• Note that this review specifies suggested dietary guidance in which evidence of a dietary influence on cancer risk is substantial, but not necessarily conclusive.
• The review provides guidance based upon compelling evidence for cancer risk associated with food consumption in six areas.

Dietary guidance to reduce cancer risk has “sufficiently compelling” evidence to recommend avoidance of red meat, limited intake of alcohol and dairy products, and increased consumption of fruits, vegetables, and soy products, authors of a review concluded.

The evidence encompasses a wide range of cancers, including prostate, breast, head and neck, colorectal, and pancreatic cancer. Based on findings from a World Cancer Research Fund/American Institute for Cancer Research (WCRF/AICR) report, the data on causal influences of dietary factors are not necessarily conclusive in every case, but substantial enough to support all of the recommendations.

The dietary guidance adheres to the “precautionary principle” common to the fields of toxicology and environmental health, according to Neal Barnard, MD, of George Washington University in Washington, D.C., and co-authors, who reported their findings and recommendations online in the Journal of the American College of Nutrition. As an example, they cited the European Union’s use of the principle “when there are reasonable grounds for concern that potential hazards may affect the environment or human, animal, or plant health and when at the same time the available data preclude a detailed risk evaluation.”

“Guidelines, up until now, have been extremely conservative, which is to say that unless something was nailed down very clearly — like, cigarettes cause cancer — by and large, the guidelines avoided it,” Barnard, founder and president of the Physicians Committee for Responsible Medicine, told MedPage Today. “There are a lot of areas where we have a lot of evidence, and we need to start acting on that evidence now.”

The authors undertook the review in an effort to address “areas in nutritional science in which scientific evidence has been insufficient for authorities to issue guidance with confidence.” The findings and recommendations should help inform day-to-day decision-making of clinical practitioners and patients, who “cannot wait for the evolution of scientific consensus.”

The review focused on six areas wherein the evidence for dietary influences on cancer risk falls sort of conclusive, despite the accumulation of a substantial evidence base. The authors relied heavily on the 2007 WCRF/AICR report, Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, and supplemented the report with information published since then.

For each of the six areas of dietary influence, the authors summarized the data and discussed the potential advantages and disadvantages associated with following the recommendations.

• Limiting or avoiding dairy products may reduce the risk of prostate cancer. The WCRF/AICR report took into consideration 10 cohort studies, 13 case-control studies, and two ecological studies. Meta-analysis of eight cohort studies yielded a summary effect estimate of 1.06 (95% CI 1.01-1.11) per serving/day. The highest estimates came from the Health Professionals Follow-up Study and a meta-analysis of 11 case-control studies, both showing ≥60 increase in the risk of prostate cancer with higher dairy consumption. The authors cited evidence that limiting or avoiding dairy products may help reduce the risk of cardiovascular disease and reduce symptoms of lactose intolerance. Limiting dairy consumption may reduce calcium intake, which can adversely affect bone integrity and eliminate a possible beneficial effect on the risk of colorectal cancer.
• Limiting or avoiding alcohol may reduce the risk of cancers of the mouth, pharynx, larynx, esophagus, colorectal area, and breast. The type of alcohol does not appear to be important. The influence of quantity may vary by type of cancer. Men appear to be at greater risk. Avoidance may also reduce cancer risks associated with heavy drinking, including liver cirrhosis and cancer, pancreatitis, pregnancy complications, obesity, accidents, and suicide. Moderate alcohol use may help lower the risk of cardiovascular disease.
• Avoiding red and processed meat may reduce the risk of colorectal cancer. Authors of an update to the WCRF/AICR report concluded that evidence has “convincingly” linked consumption of red and processed meat to an increased risk of colorectal cancer. Other potential benefits associated with avoidance of red and processed meat include reduced risk of hypertension, diabetes, stroke, and cardiovascular disease. A potential downside of avoidance is missing out of a concentrated source of protein, iron, and zinc, although all of those nutrients can be obtained from other food sources, especially plant-based foods.
• Avoiding meats cooked at high temperatures (grilled, fried, and broiled) may reduce the risk of colorectal, breast, prostate, kidney, and pancreatic cancers. In this context, meat includes poultry and fish, in addition to red meat. The risk relates to the production of potentially carcinogenic heterocyclic amines and polycyclic aromatic hydrocarbons by high-temperature cooking methods.
• Consumption of soy products in adolescence may reduce the risk of breast cancer in adulthood, and consumption by breast cancer survivors may reduce the risk of recurrence and breast cancer mortality. The guidance is based on recent evidence showing as much as a 92% reduction in the risk of breast cancer associated with soy consumption by various populations of women. Soy can also be used as a reduced-fat alternative to meat and dairy products. Bernard and co-authors offer one caveat: Consuming large amounts of concentrated soy protein has been associated with increased production of insulin-like growth factor 1, suggesting that natural soy products are preferred over soy protein concentrates and isolates.
• A diet that emphasizes consumption of fruits and vegetables may reduce the risk of several common cancers. The authors cite studies and meta-analyses showing cancer risk reductions ranging from 6% to almost 50% in association with increased intake of fruits and vegetables. Added benefits may come in the form of a reduced risk of cardiovascular disease.

“These suggested dietary recommendations are not a comprehensive dietary plan,” the authors noted. “Rather, they relate to specific areas where evidence is sufficiently compelling to merit dietary changes.”