This study aims to establish the metabolic/molecular response in both adipose tissue and skeletal muscle as well as sensory experiences (pain, fatigue, drive) to prolonged fasting of 3 days duration. Participants will undergo sequential meal assessment before and after a 3 day fast with measures taken throughout each fasting day.

This study will employ a single-arm, non-randomised design in which participants will act as their own control prior to undergoing 3 days of fasting. The control phase includes 3 days of habitual energy balance assessment before coming into the laboratory for baseline meal testing with blood, muscle, and adipose samples as well as anthropometric assessment.

Participants will then be provided with a standardised lunch to be consumed around midday and a standardised dinner to be consumed at 2000 h.

Participants will then be asked to refrain from caloric intake for 3 days until they return to the laboratory for follow up measures.

Participants will be provided with monitoring equipment throughout the 3 days of fasting and will be asked to provide a blood sample at 0800 h each day alongside self recording of blood pressure, weight, and urinary ketones each day. On the middle day (day 2) participants will also be asked to come into the lab to provide a blood sample at 2000 h.

The follow up visit will be identical to that performed before the fast, including meal testing, blood, muscle, and adipose samples.

Inclusion Criteria:

• • Body mass index 20.0-29.9 kg∙m-2

  • Age 18-45 years
  • Willing to abstain from food intake for just over 3 days (~82 hours)
  • Able and willing to provide informed consent and safely comply with study procedures
  • Females to maintain record of regular menstrual cycle phase or contraceptive use
  • No anticipated changes in lifestyle during the study (e.g. holidays or exercise programmes)

Exclusion Criteria:

• • Any reported condition or behaviour deemed either to pose undue personal risk to the participant or introduce bias

  • Any diagnosed metabolic disease (e.g. type 1 or type 2 diabetes)
  • Any reported use of substances which may pose undue personal risk to the participants or introduce bias into the experiment
  • Pregnancy
  • Breastfeeding
  • Lifestyle not conforming to standard sleep-wake cycle (e.g. shift worker)
  • History of eating disorders (e.g. anorexia)
  • Any reported recent (<6 months) change in body mass (± 3%)
  • Uncontrolled hyperthyroidism
  • Advanced cerebrovascular insufficiency or dementia
  • Advanced liver or kidney insufficiency
  • History of migraine and or headache
  • Psychotic disorders
  • Unstable or severe coronary artery disease
  • Retinal detachment
  • Duodenal or stomach ulcer
  • Cancer and malignant disease
  • Regular use of Non-steroidal anti-inflammatory drugs (NSAIDs)
  • Use of Systemic corticoids
  • Use of Antihypertensives (especially beta-blockers and diuretics)
  • Use of Antidiabetics
  • Use of Anti-coagulants
  • Use of Psychotropics (especially neuroleptics and lithium)
  • Use of Anticonvulsants.