[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative) › Prolonged fasting and starvation

[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# Prolonged fasting and starvation

- Regulation
- Exam priority: medium

Without food the body spends glycogen, then fat, and spares muscle by switching the brain to ketones; in the end it burns vital protein.

Updated 2026-11-01

Also called: Malnutrition, Refeeding syndrome

**See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/)

## Key points

- Phases: glycogen (≈24 h) → gluconeogenesis from amino acids (days) → ketosis with protein sparing (weeks) → terminal proteolysis.
- After ≈2–3 weeks the brain gets up to ≈2/3 of its energy from ketones.
- Active T₃ falls and basal metabolic rate drops: an adaptation to save energy.
- Marasmus: total energy deficit, no edema. Kwashiorkor: protein deficit with edema (low albumin) and fatty liver.
- Refeeding syndrome: insulin drives phosphate, potassium and magnesium into cells; hypophosphatemia is the hallmark.

## What it is

Prolonged fasting is an orderly sequence designed to spare protein. Glycogen runs out in about a day; over the first weeks gluconeogenesis uses amino acids, but ketones increasingly replace glucose in the brain and protein loss falls. When fat runs out, protein loss speeds up and death follows. In children, acute malnutrition is still a problem in parts of Colombia such as La Guajira.

## How it works, step by step

- Once liver glycogen is gone, glucose depends on gluconeogenesis from alanine, lactate and glycerol.
- Low insulin releases lipolysis; fatty acids fuel muscle and heart.
- The liver turns acetyl-CoA into ketones; their level rises over days.
- The brain adapts its transporters and enzymes and uses more ketones: glucose and protein demand fall.
- T₃ falls and sympathetic tone drops: less energy expenditure.
- Once fat is gone, muscle, heart and diaphragm are broken down; death usually comes from infection or heart failure.

### Starling forces along a capillary

**Arteriolar NFP** +22 mmHg **Venular NFP** +5 mmHg **Filtration** 13.5 **Reabsorption** 0.0 **Net filtration** 13.5 a.u. (lymph capacity 7.5)

Hypoalbuminemia (cirrhosis, nephrotic syndrome, malnutrition): πc falls and the force that holds water in the vessel is lost: generalized edema and ascites.

Edema: net filtration (13.5 a.u.) is more than the lymph can return (7.5 a.u.), so fluid collects in the interstitium.

Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.

- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema

In kwashiorkor low albumin lowers oncotic pressure: see why edema forms.

## Marasmus and kwashiorkor

| Feature | Marasmus | Kwashiorkor |
| --- | --- | --- |
| [Main deficit](https://durgical.com/en/conditions/marasmus/) | Total energy | Protein (with relative energy) |
| [Edema](https://durgical.com/en/conditions/kwashiorkor/) | No | Yes (low albumin) |
| Fat and muscle | Severely wasted («skin and bones») | Low muscle, fat partly kept |
| Liver | Normal | Fatty and enlarged |
| Skin and hair | Loose skin | Dermatosis, pale, brittle hair |

## Clinical notes

- Rapid refeeding of a malnourished patient: hypophosphatemia, hypokalemia, hypomagnesemia, arrhythmias and heart failure.
- Thiamine deficiency is unmasked when glucose is given (Wernicke encephalopathy).
- Anorexia nervosa shows bradycardia, hypotension, hypothermia and low T₃: fasting adaptations.

## Memory aid

«Glycogen a day, fat for weeks, protein last»: the order protects what is vital.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does urea excretion fall after several weeks of fasting?

The brain uses ketones and needs less glucose; gluconeogenesis from amino acids falls and so does urea production. This is protein sparing.

## Why does a child with kwashiorkor have edema while a marasmic child does not?

In kwashiorkor low albumin lowers plasma oncotic pressure and fluid moves into the interstitium; the marasmic child keeps albumin better.

## What causes hypophosphatemia when a starved patient is refed?

Carbohydrate triggers insulin, which drives phosphate into cells where it is used for ATP and glycolysis; total body stores were already empty.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/prolonged-fasting-and-starvation/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Liver](https://durgical.com/en/anatomy/liver/)

## Related topics

- [Fed and fasting states: the hormonal switch](https://durgical.com/en/physiology/fed-and-fasting-states-the-hormonal-switch/)
- [Nitrogen balance and body protein](https://durgical.com/en/physiology/nitrogen-balance-and-body-protein/)
- [Energy balance, food intake and obesity](https://durgical.com/en/physiology/energy-balance-food-intake-and-obesity/)

## Related conditions

- [Marasmus](https://durgical.com/en/conditions/marasmus/)
- [Kwashiorkor](https://durgical.com/en/conditions/kwashiorkor/)
- [Refeeding syndrome](https://durgical.com/en/conditions/refeeding-syndrome/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)

## In Biochemistry

- [Fed, fasting and starvation states](https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/)
- [Ketogenesis (ketone body synthesis)](https://durgical.com/en/biochemistry/pathways/ketogenesis/)
- [Gluconeogenesis](https://durgical.com/en/biochemistry/pathways/gluconeogenesis/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [24.5 Metabolic States of the Body](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-5-metabolic-states-of-the-body) (OpenStax (Rice University), 2022); [Malnutrition: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000404.htm) (MedlinePlus, US National Library of Medicine, 2026); [Kwashiorkor: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001604.htm) (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 72). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/physiology/prolonged-fasting-and-starvation/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/prolonged-fasting-and-starvation/