Metabolic conditions, such as heart disease, type 2 diabetes, and metabolic syndrome, are among the leading causes of disability and mortality worldwide. Identifying innovative lifestyle strategies to reduce metabolic risk remains a public health priority. Further research is essential to understand how low carbohydrate intake influences human metabolism and how intermittent carbohydrate restriction impacts weight loss and metabolic health markers.

This study will investigate two interventions that restrict dietary carbohydrate intake on different schedules, examining their effects on metabolic health and energy balance in adults with abdominal overweight or obesity. The physiological mechanisms potentially underlying these effects will be explored by measuring a range of lifestyle and metabolic health parameters in both free-living and laboratory settings. A digital behaviour-change intervention will be incorporated after the dietary intervention to assess whether any metabolic effects can be sustained over time.

The study employs a three-arm parallel-group design consisting of a two-week lifestyle monitoring phase, a four-week dietary intervention, and a four-week digital intervention, with four laboratory visits throughout each phase. During the dietary intervention, participants will be allocated to one of three groups: (i) carbohydrate restriction (<8% of energy intake) on two consecutive days per week, (ii) carbohydrate restriction after 4 pm each day, or (iii) no dietary changes.

Dietary intake, physical activity, and glucose levels will be monitored through wearable devices in free-living conditions. The digital intervention will use a mobile health application that provides individualised lifestyle recommendations and education based on data collected in free-living conditions. Laboratory-based measures will include anthropometry, body composition scans, indirect calorimetry, blood pressure monitoring, fat biopsies, and postprandial sampling of blood and expired air.

Inclusion Criteria:

1. Waist circumference of ≥94 cm (37 inches) if male and ≥80 cm (31.5 inches) if female, or BMI above 25 kg/m2
2. Fat mass index (FMI) of >6 kg/m2 for males, and >9 kg/m2 for females
3. Aged between 18-65 years
4. Has maintained a stable weight in the last three months (<3% change in body mass)
5. Keeps track of menstrual cycle regularity or oral contraceptive use (females only)

Exclusion Criteria:

1. Has a body weight of ≥120kg
2. Plans to undertake other lifestyle modifications during the study to manage weight (e.g. changes in dietary intake or activity levels)
3. Current or previous eating disorder
4. Diagnosed with major chronic conditions (e.g. type 2 diabetes, coronary heart disease, cancer, chronic kidney disease, etc.)
5. Use of medication that may interfere with study outcomes (e.g. glucose or lipid lowering medications)
6. Currently or recently pregnant (within last 6 months), planning to get pregnant or currently lactating
7. Has donated more than 500ml of blood in the last 3 months prior to the initial laboratory visit
8. Insufficient mental capacity or language skills to independently understand and follow the study protocol
9. Dietary restrictions to ingredients in test meals (e.g. gluten and lactose)
10. Has an irregular sleeping pattern (e.g. due to undertaking night-shift work)
11. Any condition, concurrent intervention or behaviour deemed either to pose undue personal risk to the participant or to introduce bias into the experiment