Kate M. O'Rourke

Disclosures

July 09, 2015

Every day, headlines bombard readers with various diets that purportedly prevent cancer: "The Diet That Stops Cancer" and "Eating Your Way Out of Cancer." But what do the hard data really say about lifestyle choices preventing cancer?

Michelle Harvie, SRD, PhD, of the Genesis Prevention Center and Nightingale Breast Screening Center at University Hospital of South Manchester, England, speaking at the 2015 annual meeting of the American Society of Clinical Oncology (ASCO), pointed out that most of the cancer prevention recommendations have been sparked by findings from observational studies rather than randomized trials.

"Because most of the data come from observational studies, rather than randomized trials, we can't prove causality, but certainly the cohort studies show various associations," Dr Harvie said.

To prevent cancer, the American Cancer Society (ACS), World Cancer Research Fund, and American Institute for Cancer Research recommend several lifestyle choices, including being as lean as possible without becoming underweight; being physically active for at least 30 minutes per day; eating a mostly plant-based diet; limiting red meats and avoiding processed meats; limiting alcoholic drinks; and not taking supplements to protect against cancer (Table 1).[1] Recent studies support most, but not all, of these recommendations.

Table 1. WCRF/AICR/ACS Cancer Prevention Recommendations

Be as lean as possible without becoming underweight.
Be physically active for at least 30 minutes every day.
Avoid sugary drinks.
Limit consumption of energy-dense foods.
Eat more vegetables; fruits; whole grains; and legumes such as beans.
Limit red meats (ie, beef, pork, and lamb) and avoid processed meats.
Limit alcoholic drinks to two for men and one for women a day.
Don't use supplements to protect against cancer.
Limit consumption of salty foods and foods processed with salt.

ACS = American Cancer Society; AICR = American Institute for Cancer Research; WCRF = World Cancer Research Fund

To prevent breast cancer, the ACS recommends at least 150 minutes of moderate-intensity physical activity per week, alcohol intake of one or fewer drinks per day, and maintaining a body mass index (BMI) less than 25 kg/m2. In a study of 2905 women at high-risk for breast cancer, adherence to these three ACS recommendations reduced the risk for breast cancer by 44%.[2]

The Women's Health Initiative observational study, which involved almost 64,000 women, demonstrated that a healthy diet involving more fruit and vegetables, less meat, and less alcohol decreased the risk for cancer by 20% in women with a BMI less than 25 kg/m2 and 30% in women with a BMI between 25 and 29.9 kg/m2.[3] It did not, however, reduce cancer risk in obese women.

Perhaps this shouldn't be all that surprising, given that BMI itself is a risk factor for cancer. Too much body fat triggers insulin resistance, raising levels of insulin and growth factors that promote cancer. Fat also increases estrogen production, which can fuel some cancers, and fat secretes cytokines that promote inflammation.

In a recent dose/response meta-analysis of 50 prospective observational studies, researchers concluded that avoiding adult weight gain confers protection against certain types of cancer, particularly among nonusers of hormone replacement therapy (HRT).[4] The study found that for each 5-kg increase in adult weight gain, the relative risk was increased 11% for postmenopausal breast cancer among no or low HRT users; 39% and 9% for postmenopausal endometrial cancer among HRT nonusers and users, respectively; and 13% for postmenopausal ovarian cancer among no or low HRT users. For each 5-kg increase in men, the risk for colon cancer increased by 9%. The relative risk for kidney cancer comparing highest and lowest level of adult weight gain was 1.42.

"One of the biggest problems we have is gaining weight as we get older," said Dr Harvie.

Observational studies continue to show that physical activity can reduce the risk for breast, colorectal, and endometrial cancer, said Dr Harvie. In a recent study, researchers pooled data from six population-based prospective cohorts in the United States and Europe with self-reported physical activity data.[5] In the analysis of 660,000 men and women, those who achieved the minimum recommended physical activity target—defined as 7.5 to < 15 MET hours per week—had a 20% lower risk of dying of cancer than physically inactive persons. Those who were more active had additional benefits.

