Nutrition ranks high among the modifiable predictors of cancer— partially, due to its impact on obesity and diabetes, which are established risk factors for the most commonly occurring cancers. Diet is particularly important because unlike family history and genetic factors, it can be modifiable, potentially altering our cancer risk. In theory, changing our diet should give us greater control over our health and help us avoid the physical, emotional, and psychosocial consequences of a cancer diagnosis. However, the general public, clinicians, and public health professionals face challenges in accurately understanding the role of nutrition in predicting cancer risk due to perceived and real (conflicting findings in the scientific literature) inconsistencies.
An emerging field in the area of nutrition and cancer is the role of whole grains in cancer prevention. In a world where carbohydrates, particularly refined sources, are increasingly viewed as the culprit for obesity and associated chronic disease, are whole grains the safest carbohydrate to recommend for cancer prevention? Currently, consuming a plant-based diet containing whole grain foods such as whole-grain bread, pasta, breakfast cereals, and oats is part of the American Cancer Society and the American Institute for Cancer Research’s recommendations for cancer prevention. These recommendations are primarily based on a comprehensive review of existing studies on diet and nutrition in relation to cancer. Interestingly, these recommendations are similar to those for the prevention of other chronic diseases such as diabetes and heart disease, yet cancer prevention guidelines are more qualitative in nature with no quantitative guidelines currently in place, such as consuming half of grains as whole grains, as we see in the Dietary Guidelines for Americans or the DASH eating pattern recommended by the American Heart Association for improved heart health. Furthermore, these cancer prevention guidelines are not well integrated into preventive-care practice.
One main challenge in providing nutritional guidance to cancer patients is the inconsistencies in study findings. For whole grains, while most existing evidence points to a protective impact of whole grains on obesity, diabetes, cardiovascular disease and even mortality, most studies are suggestive of no association with cancer. However, there is some suggestion of a potential protective impact on the risk of gastrointestinal cancers, suggesting that the impact of whole grains may be more pronounced for certain cancer sites but this requires further investigation. Conflicting evidence can be attributed to methodological challenges inherent to nutritional epidemiology studies in oncology. For instance, traditional dietary assessment tools where not designed to measure whole grain intake, and therefore tend to underestimate intake. This, coupled with the low, narrow range of whole grain intake within the US population makes associations between whole grain and cancer difficult to assess. Another important issue to consider is the regulation of whole grain labeling. While significant progress has been made in the recent years in quantifying the whole grain content of food for food and nutrient databases, the labels of many products can be misleading. For instance, products that are labeled “made with whole grain” or even “multigrain” may in reality contain minuscule amounts of whole grains. This makes it challenging to quantify whole grain intake in research studies, and also makes it more difficult for consumers, who are advised to increase their whole grain intake for chronic disease prevention, to identify and consume whole grain products.
Another important challenge is the absence of evidence on the potential differential impact of dietary exposures, particularly whole grains, on cancer across the life-course (in utero, childhood, adolescence, early-, mid-, and late-adulthood). This issue could potentially be of great relevance to tailoring dietary advice to life stage and intervening to advise individuals at critical periods during which dietary modification may be of importance to combat a future cancer diagnosis. In addition, there is a dearth of literature on the issue of potential racial differences in the associations between nutrition and cancer. Most studies on whole grains, for instance, have been conducted in primarily Caucasian populations. This necessitates additional research in other racial and ethnic groups to tailor public health guidance and individual-level counseling to these population groups that may experience unique social, economic, and cultural barriers that influence the adoption and maintenance of healthy behaviours such as consuming more whole grains.
Finally, it is important to keep in mind that diet is only one factor, albeit an important one, in the multitude of exposures such as hormones, body composition, physical activity, stress, aging, and broader psychosocial and environmental factors that impact cancer risk. Furthermore, the effect of a particular food or nutrient on cancer in most studies is mild to modest at best. Therefore, dietary advice on cancer should emphasize the importance of a comprehensive lifestyle approach that involves consuming a healthy diet, in the context of which whole grain consumption may be of benefit. Despite these challenges in providing nutritional guidance to cancer patients, in general, and on the issue of whole grains, in particular, the future of a clearer understanding of the role nutrition and diet in cancer risk is promising. The evolution of dietary assessment tools, identification of biomarkers, genetic information, diverse study cohorts, and refinement of data analysis techniques will enable us to develop an increasingly clearer understanding of the complexities of the relationships between diet, nutrition, and cancer. Furthermore, these advances are moving the field towards personalized nutrition, which will enable the tailoring of food and nutrient intake to individuals based on their genetic make-up, microbial environment, and metabolic response to foods.
The field of nutrition and cancer is ever-evolving, yet dietary guidance has not significantly changed over the years and continues to be in line with guidance for prevention of other chronic diseases, despite the inconsistencies in study findings on diet–cancer relationships. Although certain nutrients may be more beneficial or more detrimental to the risk of developing one chronic disease compared to others, ultimately, there is only one “good” diet. A healthy diet continues to be one that provides a variety of plant foods such as fruits, vegetables, legumes, and whole grains and restricts energy-dense and processed foods and drinks. On the issue of how to best communicate the status of the evolving science on whole grains and cancer to the public, it is important to note that evidence-based recommendations for cancer are constantly evolving, but for the time being, the dietary advice is this: whole grains are good to eat and it doesn’t hurt that their food sources can be tasty too.
Featured image credit: Breads, cereals, oats. CC0 Public Domain via Pixabay.