Abstract: We included 25 RCTs involving 7259 participants including 977 (13.5%) men and 6282 (86.5%) women. Mean age reported ranged from 52.6 to 71 years, and range of age of included participants was 23 to 85 years. The trials reported 27 comparisons and included participants who had survived breast cancer (17 trials), colorectal cancer (2 trials), gynaecological cancer (1 trial), and cancer at mixed sites (5 trials). For overall survival, dietary intervention and control groups showed little or no difference in risk of mortality (hazard ratio (HR) 0.98, 95% confidence interval (CI) 0.77 to 1.23; 1 study; 3107 participants; low-certainty evidence). For secondary malignancies, dietary interventions versus control trials reported little or no difference (risk ratio (RR) 0.99, 95% CI 0.84 to 1.15; 1 study; 3107 participants; low-certainty evidence). Co-morbidities were not measured in any included trials. Subsequent outcomes reported after 12 months found that dietary interventions versus control probably make little or no difference in energy intake at 12 months (mean difference (MD) -59.13 kcal, 95% CI -159.05 to 37.79; 5 studies; 3283 participants; moderate-certainty evidence). Dietary interventions versus control probably led to slight increases in fruit and vegetable servings (MD 0.41 servings, 95% CI 0.10 to 0.71; 5 studies; 834 participants; moderate-certainty evidence); mixed results for fibre intake overall (MD 5.12 g, 95% CI 0.66 to 10.9; 2 studies; 3127 participants; very low-certainty evidence); and likely improvement in Diet Quality Index (MD 3.46, 95% CI 1.54 to 5.38; 747 participants; moderate-certainty evidence). For anthropometry, dietary intervention versus control probably led to a slightly decreased body mass index (BMI) (MD -0.79 kg/m², 95% CI -1.50 to -0.07; 4 studies; 777 participants; moderate-certainty evidence). Dietary interventions versus control probably had little or no effect on waist-to-hip ratio (MD -0.01, 95% CI -0.04 to 0.02; 2 studies; 106 participants; low-certainty evidence). For QoL, there were mixed results; several different quality assessment tools were used and evidence was of low to very low-certainty. No adverse events were reported in any of the included studies. Evidence demonstrated little effects of dietary interventions on overall mortality and secondary cancers. For comorbidities, no evidence was identified. For nutritional outcomes, there was probably little or no effect on energy intake, although probably a slight increase in fruit and vegetable intake and Diet Quality Index. Results were mixed for fibre. For anthropometry, there was probably a slight decrease in body mass index (BMI) but probably little or no effect on waist-to-hip ratio. For QoL, results were highly varied. Additional high-quality research is needed to examine the effects of dietary interventions for different cancer sites, and to evaluate important outcomes including comorbidities and body composition. Evidence on new technologies used to deliver dietary interventions was limited.

Plain-language summary: The quality of evidence is generally low to very low. Most studies did not evaluate dietary interventions for key review outcomes, particularly mortality and morbidity. However, a few study outcomes with moderate-certainty evidence focused on dietary intake and physical measurements. Included studies compared dietary interventions versus control or usual care. We pooled data from similar randomised controlled trials (RCTs) to provide a summary estimate of the effects of an intervention, and we judged how confident (certain) we were of these findings by using an established method (GRADE). We identified 25 RCTs involving 27 different comparisons. For some outcomes, we found absence of evidence for dietary interventions. We found some evidence showing that dietary interventions probably did not modify energy intake; however, some evidence shows what is probably a slight increase in fruit and vegetable intake (moderate-certainty evidence). Evidence on dietary fibre was mixed for different advice on weight reducing or healthy eating. Dietary interventions compared to control probably improved the Diet Quality Index (moderate-certainty evidence). For physical measurements, we found a probable reduction in body mass index (BMI) with dietary interventions compared to controls (moderate-certainty evidence) but little evidence showing any change in waist-to-hip ratio (low-certainty evidence). For quality of life (QoL), results were mixed due to the wide variety of tools used. No adverse events were reported. Available evidence shows that dietary interventions can be helpful in modifying fruit and vegetable servings and diet quality; modification of fibre intake was variable, and some benefits were seen for anthropometric measurements, including BMI. Most of the evidence is based on women with breast cancer, so more research is needed for patients with other cancers. Gaps identified in the evidence involved the use of new technologies, comorbidities, and body composition data.