Long-Term Randomized Controlled Trials of Diet Intervention Reports and Their Impact on Cancer: A Systematic Review.

backgroundMost randomized controlled trials (RCTs) assessing the impact of diet on cancer have been short term (<1 year), mostly evaluating breast cancer survivors. Given the many-year interval that is generally required for an intervention to have an impact on cancer risk or prognosis, as well as the fact that lifestyle strategies such as diet modification frequently fail due to lack of adherence over the long term, we focused this systematic review only on longer-term (≥1 year) intervention reports. Diet intervention reports focused on reducing cancer risk in overweight and obese individuals target caloric restriction (every day, some days, or most hours of each day).

methodsThis study is a systematic review of RCTs lasting at least 1 year, testing dietary interventions with a primary or secondary endpoint of cancer or a biomarker linked to cancer.

resultsFifty-one reports met our review criteria. Twenty of fifty-one (39%) reports are RCTs where the primary endpoint was cancer or a cancer-related biomarker, while the other reports evaluated reports where cancer or a cancer-related biomarker was a secondary endpoint. Thirteen of twenty (65%) primary reports evaluated isocaloric, and the remaining eight evaluated low-calorie diets. All but one of the primary and two secondary isocaloric diet reports evaluated the benefit of a low-fat diet (LFD), with the other three evaluating a Mediterranean diet (MedD). More LCD vs. isocaloric diet primary reports (71% vs. 38%) demonstrated cancer or cancer-related biomarker benefit; the difference in chance of benefit with secondary reports was 85% for LCD vs. 73% for isocaloric diets. Three of three MedD reports demonstrated benefit. Sixty-nine percent (20/29) of the secondary reports came from two large reports: the WHI diet modification trial (15 secondary reports) and the polyp prevention trial (5 secondary reports). Nineteen of twenty-two (86%) primary reports enrolled only women, and three enrolled both men and women. No study that met our criteria enrolled only men, comprising 1447 men in total vs. 62,054 women. Fifteen of twenty (75%) primary reports focus on healthy women or women with breast cancer. Adherence findings are discussed when provided.

conclusionsMore long-term RCTs evaluating cancer and cancer-related biomarker endpoints are needed, especially for cancers at sites other than the breast.