Colorectal cancer is the third most frequently diagnosed cancer in males and the second in females worldwide (Torre et al., 2015). There were approximately 1.4 million cases of colorectal cancer, and 693,900 deaths in 2012, with the incidence rate being higher in men compared to women (Torre et al., 2015). Colorectal cancer is most prevalent in Australia, New Zealand, Europe and North America (Torre et al., 2015). Meanwhile, rates are lowest in Africa and South-Central Asia, which may be due to the increased dietary fibre intake in these regions (Torre et al., 2015; Yao et al., 2017). Cancer risk derives from the gut epithelium interacting with a multitude of nutrient and non-nutrient compounds which may be beneficial or detrimental to the gut, as the gastrointestinal tract can be very sensitive to different diets (Eid et al., 2013/2014). Dietary fibres are resistant to digestion and absorption in the small intestine but endure complete or partial fermentation in the large intestine (Eid et al., 2013/2014). Dietary fibres promote physiological benefits such as laxation, lowering blood cholesterol and blood glucose attenuation (Eid et al., 2013/2014). This paper will focus on the connection between the consumption of dietary fibre and the risk of colorectal cancer.
Dietary Fibre as a Functional Food
Dietary fibres are non-glycemic carbohydrates, and are therefore not a source of absorbable glucose (Kendall et al., 2010). They consist of particulate, viscous, soluble and insoluble fibres; viscous fibres promote most of the clinical benefits associated with fibre (Kendall et al., 2010). The viscosity of the fibre slows digestion of nutrients by preventing bulk diffusion of foods across the intestinal lumen, thereby lowering postprandial glucose and insulin responses (Kendall et al., 2010). Insoluble fibre increases faecal bulk and reduces transit time, resulting in the reduction of toxin concentrations and potential carcinogen exposure to the colonic epithelium (Eid et al., 2013/2014). Although a high-fibre diet is recommended to prevent chronic diseases, dietary fibre recommendations also vary worldwide, which reflects the lack of knowledge regarding their health benefits (Eid et al., 2013/2014).
The use of dietary fibre as a functional ingredient to enhance foods has steadily increased to provide health benefits to consumers (Quiros-Sauceda et al., 2014). Functional foods are foods enriched with ingredients that can promote physiological benefits (Quiros-Sauceda et al., 2014). Dietary fibre provides added benefits to nutritional gains and improves the functionality of a food (Quiros-Sauceda et al., 2014). This nutrient plays an important role in gastrointestinal health, and assists in reducing blood cholesterol and blood glucose (Quiros-Sauceda et al., 2014). Dietary fibre improves functional properties such as increasing water holding capacity, emulsification, viscosity, adsorption, or fermentability (Quiros-Sauceda et al., 2014). It also plays a role in the bioavailability of other substrates, including phenolic compounds, and may enhance the availability of these antioxidants in the body (Quiros-Sauceda et al., 2014). Ultimately, dietary fibre performs many important tasks for the body including trapping toxins and stimulating intestinal microflora proliferation (Quiros-Sauceda et al., 2014). Not only is dietary fibre beneficial for health, it also assists with food processing (Quiros-Sauceda et al., 2014).
Currently, there is no health claim in Canada that verifies that a high intake of dietary fibre decreases the risk for cancer. However, there is a health claim in the United States supporting that a high intake of fruits and vegetables may reduce the risk of some types of cancers (Health Canada, 2011). There may be a correlation between a diet rich in fruits and vegetables containing dietary fibre and the risk reduction of some cancers. Several studies have been performed to determine whether dietary fibre decreases the risk for colorectal cancer. The World Cancer Research Foundation states that there is substantial consistent evidence that foods containing dietary fibre protect against colorectal cancer (World Cancer Research Fund, 2011). 13 of the 18 studies examining the benefits of dietary fibre on colorectal cancer showed a decreased risk for this cancer when an increased intake of dietary fibre was observed (World Cancer Research Fund, 2011). Meta-analyses performed by the Continuous Update Project and Second Expert Report revealed a 10% decrease in the risk of colorectal cancer when 10 g of fibre was consumed each day (World Cancer Research Fund, 2011). Conflicting results from these studies indicate that more research must be conducted provided that only 13 of the 18 studies reached the consensus of increased consumption of dietary fibre decreases the risk of colorectal cancer (World Cancer Research Fund, 2011).
