Prevent 6:1 trial: pI rotocol for a feasibility trial of a 6:1 intermittent diet for weight gain prevention in women at increased risk of breast cancer.

INTRODUCTION: Weight gain in adulthood is a common potentially modifiable breast cancer risk factor. Intermittent 5:2 diets (two low-calorie days/week) have proven efficacy for weight loss among people with overweight or obesity and can promote sustained awareness and mindfulness of diet choice and help appetite control.This trial aims to test whether a less intensive 6:1 intermittent diet programme (one low-calorie day/week) is a feasible intervention to promote healthy eating and prevent weight gain in women at increased risk of breast cancer. METHODS AND ANALYSIS: . Participants will be entered to a 16-week 6:1 diet programme involving 1 day consuming 1000 kcal and healthy eating for 6 days a week. Participants will also be advised to meet physical activity recommendations for health (≥150 min of moderate intensity physical activity/week and resistance exercise two times per week). The programme will be supported remotely by dietitian calls at baseline, week 1, 4, 8, 12 and 16. Participants will also be provided access to a trial-specific private monitored Facebook group which includes information and the opportunity for peer support.Co-primary outcomes are: (a) uptake to the trial, (b) retention rate, (c) adherence to the 6:1 diet and (d) participant feedback on acceptability of the programme. Secondary outcomes include characteristics of those recruited and completing the programme and a preliminary evaluation of benefits and harms. This includes changes in body weight and body composition (bioelectrical impedance), diet quality, physical activity, binge eating, sleep quality (Pittsburgh Sleep Quality Index), menstrual cycle length and potentially diet-related adverse events, that is, fatigue, constipation, dizziness, headache, indigestion. Exploratory outcomes include the impact of low-calorie days on dietary intake and physical activity across the week and any differences in adherence to the low calorie days across the menstrual cycle. ETHICS AND DISSEMINATION: This trial has been approved by South Central-Berkshire B Research Ethics committee (rec reference 24/SC/0321). Findings will be disseminated via peer-reviewed journals, national and international cancer prevention and obesity conferences and cancer prevention charitable bodies. TRIAL REGISTRATION NUMBER: ISRCTN14330469.