1 Basic concepts

1.1 Definition and scope

Metabolism refers to the totality of chemical reactions that sustain life. These reactions convert nutrients into usable energy and into the molecular building blocks needed for growth, repair, reproduction, and routine cell maintenance. In practice, metabolism includes both the handling of fuels such as glucose and fatty acids and the processing of structural materials such as amino acids and nucleotides.

The term is used in biochemistry, physiology, nutrition, and medicine. In a clinical setting, it may describe the body’s overall energy handling or a specific pathway, such as glucose metabolism or lipid metabolism. Because metabolic reactions are interconnected, a change in one pathway often influences many others.

1.2 Metabolic pathways

Metabolism is organized into pathways, which are ordered sequences of enzyme-catalyzed reactions. Each step typically transforms one molecule into another, allowing the cell to control chemical flow with precision. Pathways may function independently, but most are linked through shared intermediates and common regulatory signals.

1.2.1 Catabolism

Catabolism is the breakdown of complex molecules into simpler ones. This process releases energy, often captured in adenosine triphosphate and reduced coenzymes. Examples include the digestion and oxidation of carbohydrates, fats, and proteins. Catabolic reactions are especially active when cells need fuel for movement, synthesis, or maintenance.

1.2.2 Anabolism

Anabolism consists of pathways that build complex molecules from smaller precursors. These reactions require energy and reducing power. They support the synthesis of proteins, lipids, glycogen, and nucleic acids, as well as the replacement of damaged cell components. Anabolism predominates during growth, tissue repair, and nutrient abundance.

1.3 Energy transfer

Cells require a transferable form of energy to power chemical work. Metabolism meets this need by coupling energy-releasing reactions to energy-consuming ones. This transfer depends on phosphate bonds, electron carriers, and tightly regulated enzymatic steps.

Adenosine triphosphate is the principal energy currency of the cell. When one phosphate group is removed, energy becomes available for transport, mechanical movement, and biosynthesis. Other related molecules, such as GTP and creatine phosphate, also contribute to short-term or specialized energy transfer.

1.3.2 Redox reactions

Redox reactions involve the transfer of electrons. Oxidation removes electrons, while reduction gains them. These reactions are central to metabolism because they help capture energy from nutrients and move it through cellular pathways. Coenzymes such as NAD and FAD serve as common electron carriers.

1.4 Homeostasis

Homeostasis is the maintenance of stable internal conditions despite changing external demands. Metabolism contributes to homeostasis by balancing energy intake, storage, and expenditure. It also helps regulate blood glucose, pH, temperature, and the availability of essential molecules. When regulation fails, metabolic disease may develop.

2 Biochemical foundations

2.1 Enzymes

Enzymes are biological catalysts that accelerate metabolic reactions without being consumed. They increase the rate of specific chemical transformations and allow pathways to proceed under physiological conditions. Most enzymes are proteins, although some RNA molecules can also catalyze reactions.

2.1.1 Enzyme kinetics

Enzyme kinetics describes the rate of enzymatic reactions and the factors that influence them. Substrate concentration, temperature, pH, and inhibitor presence can all affect reaction speed. Kinetic behavior helps explain why pathways respond differently to nutrient supply and why some defects produce measurable changes in metabolism.

2.1.2 Cofactors and coenzymes

Many enzymes require additional nonprotein components for activity. Cofactors may be metal ions such as magnesium or zinc. Coenzymes are organic helpers, often derived from vitamins, that participate directly in chemical transfer. Their presence is essential in reactions involving electrons, carbon fragments, or chemical groups.

2.2 Cellular compartments

Metabolic reactions are distributed among distinct cellular locations. Compartmentalization improves efficiency, prevents incompatible reactions from interfering with one another, and helps coordinate pathways according to cellular needs.

2.2.1 Cytosol

The cytosol is the fluid portion of the cell outside membrane-bound organelles. Many early steps in carbohydrate metabolism and biosynthesis occur here. It provides a flexible environment for rapid metabolic adjustments.

