Intermittent fasting (IF) is proposed as a viable alternative to moderate calorie restriction (CR) for weight loss and metabolic health, but few long term randomized trials have been conducted. This protocol paper describes the rationale and detailed protocol for DIRECT study (Daily versus Intermittent Restriction of Energy: Controlled Trial to Reduce Diabetes Risk), comparing long term effectiveness of IF versus CR on metabolic health in individuals who are at increased risk of developing type 2 diabetes.

Anticipated 260 non-diabetic men and women aged 35–75 years, BMI 25−50 kg/m² with score ≥12 on the Australian Diabetes Risk (AUSDRISK) calculator will be recruited into this open-label, multi-arm, parallel group sequential randomized controlled trial. Participants will be randomized to one of three groups for 18 months: IF (30% of energy needs on fast days), CR (70% of energy needs daily), or standard care (SC) group. All participants will visit the clinic fortnightly for weight assessments during active intervention phase (6 months), followed by a 12-month follow-up phase. IF and CR groups will receive further diet counselling by dietitian. Two primary outcomes are the changes in glycated haemoglobin (HbA1c) and postprandial glucose area under the curve (AUC) at week 24 post-randomization. Secondary outcomes include changes in weight, body composition via dual-energy X-ray absorptiometry, gastro-intestinal hormones, cardiovascular risk factors, and dietary record by a smartphone-based application.

This study will provide substantial evidence as to whether IF is an effective nutrition intervention for glycaemic control in a population at risk of developing type 2 diabetes.