Most individuals spend the majority of the day in a fed state, making postprandial blood glucose and lipid regulation a central component of metabolic health. Short periods of fasting or reduced carbohydrate intake can occur frequently in daily life, whether through intentional dietary practices (e.g. intermittent fasting), cultural or religious traditions, or unstructured eating patterns. These periods can induce measurable metabolic changes, but the acute effects of a single 24-hour period of fasting or carbohydrate restriction on postprandial metabolism upon refeeding remain poorly characterised.

This study will examine how 24 hours of either complete fasting or low carbohydrate intake without energy restriction influence postprandial blood glucose and lipid concentrations, as well as related metabolic markers, in healthy adults, in comparison to a habitual high carbohydrate control diet. Participants will complete all three conditions in a randomised order, consuming a standardised meal test after each 24-hour condition. Repeated blood sampling and gas exchange measurements will be carried out to assess postprandial metabolic responses.

The aim of this study is to characterise whether acute periods of fasting and low carbohydrate restriction elicit distinct or comparable alterations in postprandial metabolic responses. A clearer understanding of these acute physiological effects can help inform how variations in eating patterns influence glucose and lipid handling in individuals without metabolic disease.

Inclusion Criteria:

• Aged between 18-40 years
• Have a BMI of 18.5-29.9 kg/m2

Exclusion Criteria:

1. Diagnosed with any major chronic conditions (e.g. type 2 diabetes, coronary heart disease, cancer, chronic kidney disease, etc.)
2. Use of medication that may interfere with study outcomes (e.g. glucose or lipid-lowering medications)
3. Currently or recently pregnant (within last 6 months), planning to get pregnant or currently lactating
4. Has a habitually low carbohydrate intake (<150g carbohydrates/day)
5. Current or previous eating disorder
6. Has donated more than 500ml of blood in the last 3 months prior to the initial laboratory visit
7. Insufficient mental capacity or language skills to independently understand and follow the study protocol
8. Has an irregular sleeping pattern (e.g. due to undertaking night-shift work)
9. Any other condition, concurrent intervention or behaviour deemed either to pose undue personal risk to the participant or to introduce bias into the experiment