2.2.2 Mitochondria

Mitochondria are major sites of aerobic energy production. They house pathways involved in the oxidation of fuels and the generation of ATP. Their membrane structure supports electron transport and the coupling of oxidation to phosphorylation.

2.2.3 Endoplasmic reticulum

The endoplasmic reticulum participates in lipid synthesis, membrane assembly, and the processing of certain macromolecules. In specialized cells, it also contributes to detoxification and calcium handling. Its metabolic role is especially important in tissues with high synthetic demand.

2.3 Key biomolecules

Metabolism centers on a limited set of major biomolecule classes. Each class has distinct structural features and biological roles, yet all can be transformed, stored, or reused through metabolic pathways.

2.3.1 Carbohydrates

Carbohydrates are a primary source of rapid energy and a common storage form of fuel. They include simple sugars, disaccharides, and polysaccharides such as glycogen and starch. They also serve structural and recognition functions in cells.

2.3.2 Lipids

Lipids are hydrophobic molecules that store energy efficiently and form biological membranes. They include fatty acids, triglycerides, phospholipids, and sterols. Because they are energy-dense, lipids are a major long-term fuel reserve.

2.3.3 Proteins

Proteins are polymers of amino acids with diverse functions, including catalysis, transport, signaling, and structural support. Although they are not the body’s preferred storage fuel, proteins can be broken down when necessary and their components redirected into other pathways.

2.3.4 Nucleic acids

Nucleic acids, principally DNA and RNA, store and transmit genetic information. Their synthesis and turnover depend on nucleotide metabolism. In growing or repairing tissues, nucleic acid production is closely linked to overall metabolic activity.

3 Major metabolic pathways

3.1 Carbohydrate metabolism

Carbohydrate metabolism governs the use, storage, and production of glucose and related sugars. It is essential for maintaining energy supply, especially in tissues with high glucose demand.

3.1.1 Glycolysis

Glycolysis is the breakdown of glucose into pyruvate through a series of enzymatic steps. It can proceed without oxygen and yields ATP and reducing equivalents. The pathway is a central route for rapid energy production and for generating intermediates used in other processes.

3.1.2 Glycogenesis

Glycogenesis is the formation of glycogen from glucose. This storage pathway occurs mainly in the liver and skeletal muscle. It allows excess glucose to be temporarily stored in a compact and readily mobilized form.

3.1.3 Glycogenolysis

Glycogenolysis is the breakdown of glycogen to release glucose units. In the liver, this helps maintain blood glucose levels between meals. In muscle, it provides a local fuel source during activity.

3.1.4 Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from noncarbohydrate precursors such as lactate, glycerol, and certain amino acids. It becomes especially important during fasting and prolonged energy shortage. The liver is the primary site, with the kidney also contributing under some conditions.

3.2 Lipid metabolism

Lipid metabolism includes the storage, mobilization, synthesis, and oxidation of fats. Because lipids are an efficient energy reserve, their metabolism is central to long-term fuel balance.

3.2.1 Fatty acid oxidation

Fatty acid oxidation is the degradation of fatty acids to produce acetyl coenzyme A, ATP, and reducing equivalents. It is a major source of energy during rest, prolonged exercise, and fasting. The process occurs mainly in mitochondria.

3.2.2 Lipogenesis

Lipogenesis is the synthesis of fatty acids and triglycerides from acetyl units, often when energy intake exceeds immediate needs. It supports storage in adipose tissue and contributes to membrane and lipid production. This pathway is stimulated after meals.

3.2.3 Ketone body metabolism

Ketone body metabolism involves the production and use of ketone bodies as alternative fuels. The liver produces ketones from fatty acid-derived substrates, while other tissues can oxidize them for energy. This system becomes more important when glucose availability is limited.

3.3 Protein and amino acid metabolism

Protein and amino acid metabolism manages the constant turnover of proteins and supplies precursors for energy and biosynthesis. Amino acids may be reused, converted into other compounds, or oxidized when needed.