Whereas recent studies continue to support weight management and exercise as factors influencing the development of cancer, the same cannot be said about some of the specific food recommendations. A recent meta-analysis of prospective cohort studies involving almost 1 million people concluded that higher consumption of fruits and vegetables was associated with a lower risk for all-cause mortality and cardiovascular mortality, but not cancer-specific death.[6]

Recent studies have also shown that red meat may not be as bad as we think, at least in terms of causing cancer. An analysis of nearly 450,000 individuals from the European Prospective Investigation Into Cancer and Nutrition suggests that individuals should only be concerned about processed meats increasing the risk for cancer.[7,8] The analysis found that for every additional 50 g of processed meat an individual has on a daily basis, their risk for cancer goes up by 11%, but the study did not find an association between red meat intake and cancer risk.

"Processed meat seems to be the problem, rather than red meat," said Dr Harvie. "The state of the epidemiologic science on red meat consumption and colorectal cancer is best described in terms of weak associations, heterogeneity, and inability to disentangle effects from other dietary and lifestyle factors, lack of a clear dose/response effect, and weakening evidence over time."

New studies still support moderate drinking to avoid cancer. Heavy drinking, more than five drinks per day, has been linked very strongly to 10 different cancers: oral/pharyngeal (hazard ratio [HR], 5.13), esophageal squamous (HR, 4.95), breast (HR, 1.61), laryngeal (HR, 2.65), colorectal (HR, 1.44), liver (HR, 2.07), stomach (HR, 1.2), gall bladder (HR, 2.07), pancreatic (HR, 1.19), and lung (HR, 1.11).[9,10,11] Light drinking, one drink per day, has been shown to increase the risk for oral/pharyngeal (HR, 1.17), esophageal squamous (HR, 1.3), and breast cancer (HR, 1.05).[9,10,11]

"Light drinking, one drink a day, can increase risk for the head and neck cancers, esophageal cancer, and breast cancer—but what we shouldn't forget before we go around advocating no alcohol at all is that no alcohol at all is linked to overall mortality, probably because of increased cardiovascular events," said Dr Harvie.

Recent data also support not taking vitamin supplements to prevent cancer, and this recommendation is supported by randomized clinical trials rather than observational studies. "Randomized studies have in good faith tested whether adding vitamin supplements to healthy people can lower their risks for cancer, and we have a number of very sobering messages from these studies," said Dr Harvie. Folic acid increases the risk for cancer overall (HR, 1.07), and specifically for prostate (HR, 1.24) and colorectal cancer (HR, 1.28).[12] Beta-carotene can increase the risk for lung cancer (HR, 1.20) and stomach cancer (HR, 1.54).[13] Selenium increases the risk for nonmelanoma skin cancer (HR, 1.44), and vitamin E increases the risk for prostate cancer (HR, 1.17).[14,15]

"Weight gain, lack of exercise, and high alcohol intake are the key factors we should be focusing on," said Dr Harvie. "The evidence for diet quality is not so strong, but we need to explore that better and determine the underlying mechanisms."

At the annual ASCO meeting this year, Donald Abrams, MD, chief of hematology/oncology at San Francisco General Hospital and professor of clinical medicine at the University of California, San Francisco, pointed out that studies have also shown that exercise can improve the outcomes of patients after they develop cancer. In a recent a meta-analysis of 16 breast cancer studies and seven colorectal cancer studies with approximately 50,000 survivors, the most active breast cancer survivors had lower rates of death from breast cancer (relative risk [RR], 0.72) and the most active colorectal cancer survivors had lower rates of death from colorectal cancer (RR, 0.61). Survivors reporting an increase in activity after diagnosis had a lower risk for death than those who did not (RR, 0.61).[16]

"After treatment, cancer survivors should follow the recommendations for cancer prevention," said Dr Abrams.

Dr Harvie and Dr Abrams have no relevant disclosures.

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