General metabolism

Last updated: April 26, 2023

Summarytoggle arrow icon

Metabolism is the sum of chemical reactions continuously taking place in the body to maintain its proper function. These reactions include water metabolism, electrolyte metabolism, and the conversion of nutrients into energy and substances (e.g., proteins, lipids, nucleic acids) that can be processed by cells. The conversion of nutrients involves the breakdown of substances (catabolism), which generally releases energy, or the synthesis of substances (anabolism), which generally consumes energy. Metabolic reactions are organized into pathways (e.g., glycolysis, lipolysis), with each step involving a specific enzyme to catalyze the reaction. Dysregulation of these pathways can lead to metabolic disorders such as glycogen storage diseases.

This article provides an overview of the most important metabolic pathways, as well as the most common metabolic disorders.

Water metabolismtoggle arrow icon


  • Percentage of body water [1]
    • Adults: 50–60% of the total body weight
    • Infants: ∼ 75% of the total body weight
  • Composition [1]
    • Approx. 65% intracellular fluid volume (ICFV)
    • Approx. 35% extracellular fluid volume (ECFV), which is composed of:
      • 75% interstitial fluid volume (ISFV)
      • 25% plasma volume (PV)
  • Distribution: via osmosis (e.g., through aquaporin channels) and via diffusion (mostly dependent on Na+ and Cl- concentrations)
  • Tolerance of imbalance: less than 1% difference to physiological osmolality concentration (285–290 mOsm/kg) [2]


Clinical significance [1]


Electrolyte metabolismtoggle arrow icon

Electrolytes that play an important role in water metabolism, electrical cell stability, and action potentials in nerves and muscles are sodium (Na+), potassium (K+), chloride (Cl-), bicarbonate (HCO3-), calcium (Ca2+), magnesium (Mg2+), and phosphate (H2PO4-, HPO42-). More detailed information about electrolyte imbalances is provided in the following articles:

Pathway overviewtoggle arrow icon

This section deals with the most important metabolic pathways.

Carbohydrate metabolismtoggle arrow icon

For details on the individual pathways, see the articles “Carbohydrates”, “Glycolysis and gluconeogenesis”, and “Glycogen metabolism”.

Lipid metabolismtoggle arrow icon

For details on this topic, see the article “Lipids and their metabolism.”

Overview of lipid metabolism
Pathway Precursor(s) End product(s) Rate-limiting enzyme Individual steps Pathway regulation
Stimulation Inhibition
  • N/A
  • N/A
Cholesterol synthesis

Protein metabolismtoggle arrow icon

For details on this topic, see the article “Amino acids”.

Metabolism of exercise and starvationtoggle arrow icon

Metabolism of exercise

The energy required for physical activity is derived from a combination of aerobic and anaerobic metabolism. Short-duration high-intensity exercise promotes anaerobic energy production while long-duration lower-intensity exercise favors aerobic energy production. For more information, see “Pathways of ATP synthesis.”

ATP sources during exercise [8][9][10]
Duration of exercise Primary source of ATP Characteristics
1–3 sec
  • Freely available stored ATP
  • Immediate energy release
  • Highly limited total capacity
1–6 sec
6–30 sec
30–120 sec
120–180 sec
> 180 sec

Metabolic states of the body

There are 3 different metabolic states of the body: postprandial state, fasting state, and starvation.

  • During fasting and starvation, metabolic processes provide the energy vital for protein preservation and normal cell and tissue function.
  • The metabolic processes during fasting and starvation are primarily regulated by the following:
  • The greater the substrate stores (e.g., adipose tissue), the longer physical function and, ultimately, life can be maintained.

Postprandial and fasting state

Postprandial vs. fasting state of metabolism
Postprandial (fed or absorptive state) Fasting (in between meals or postabsorptive state)
Hormone involved
Hormone-sensitive tissue
Pathway involved
Hormone-resistant tissue

Metabolism of starvation

Energy sources during starvation
Starvation days 1–3 Starvation after day 3
Biochemical reactions and substrates
Source of energy For brain
  • Glucose
For rest of body

RBCs rely on glucose as a source of energy and are resistant to both insulin and glucagon.

Clinical significancetoggle arrow icon

This section gives an overview of the most common metabolic disorders.

Disorders affecting the carbohydrate metabolismtoggle arrow icon

Disorders that affect the carbohydrate metabolism are predominantly inherited genetic conditions that are inherited in an autosomal recessive fashion. Lactose intolerance, however, is generally caused by a genetic polymorphism of the lactase-coding gene or develops secondary to other conditions (e.g., gluten-sensitive enteropathy). For details, see the article “Lactose intolerance” and “Inborn errors of carbohydrate metabolism”.