3.3.1 Transamination

Transamination is the transfer of an amino group from one amino acid to a keto acid. It is a key step in amino acid interconversion and in preparing nitrogen for disposal. The reaction is widely used in liver metabolism.

3.3.2 Urea cycle

The urea cycle converts toxic ammonia into urea for excretion. It is a crucial detoxification pathway in the liver. By safely removing excess nitrogen, it allows amino acid breakdown to continue without accumulation of harmful byproducts.

3.3.3 Amino acid catabolism

Amino acid catabolism is the breakdown of amino acids into intermediates that enter carbohydrate or lipid pathways. Some amino acids are glucogenic, some ketogenic, and some can contribute to both types of metabolism. Their fate depends on tissue state and nutritional conditions.

3.4 Nucleotide metabolism

Nucleotide metabolism covers the synthesis, interconversion, and degradation of purines and pyrimidines. These compounds are essential for genetic material, energy transfer, and signaling.

3.4.1 Purine metabolism

Purine metabolism includes the formation and recycling of adenine and guanine nucleotides. It is tightly regulated because purine balance affects DNA synthesis and cellular energy status. Breakdown products are ultimately excreted.

3.4.2 Pyrimidine metabolism

Pyrimidine metabolism produces and recycles cytosine, thymine, and uracil nucleotides. These pathways support DNA replication, RNA synthesis, and some coenzyme formation. Like purine pathways, they are closely linked to cell growth.

4 Regulation of metabolism

4.1 Hormonal control

Hormones coordinate metabolic activity across tissues. They transmit information about nutrient availability, stress, and energy demand, allowing the body to shift between storage and mobilization.

4.1.1 Insulin

Insulin promotes nutrient uptake and storage, especially after meals. It stimulates glucose use, glycogen synthesis, fat accumulation, and protein building. In general, it favors anabolic processes and lowers circulating glucose.

4.1.2 Glucagon

Glucagon acts mainly during fasting to raise blood glucose. It stimulates glycogen breakdown and gluconeogenesis in the liver. Its actions generally oppose those of insulin.

4.1.3 Epinephrine

Epinephrine supports rapid energy mobilization during stress or exercise. It enhances glycogen breakdown and fat release, making fuels available quickly. Its effect is broad but especially important in skeletal muscle and liver.

4.1.4 Thyroid hormones

Thyroid hormones influence the overall pace of metabolism. They affect oxygen use, heat production, and the turnover of many biomolecules. Changes in their levels can alter metabolic rate across many tissues.

4.2 Nutritional state

Metabolic priorities shift according to feeding status. The body alternates between storing incoming nutrients and drawing on reserves to maintain function.

4.2.1 Fed state

In the fed state, nutrients are abundant and storage pathways are active. Glucose is used immediately or stored as glycogen, while excess energy may be converted into fat. Protein synthesis is also favored.

4.2.2 Fasting state

During fasting, stored fuels are mobilized to maintain blood glucose and organ function. Glycogen is depleted first, followed by increased fat oxidation and gluconeogenesis. The body becomes more reliant on internal reserves.

4.2.3 Starvation

Starvation is a prolonged state of energy deficit in which fuel use becomes highly conserved. The body reduces expenditure and increases reliance on fat and ketone production. Protein breakdown is limited as much as possible to preserve lean tissue.

4.3 Allosteric and genetic regulation

Metabolic pathways are controlled not only by hormones but also by local molecular signals and changes in gene activity. This layered regulation ensures both rapid adjustment and long-term adaptation.

4.3.1 Feedback inhibition

Feedback inhibition occurs when the end product of a pathway suppresses an earlier step. This prevents unnecessary synthesis and helps maintain balance. It is a common mechanism in biosynthetic pathways.

4.3.2 Gene expression control

Cells can increase or reduce the production of metabolic enzymes by changing gene expression. This allows adaptation to diet, hormones, and environmental conditions. Gene regulation is slower than direct enzyme control but provides lasting effects.

