors. Remarkably, besides some unhealthy habits (such as tobacco use, alcohol abuse, and sedentary lifestyle), consumption of harmful foodstuff and nutrients increases cancer risk; coherently, healthy dietary behaviors, which involve consumption of healthy foods (fruits, vegetables, cereals, legumes, fish, olive oil) and nutrients (antioxidants, phytochemicals, fiber, vitamins, mono-and poly-unsaturated fatty acids), are worldwide recognized as a valid strategy for primary cancer prevention. Scientific organizations of several countries encourage WG intake in maintaining health and reducing risk of chronic diseases, such as type 2 diabetes, cardiovascular disease, and cancer [119,[194][195][196][197]. It has been estimated, in fact, that low WG intake resulted in almost 270,000 avoidable deaths and almost 4 million disability-adjusted life years in the European Union in 2015 [198].
Herein, we focused on inverse relationship between WGs, whose consumption is increasingly recommended, and gastrointestinal cancer onset and outcomes. What emerged is that WGs, unlike refined counterparts, consistently protect against gastrointestinal cancer, especially colorectal type; such differences can mainly be ascribed to reduction (or loss) of beneficial nutrients and phytochemicals during milling process. WG is indeed a complex food matrix containing different bioactive compounds, which synergistically act in chronic disease prevention. It is therefore difficult to identify which constituent is responsible for protection; for this reason, attention should be shifted not to single compounds, but instead to WG food matrix. For example, some WG positive effects essentially depends on fiber, but fiber varies from grain to grain and is present in other food items (vegetable, fruits, and legumes) that are equally consumed by high WG users. Thereby, although both fiber and WGs have healthy benefits, they are not interchangeable and consumers should pay particular attention to high-fiber products, sometimes containing bran or other added fiber without actually having much, if any, WG.
Noteworthily, WG consumers are more likely to consume less sugar, alcohol, fat, red meat, and indulgent foods, while consuming more fruits, vegetables, and fish; moreover, they have high education and socioeconomic status, as well as healthy lifestyle (physically active, no smoking). For this reason, most of studies investigated WG effects after accurate statistical adjustments for all of these possible confounders, thus removing potential bias from data and providing authentic and real relationship between WG intake and gastrointestinal cancer. Nonetheless, several elements point out that we are far from a solid, scientific-based knowledge for developing individualized WG-based regimens to prevent and manage cancer. WG consumption, indeed, reduces risk of digestive tract tumors with significant heterogeneity because of additional confounding factors, including differences in (i) type, duration, quality, and sample size of investigations; (ii) methods of collecting WG intake (food-frequency questionnaires vs. more quantitative questionnaires); (iii) type of WG foods; (iv) racial and ethnic demographic groups displaying different nutritional habits.
Despite all these limitations, beneficial effects of WGs cannot be denied, and therefore programs aimed at increasing WG consumption should be implemented through a broad partnership involving both public (Government authorities) and private (industries) stakeholders. Several barriers to WG consumption should, indeed, be removed by effective strategies: (i) univocal, quantitative, and international recommendations; (ii) nutritional education programs; (iii) improvement of sensory characteristics and increase of variety of WG foods (in order to satisfy different eating habits of consumers of all ages); (iv) better identification of WG-containing products; (v) reduction of purchase costs.