Due to inconsistent findings regarding fibre intake and lower incidence of colorectal cancer, Zhong et al., 2014 decided to perform a case-control study to determine whether consuming a high intake of dietary fibre reduces the risk of colorectal cancer. This study examined the relationship between lifestyle factors and colorectal cancer risk in China (Zhong et al., 2014). Two control groups were examined in this study, the first included individuals with no family history of cancer, and were native to a region in China, or had lived there for the past five years prior to the study (Zhong et al., 2014). The second control was a group of apparently healthy individuals from the same region (Zhong et al., 2014). The researchers established that high intakes of total dietary fibre, vegetable fibre and fruit fibre were associated with a reduction in the risk of colorectal cancer (Zhong et al., 2014). They also discovered an inverse relationship between cereal fibre intake and colorectal cancer risk exists among males but not females (Zhong et al., 2014). This may be due to the difference in bile acid composition between males and females (Zhong et al., 2014). The researchers also state that there may be confounding factors attributed to high intakes of legumes that are highly correlated with fibre, and may be contributing to the reduced risk of colorectal cancer (Zhong et al., 2014). In this study, beneficial effects of dietary fibre were seen for both colon and rectal cancers, where previous studies have only found protective effects for either type of cancer (Zhong et al., 2014).
A study performed by Song et al., 2015 examined the effect of dietary fibre consumption on the risk of colorectal cancer. This case-control study recruited Chinese individuals with colorectal cancer and without cancer, who completed a food frequency questionnaire to collect information on what these individuals were consuming (Song et al., 2015). All subjects included in this study lived in the same region, were aged 30-70 years, and completed interviews with the researchers (Song et al., 2015). All controls had not suffered from cancer previously, had no family history of colorectal cancer, and no intestinal diseases were detected after a medical checkup (Song et al., 2015). Results revealed that individuals in the control group consumed more vegetables, soy products, and dietary fibre, than the case group (Song et al., 2015). Fibre has many protective effects against cancer such as increasing stool bulk, diluting fecal carcinogens, shortening fecal transit time, and reducing carcinogen contact with the colon epithelium (Song et al., 2015). Fibre can also bind to bile acid which can produce carcinogens, and fibre can be fermented by gut flora into short chain fatty acids, which can inhibit carcinogen production (Song et al., 2015). Fibre can reduce hyperinsulinemia by slowing down starch absorption (Song et al., 2015). Ultimately, this study showed an inverse relationship between the consumption of total fibre and vegetable fibre and colorectal cancer, suggesting that fibre plays an important role in protecting against this cancer (Song et al., 2015).
Another study performed by Kunzmann et al., 2015 discovered that consuming fibre at elevated quantities reduced the risk of colorectal adenomas but not recurrent adenomas. This study evaluated the association between dietary fibre and adenoma risk in a population-based screening trial (Kunzmann et al., 2015). Individuals included in this prospective study were screened at baseline, had no evidence of distal polyps, and some received a second screening three to five years later as part of the trial (Kunzmann et al., 2015). Interestingly, this study found that cereal and fruit fiber reduced the prevalence of adenomas (Kunzmann et al., 2015). The reduction of incidence was evident for advanced adenomas, which are more likely to progress to colorectal cancer (Kunzmann et al., 2015). These results are supportive of population-wide recommendations concerning fibre intakes (Kunzmann et al., 2015).
