We are undertaking a 6-month randomised trial to compare the two approaches in 104 premenopausal women aged 30–45 years at high risk of breast cancer because of adult weight gain >7 kg. Women will be randomly assigned to either CER (75% estimated energy requirements: ~1,500 kcal 7 days/week) or IER (75% estimated energy requirements: 650 kcal for 2 days and ~1,800 kcal 5 days/week) over 6 months. Study end points will be measures of insulin sensitivity (HOMA, SHBG and testosterone), potential breast cancer growth factors (IGF axis, leptin and adiponectin), inflammatory markers (C-reactive protein and sialic acid), oxidative stress marker (urinary F2 isoprostane), weight and body composition (waist/hip circumference, fat free and total fat mass). The relative acceptability of IER and CER will be assessed using a quality of life questionnaire (RAND SF-36) and scales of behaviour change and adherence. The relative efficacy and acceptance of intermittent and chronic calorie restriction will inform future weight loss programmes to prevent breast cancer. Thirty-seven women have currently been recruited to the study (18 CER and 19 IER) and recruitment is planned to be completed by December 2006.