This study investigates a novel dietary approach called Intermittent Carbohydrate Restriction (ICR), which involves reducing carbohydrate intake on selected days of the week while allowing habitual eating on other days. Previous research suggests that intermittent dietary strategies may improve metabolic health and support better adherence compared to continuous calorie restriction.

In this single-arm feasibility study, 40 healthy adults will follow the ICR protocol for 4 weeks. Outcomes including body weight, body fat percentage, waist circumference, and resting metabolic rate will be measured before and after the intervention. Dietary adherence, changes in habitual intake, and participant experiences will be assessed through food diaries, 24-hour recalls, questionnaires, and exit interviews.

The primary aim is to evaluate the acceptability and adherence to the ICR protocol in free-living conditions. Secondary aims include exploring its effects on body composition and diet-related behaviours, with the goal of informing future long-term dietary interventions.

Intermittent energy restriction (IER) has shown to improve postprandial metabolic systems and cognitive function, with superiority in dietary adherence compared with continuous energy restriction (CER)(1-3). A gap in current research is the impact of intermittent carbohydrate restriction (ICR) compared with IER in line with the carbohydrate-insulin model. Novel research (UEC 2019 008 FHMS) has explored the acute effects of ICR on postprandial metabolic systems and found engagement of the same underlying mechanisms with superior outcomes in glucose and lipid metabolism, compared with IER. Unpublished data supports the superiority of ICR over IER in dietary adherence due to perceived increased flexibility and reduced restrictions. However, a novel intermittent carbohydrate restricted diet has not been studied chronically, specifically exploring effects adherence and compliance on free living condition and its effect on body composition and food preference. This study primarily aims to explore the acceptability and compliance to ICR, and secondarily aims to explore its effect on behaviour change and body composition in free living condition in health adults. 40 participants (female and male) will follow a single arm novel intermittent carbohydrate diet protocol for 4 weeks. Weight, height, body fat %, waist circumference and resting energy expenditure values will be measured before and after the intervention. Compliance and the effects of dieting on the habitual food intake will be measured using food diaries and 24hr food recalls. Perceptions of participants' towards ICR and their experiences of dieting will be measured using questionnaires and an exit interview. This study will provide vital information for implementing flexible and adaptable dieting strategy, which can provide improvements to physical health in the long-term.

Inclusion Criteria:

• Body Mass Index (BMI) between 18.5 and 29.9 kg/m² (normal to overweight range; avoids confounding from obesity-related metabolic disturbances).
• Generally healthy (self-reported good health with no diagnosed metabolic, cardiovascular, or endocrine disorders).
• Not currently following a weight-loss diet or carbohydrate-restricted diet (to ensure dietary baseline is not already influenced by similar patterns).
• Willing and able to follow the ICR protocol for 4 weeks (includes ability to prepare meals and follow dietary instructions independently).
• Able to attend in-person laboratory assessments at baseline and follow-up (for measurements of REE, body composition, etc.).
• Fluent in English (to ensure comprehension of dietary instructions, consent forms, and interviews).
• Provides written informed consent (as per ethical standards and good clinical practice).

Exclusion Criteria:

• Diagnosed metabolic or endocrine disorders (e.g., diabetes, hypothyroidism, PCOS), to avoid confounding effects on metabolic outcomes and dietary adherence.
• History of cardiovascular disease, renal disease, or gastrointestinal disorders (e.g., IBS, coeliac disease), which may affect dietary tolerance or nutrient absorption.
• Currently pregnant, breastfeeding, or planning pregnancy during the study (due to potential risks and altered metabolic/nutritional needs).
• BMI <18.5 or ≥30 kg/m² (to exclude underweight or obese individuals where safety and generalizability may differ).
• Currently following a restrictive or structured diet (e.g., ketogenic, low-carb, intermittent fasting, or weight-loss diets), to ensure a clear baseline dietary behavior.
• History or presence of eating disorders (e.g., anorexia nervosa, bulimia, binge eating), for participant safety and ethical considerations.
• Use of medications or supplements known to affect metabolism or appetite (e.g., corticosteroids, GLP-1 agonists, weight-loss medications).
• Smoking or excessive alcohol consumption (defined as >14 units/week), which may interfere with dietary adherence and metabolic assessments.
• Participation in another clinical or dietary study within the past 3 months (to avoid overlapping interventions or residual effects).
• Inability or unwillingness to comply with study procedures (including dietary tracking, visit attendance, or sample collection).