Analyses within the European Prospective Investigation into Cancer and Nutrition (EPIC) study originally showed that dietary fibre intake lowered cancer risk, however results from some large cohort studies did not support this finding (Murphy et al., 2012). Since then, further research has been completed using the EPIC prospective cohort study to strengthen the evidence that a high dietary fibre intake can help prevent colorectal cancer (Murphy et al., 2012). This study aimed to examine whether the original inverse association persisted after a longer follow-up and an increased number of colorectal cancer cases to contribute a more accurate estimation of the association by cancer sub-site and dietary fibre source (Murphy et al., 2012). The researchers also wanted to further analyze the fibre-colorectal cancer relationship by looking at possible interactions such as age, sex, and other lifestyle and dietary variables (Murphy et al., 2012). This study included an abundance of individuals, mostly women aged 35 years or above from 10 different countries in Europe (Murphy et al., 2012). Dietary information over the previous 12 months was collected at the beginning of the study using a dietary questionnaire specific to the country (Murphy et al., 2012). Lifestyle questionnaires were also used to obtain information on lifestyle factors that may impact health such as smoking, alcohol consumption and physical activity levels (Murphy et al., 2012). The results from this study strengthen the results from the original study, as the results after 11 years of consuming dietary fibre, still showed that dietary fibre intake is inversely associated with colorectal cancer risk (Murphy et al., 2012). This association was of a similar degree of strength among men and women for both colon and rectal cancers (Murphy et al., 2012). Unfortunately, diet was only assessed at baseline of this study, so any dietary changes during follow-up are unaccounted for, but the consistency of the inverse association seen throughout follow-up indicates that regression dilution is unlikely to have affected the results (Murphy et al., 2012). Overall, this study showed an inverse association between dietary fibre intake and colorectal cancer, strengthening the evidence for increased consumption of fibre rich foods to prevent colorectal cancer (Murphy et al., 2012).
A Cochrane review performed by Yao et al., 2017 assessed how dietary fibre affected the recurrence of colorectal adenomatous polyps in people with a history of colorectal cancer. This review revealed that increasing fibre consumption in the Western diet for two to eight years did not lower the risk of colorectal cancer (Yao et al., 2017). However, the quality of evidence was low due to small sample sizes, potential biases, and inadequate consumption of dietary fibre (Yao et al., 2017).
A case-control study was performed by Grosso et al., 2014, to assess whether the Mediterranean diet had a beneficial effect on the body, and led to a decreased risk of colorectal cancer. The Mediterranean diet consists of an abundance of fruits and vegetables rich in dietary fibre, contains a moderate amount of low glycemic index carbohydrates, a low amount of red meat, and a large amount of olive oil (Grosso et al., 2014). Individuals included in this study were located in Southern Italy and all controls had no signs or symptoms of cancer in their medical history (Grosso et al., 2014). This study reported that the consumption of the Mediterranean diet inhibits colorectal cancer development by inducing the death of colon cancer cells and downregulating carcinogen production proteins (Grosso et al., 2014). Researchers also discovered that even when individuals had other metabolic disorders like obesity, the Mediterranean diet continued to protect them from colorectal cancer (Grosso et al., 2014). There was also a stronger association for colon cancer rather than rectal cancer, however these results may be due to different biological characteristics concerning the digestive tract mucosa between individuals (Grosso et al., 2014).
Fruit and Vegetable Intake and Colorectal Cancer
This study investigated whether the consumption of fruit and vegetable intake had a protective effect against colorectal cancer (Annema et al., 2011). This case-control study was conducted in Western Australia and was designed to find environmental and genetic risk factors for colorectal cancer (Annema et al., 2011). Data was collected through a food frequency questionnaire and a questionnaire based on lifestyle and health factors (Annema et al., 2011). They found that a higher intake of brassica vegetables (i.e. broccoli, Brussel sprouts, cauliflower, kale), was associated with a decreased risk of proximal and maybe distal colon cancer (Annema et al., 2011). A higher intake of dark yellow vegetables, which are rich in carotenoids, was associated with a lower risk of distal colorectal cancer (Annema et al., 2011). This decreased risk may be due to the glucosinolate content in brassica vegetables and the hydrolysis products that show anti-carcinogenic properties in animals (Annema et al., 2011). This protective effect may also occur because of their high content in folate and folate’s role in the synthesis of nucleic acids and in DNA methylation (Annema et al., 2011). The consumption of apples was also associated with a lowered risk of overall colorectal cancer and decreased the risk for distal colon cancer (Annema et al., 2011). The researchers do state that previous findings are not significant with regards to apple consumption and reduced colorectal cancer risk, and state that their results may be due to recall bias or to reverse causality if people with undetected colorectal cancer lower their intake of apples (Annema et al., 2011). Fruit juice intake was associated with a higher risk of rectal cancer (Annema et al., 2011). Although fruit juice may be similar in composition to whole fruits, it is low in fibre and many juices contain added sugars, which may increase the risk of colorectal cancer (Annema et al., 2011). Overall, this study found that the association between intake of fruits and vegetables and colorectal cancer may change depending on the location of the cancer (Annema et al., 2011).