5 Organs involved in metabolism

5.1 Liver

The liver is a major metabolic hub. It processes nutrients absorbed from the intestine, stores glycogen, synthesizes lipids and proteins, and handles detoxification. Many circulating metabolic signals converge on the liver.

5.1.1 Glucose buffering

The liver helps stabilize blood glucose by taking up glucose when levels are high and releasing it when levels fall. It performs both glycogen storage and gluconeogenesis. This buffering role is central to energy balance.

5.1.2 Lipid processing

The liver synthesizes, modifies, and distributes many lipid species. It packages lipids for transport and converts excess carbohydrate into fat-related molecules. It also participates in ketone production during fasting.

5.1.3 Detoxification functions

The liver modifies drugs, toxins, and metabolic waste products so they can be excreted. Enzymatic systems in liver cells transform compounds into more water-soluble forms. This protective role is essential for maintaining internal chemical stability.

5.2 Skeletal muscle

Skeletal muscle is a major site of fuel consumption, especially during physical activity. It also stores glycogen and can use multiple energy sources depending on demand.

5.2.1 Energy use during activity

During contraction, muscle rapidly consumes ATP and must continually regenerate it. It can draw on glucose, glycogen, fatty acids, and, in some circumstances, ketone bodies. Fuel choice changes with exercise intensity and duration.

5.2.2 Glycogen storage

Muscle glycogen serves as an internal reserve for local energy needs. Unlike liver glycogen, it is not used to directly support blood glucose. Instead, it fuels the muscle in which it is stored.

5.3 Adipose tissue

Adipose tissue is specialized for energy storage and endocrine signaling. It accumulates triglycerides during energy surplus and releases fatty acids during energy deficit.

5.3.1 Energy storage

Adipose tissue stores excess calories efficiently in lipid droplets. This reserve can be mobilized over long periods, making it a major buffer against fluctuating intake. The tissue also helps insulate the body and cushion organs.

5.3.2 Adipokines

Adipose tissue secretes signaling molecules known as adipokines. These influence appetite, insulin sensitivity, inflammation, and energy use. Through these signals, fat tissue participates in whole-body metabolic regulation.

5.4 Pancreas

The pancreas has both endocrine and exocrine functions. Its metabolic importance lies in hormone secretion and in the release of digestive enzymes that support nutrient breakdown.

5.4.1 Endocrine regulation

The endocrine pancreas produces hormones such as insulin and glucagon. These hormones act on distant organs to coordinate fuel storage and mobilization. Their balance is vital for stable glucose control.

5.4.2 Exocrine contributions

The exocrine pancreas secretes enzymes into the small intestine. These enzymes digest carbohydrates, fats, and proteins into absorbable units. By enabling nutrient absorption, the pancreas indirectly supports metabolism throughout the body.

5.5 Brain

The brain has a continuous and substantial energy demand. Because it has limited fuel reserves, it depends on a steady nutrient supply and specialized transport mechanisms.

5.5.1 Glucose dependence

Under ordinary conditions, the brain relies heavily on glucose. Interruptions in glucose supply quickly affect function because neural tissue has little capacity for long-term storage. This dependence makes blood glucose regulation especially important.

5.5.2 Ketone utilization

During prolonged fasting or low carbohydrate intake, the brain can use ketone bodies as an additional fuel. This metabolic flexibility helps preserve function when glucose availability is reduced. Ketone use increases after adaptation to carbohydrate shortage.

6 Metabolic rate and energy expenditure

6.1 Basal metabolic rate

Basal metabolic rate is the minimum energy required to sustain life at rest. It supports breathing, circulation, ion gradients, and other essential functions. It varies with age, body size, body composition, and hormonal state.

6.2 Resting energy expenditure

Resting energy expenditure is the energy used by the body in a resting but not strictly basal condition. It is similar to basal metabolic rate but measured under less restrictive circumstances. It reflects the combined demands of organs and tissues at rest.

