# Intermittent Fasting Physiology > Intermittent fasting physiology for MBBS Physiology: glycogen depletion, the metabolic switch, ketogenesis, Ramadan fasting and refeeding precautions. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/intermittent-fasting-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Intermittent Fasting Physiology", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/intermittent-fasting-physiology ## Direct answer The liver's 80-100 g glycogen store barely lasts a day; when it runs out — typically 12-24 hours after the last meal — the body crosses the metabolic switch: insulin falls substantially (commonly quoted near 50-70% from fed values), glucagon and catecholamines rise, glycogenolysis gives way to gluconeogenesis, and lipolysis feeds a liver that begins making ketone bodies. By 24-48 hours of fasting, ketones (beta-hydroxybutyrate and acetoacetate) rise from under 0.5 mM towards 1-2 mM and higher with prolonged abstinence, eventually covering a substantial share of the brain's energy demand and sparing muscle protein — nitrogen losses fall after the first few days. Popular regimens time-restrict this physiology: 16:8 windows, 5:2 (two very-low-calorie days weekly), alternate-day fasting, and dawn-to-sunset Ramadan fasting (12-16 hours or more by season). The honest exam position: fasting regimens achieve weight loss broadly comparable to calorie-matched continuous restriction in randomised trials, with individual adherence deciding outcomes. ## What you must remember - **Fuel timeline:** 0-4 hours absorption; 4-12 hours hepatic glycogenolysis with falling insulin; 12-24 hours gluconeogenesis from lactate, glycerol and amino acids; beyond 24-48 hours ketogenesis dominates as fat oxidation supplies acetyl-CoA the semistarved citric acid cycle cannot fully consume. - **Hormonal switch:** low insulin with preserved or raised glucagon is the permissive state; growth hormone rises protecting protein; cortisol supports gluconeogenesis; catecholamines mobilise fat. - **Brain adaptation:** after several days the brain derives a majority of its energy from ketones (commonly quoted near 60-70% during prolonged fasting), cutting glucose need and thus muscle protein sacrifice. - **Counterregulatory safety:** healthy people defend glucose throughout prolonged fasting; those on insulin or sulphonylureas, the elderly and pregnant women should not fast — the practical counselling point for patients with diabetes who wish to fast during Ramadan or navratri. - **Ramadan physiology:** dawn-to-sunset abstinence produces dehydration (often 1-2% body weight by evening in long summer fasts), daytime natriuresis-free sodium conservation, and a shifted circadian cortisol — in pregnancy, diabetic and renal-disease cohorts, guidance permits exemption, which is the medical advice framing. - **What fasting does not do:** it does not selectively burn "toxins", it does not raise basal metabolic rate (it falls modestly with weight loss), and head-to-head trials show weight loss similar to continuous calorie restriction of equal deficit. - **The danger door:** refeeding after prolonged fasting can trigger refeeding syndrome — insulin-driven shifts producing hypophosphataemia, hypokalaemia, hypomagnesaemia and thiamine depletion with fluid retention. ## Working through the two clinical faces Consider a 26-year-old practising 16:8 time-restricted eating for six months. What changes: a consistent window typically trims 200-400 kcal daily without counting, insulin exposure shortens, and fat oxidation covers more of the energy gap — modest weight loss, and in small trials improved insulin sensitivity disproportionate to weight change, findings still awaiting confirmation. Contrast a severely underweight adolescent who has been restricting and now develops tremor, oedema and heart failure on day three of hospital refeeding: phosphate 1.0 mg/dL, potassium 2.8 mEq/L, magnesium 1.2 mg/dL. That is the fasting physiology reversed too fast — the insulin surge drove ions intracellularly while thiamine-dependent carbohydrate metabolism stripped what remained. Prevention is the treatment: thiamine before the first feed, calories starting low (10-15 kcal per kg in highest-risk patients per current guidance), and daily electrolytes. Between the wellness enthusiast and the refeeding ward lies the whole topic: a switch safe when crossed slowly, dangerous when the return is rushed. ## Where students slip Treating ketones as pathology heads the list: nutritional ketosis (roughly 1-3 mM, managed by intact homeostasis) differs from diabetic ketoacidosis (often above 10-15 mM with acidosis and dehydration) by order and by insulin — the same molecules, different regulation. Second, students claim fasting "boosts metabolism"; resting metabolic rate falls in proportion to weight loss, with transient catecholamine-driven rises during fasting days that do not sum to a net advantage. Third, the brain's fuel is quoted as glucose forever: after days of fasting, ketones carry much of the cortical load — the protein-sparing logic that kept our ancestors' muscles usable. Finally, for Ramadan or vrata fasting with diabetes, the safe response is therapeutic: risk stratify, shift sulphonylurea and insulin timing to iftar, and apply sick-day rules. ## Frequently asked questions ### What is the metabolic switch in intermittent fasting? The transition, roughly 12-24 hours after eating, from glycogen-supported glucose metabolism to fatty acid oxidation and ketogenesis as insulin falls and counterregulatory hormones rise. ### What happens to the brain's fuel during prolonged fasting? Ketone bodies progressively replace glucose, covering a majority of cerebral energy need after several days and thereby reducing the muscle protein sacrificed for gluconeogenesis. ### How does nutritional ketosis differ from diabetic ketoacidosis? Nutritional ketosis runs around 1-3 mM with preserved insulin action and normal pH; ketoacidosis exceeds roughly 10 mM with acidosis, dehydration and absent insulin action. ### Is intermittent fasting superior to continuous calorie restriction? Randomised comparisons show broadly similar weight loss and metabolic outcomes at matched calorie deficits — adherence, not the eating window, is the main determinant. ### What is refeeding syndrome and its mechanism? Insulin-driven intracellular shifts of phosphate, potassium and magnesium with thiamine depletion and fluid retention when feeding resumes after prolonged fasting or malnutrition.