The Gut Microbiome and Colorectal Cancer
There is also a relation between diet, the gut microbiome and colorectal cancer (Louis et al., 2014). Diets that are low in fibre, but high in fat and sugar result in a meager gut microbiota, combined with the negative effects of the diet, are likely to increase the risk of colorectal cancer (Louis et al., 2014). On the other hand, high microbial diversity in the gut combined with low pathogen levels, result in microbial short chain fatty acids which provide energy and maintain intestinal homeostasis (Louis et al., 2014). Good bacteria in the gut tend to use fibre as an energy source, which leads to a lower risk of colorectal cancer, when a diet high in fibre is consumed (Louis et al., 2014).
The Combined Effect of Consuming Dietary Fibre and Fat on Colorectal Cancer
The relationship between dietary fibre and fat and their association with colorectal cancer has also been studied. The researchers in this study were investigating whether an intake of high dietary fibre and high intakes of omega 3 fatty acids would reduce the risk of colorectal cancer (Navarro et al., 2016). This study focused on the large prospective study, the Women’s Health Initiative (WHI) Dietary Modification Trial, which was a low fat dietary pattern intervention that included increasing the intake of whole grains, and vegetables and fruits, but found no significant benefit on colorectal cancer incidence over 8 years (Navarro et al., 2016). The WHI aimed to find strategies to prevent major chronic diseases in postmenopausal women (Navarro et al., 2016). Information such as demographic characteristics, physical activity, medical, reproductive, family history of colorectal cancer, and lifestyle factors were all collected at baseline (Navarro et al., 2016). This study also distributed a food frequency questionnaire, and measured the intake over the past three months, and follow up questionnaires were also completed in years two through nine (Navarro et al., 2016). This study did not find significant evidence to support the levels of dietary fibre and fat intake consumed in WHI (Navarro et al., 2016).
Summary
The majority of studies found that the intake of dietary fibre reduced the risk of colorectal cancer. However, these studies also concluded that there was not sufficient evidence, and more research needs to be performed to confirm their findings. There is also no health claim currently in Canada to support the intake of dietary fibre reducing the risk of cancer, although the World Cancer Research Fund acknowledges that there is substantial evidence to support this statement.
Most studies reviewed were case-control studies. This type of study always has the potential to include recall and selection bias, along with sample size discrepancies. Replication of studies in larger cohorts may help determine whether the intake of dietary fibre helps reduce the risk of colorectal cancer. Some studies included were performed on specific populations, which may also affect the results. Outcomes from certain populations in a specific geographical area may not be relevant to other populations. Subject characteristics must also be considered, as age, ethnicity, lifestyle factors, etc. may impact findings. There may also be issues when determining how much fibre was consumed by the subjects participating in the study. Food questionnaires are commonly used to collect dietary information, and unfortunately this method may not be accurate, as some individuals may misclassify dietary fibre. Each study also examined the consumption of different fibres from multiple food sources, such as oat fibre, vegetable fibre and fruit fibre, which should be considered when analyzing results from the literature.
Overall, there is an abundance of literature supporting the intake of dietary fibre to reduce the risk of colorectal cancer, but there are still confounding papers that state the opposite. Ultimately, it would be greatly beneficial to the overall population to increase their consumption of dietary fibre, as there are still considerable nutritional benefits including but not limited to; increasing stool bulk, diluting fecal carcinogens, shortening fecal transit time, and reducing carcinogen contact with the colon epithelium (Song et al., 2015). While more research still needs to be conducted to strengthen the evidence on the benefits dietary fibre has on colorectal cancer, the current evidence is oriented towards the positive aspects this nutrient has on colorectal cancer.