Overview of disorders affecting the carbohydrate metabolism
Condition Enzyme deficiency Pathophysiology Clinical features
Lactose intolerance
  • Diarrhea (often watery, bulky, and frothy)
  • Cramping abdominal pain (often periumbilical or in the lower abdomen)
  • Abdominal bloating, flatulence
  • Nausea
Fructose intolerance Hereditary fructose intolerance
Essential fructosuria
  • Asymptomatic
Galactosemia Galactokinase deficiency
  • ↓ Conversion of galactose to galactose-1-phosphate → accumulation of galactitol in tissues
Classic galactosemia
Uridine diphosphate galactose-4-epimerase deficiency
  • Mostly asymptomatic

Glycogen storage diseases Type I (von Gierke disease) Type 1a
Type 1b
Type II (Pompe disease)
  • Hydrolyzation of α-1,4- and α-1,6-linkages in the acidic environment of the lysosome → impaired glycogenolysis
Type III (Cori disease)
Type IV (Andersen disease)
Type V (McArdle disease)
Type VI (Hers disease)

Disorders affecting the fatty acid metabolismtoggle arrow icon

Medium-chain acyl-CoA dehydrogenase deficiency, primary carnitine deficiency, and carnitine palmitoyltransferase II deficiency are all genetic conditions that are inherited in an autosomal recessive fashion. For details on these conditions, see “Disorders of fatty acid metabolism”.

Overview of disorders affecting the fatty acid metabolism
Condition Deficiency Pathophysiology Clinical features
Medium-chain acyl-CoA dehydrogenase deficiency (MCAD deficiency)
Primary carnitine deficiency
  • Carnitine transporters
Carnitine palmitoyltransferase II deficiency (CPT II deficiency)
  • Carnitine palmitoyltransferase II

Disorders affecting the amino acid metabolismtoggle arrow icon

All conditions that affect the amino acid metabolism are genetic conditions that are inherited in an autosomal recessive fashion. For details on these conditions, see the article “Inborn errors of metabolism”.

Overview of disorders affecting the amino acid metabolism
Condition Deficiency Causes Pathophysiology Clinical features
Hartnup disease
Maple syrup urine disease
Organic acidemias Propionic acidemia
Methylmalonic acidemia
  • Histidase
  • Mostly asymptomatic
Pyruvate dehydrogenase complex deficiency

Disorders affecting the purine metabolismtoggle arrow icon

For details, see “Purine salvage deficiencies”.

Overview of disorders affecting the purine metabolism
Condition Enzyme deficiency Causes Pathophysiology Clinical features
Lesch-Nyhan syndrome
Adenosine deaminase deficiency

Disorders affecting the urea cycletoggle arrow icon

For details, see “Urea cycle disorders”.

Overview of disorders affecting the urea cycle
Condition Enzyme deficiency Causes Pathophysiology Clinical features
Ornithine transcarbamylase deficiency
  • Symptoms commonly manifest in the first days of life but can develop at any age.
  • Nausea, vomiting, irritability, poor feeding
  • Delayed growth and cognitive impairment
  • Metabolic encephalopathy
Arginase deficiency
Carbamoyl phosphate synthetase 1 (CPS1) deficiency
N-acetylglutamate synthase deficiency

Miscellaneoustoggle arrow icon

For details, see the article “Inborn errors of metabolism”.

Overview of miscellaneous disorders
Condition Defect Causes Pathophysiology Clinical features
Alpha-1 antitrypsin deficiency
  • Gene mutation → conformational change in the structure of AAT protein → dysfunctional (or absent) AAT
Mitochondrial myopathies

Mitochondrial Encephalomyopathy, Lactic Acidosis, Stroke-like episodes (MELAS)

Myoclonic epilepsy with ragged red fibers (MERRF)
Chronic progressive external ophthalmoplegia (CPEO)
Kearns-Sayre syndrome
Leber hereditary optic neuropathy (LHON)
  • Painless acute or subacute bilateral vision
Leigh syndrome
Pyruvate dehydrogenase complex deficiency
Orotic aciduria

Referencestoggle arrow icon

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  2. Morrison S, Ward P, duManoir GR. ENERGY SYSTEM DEVELOPMENT AND LOAD MANAGEMENT THROUGH THE REHABILITATION AND RETURN TO PLAY PROCESS.. International journal of sports physical therapy. 2017; 12 (4): p.697-710.
  3. Gastin PB. Energy system interaction and relative contribution during maximal exercise.. Sports Med. 2001; 31 (10): p.725-41.doi: 10.2165/00007256-200131100-00003 . | Open in Read by QxMD
  4. Bhagavan NV, Ha C-E. Carbohydrate Metabolism II. Elsevier ; 2015: p. 205-225
  5. Liao Y, Davies NA, Bogle IDL. Computational Modeling of Fructose Metabolism and Development in NAFLD. Frontiers in Bioengineering and Biotechnology. 2020; 8.doi: 10.3389/fbioe.2020.00762 . | Open in Read by QxMD
  6. Tobias A, Mohiuddin SS. Physiology, Water Balance. StatPearl. 2020.
  7. Weitzman RE, Kleeman CR. The clinical physiology of water metabolism. Part I: The physiologic regulation of arginine vasopressin secretion and thirst.. West J Med. 1979; 131 (5): p.373-400.
  8. McKinley MJ, Johnson AK. The Physiological Regulation of Thirst and Fluid Intake. Physiology. 2004; 19 (1): p.1-6.doi: 10.1152/nips.01470.2003 . | Open in Read by QxMD
  9. Brinkman JE, Dorius B, Sharma S. Physiology, Body Fluids. StatPearls. 2021.
  10. Kavouras SA, Anastasiou CA. Water Physiology. Nutrition Today. 2010; 45 (6): p.S27-S32.doi: 10.1097/nt.0b013e3181fe1713 . | Open in Read by QxMD

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