6.3 Physical activity

Physical activity can markedly increase energy expenditure. Muscle contraction, posture, and movement all require ATP and consume fuel. The amount of energy used depends on intensity, duration, and individual fitness.

6.4 Thermogenesis

Thermogenesis is heat production by the body. It can occur as a byproduct of metabolism or as a regulated process that helps maintain body temperature. Different forms of thermogenesis contribute to total energy use.

6.4.1 Diet-induced thermogenesis

Diet-induced thermogenesis is the rise in energy expenditure after eating. It reflects the energy cost of digestion, absorption, and nutrient processing. The magnitude varies by meal composition and individual physiology.

6.4.2 Brown adipose tissue

Brown adipose tissue produces heat efficiently through specialized mitochondrial activity. It is particularly important in infants and also contributes to heat generation in adults. Its metabolic role is distinct from that of white fat, which primarily stores energy.

7 Clinical metabolism

7.1 Metabolic disorders

Metabolic disorders arise when pathways that regulate fuel use, storage, or synthesis are disrupted. They may involve hormone signaling, enzyme activity, transport processes, or organ function. Symptoms often reflect disturbed energy balance or abnormal accumulation of metabolites.

7.1.1 Diabetes mellitus

Diabetes mellitus is a disorder of glucose regulation characterized by chronic elevation of blood sugar. It results from insufficient insulin action, impaired insulin production, or both. Over time, it can affect many organs and metabolic pathways.

7.1.2 Obesity

Obesity involves excessive body fat accumulation. It is associated with altered energy balance and changes in lipid and glucose metabolism. The condition can influence hormone signaling and increase the burden on several organ systems.

7.1.3 Dyslipidemia

Dyslipidemia refers to abnormal levels or patterns of blood lipids. It may involve elevated triglycerides, altered cholesterol fractions, or both. Because lipids are central to energy transport and storage, these changes can have broad metabolic consequences.

7.2 Inborn errors of metabolism

Inborn errors of metabolism are inherited conditions caused by defects in enzymes, transporters, or related proteins. They often appear in infancy or childhood, though some are recognized later. Clinical effects depend on which pathway is affected and how severe the block is.

7.2.1 Amino acid disorders

Amino acid disorders impair the processing of specific amino acids. They may cause toxic accumulation of substrate or deficiency of downstream products. Early recognition is important because dietary management can reduce harm.

7.2.2 Organic acid disorders

Organic acid disorders involve the buildup of acidic intermediates from amino acid or energy metabolism. These conditions can lead to metabolic acidosis and systemic illness. They often require prompt treatment during acute decompensation.

7.2.3 Fatty acid oxidation disorders

Fatty acid oxidation disorders prevent efficient use of fatty acids for energy. Affected individuals may struggle during fasting or illness, when fat use becomes important. Symptoms often reflect low energy availability and accumulation of unmetabolized lipids.

7.3 Metabolic acidosis and alkalosis

Metabolic acidosis and alkalosis are disturbances of acid-base balance caused by metabolic processes. Acidosis reflects excess acid or loss of base, while alkalosis reflects excess base or loss of acid. These states can arise from respiratory, renal, or metabolic causes, and they influence enzyme function and organ performance.

7.4 Nutritional deficiencies and excesses

Insufficient or excessive intake of nutrients can disrupt metabolism. Deficiencies may impair coenzyme production, energy generation, or tissue synthesis. Excess intake can overload storage pathways and contribute to imbalances in glucose, lipids, or other metabolites.

8 Diagnostic evaluation

8.1 Blood tests

Blood tests provide direct information about circulating metabolites, organ function, and endocrine status. They are commonly used to assess metabolic health and to monitor treatment response.

8.1.1 Glucose measurement

Glucose measurement evaluates current blood sugar levels and related control over time. It is central to detecting diabetes and hypoglycemia. Testing may be done in fasting or nonfasting states.

8.1.2 Lipid profile

A lipid profile measures major blood lipid fractions. It helps identify patterns associated with abnormal fat metabolism. The results are used in risk assessment and treatment planning.

8.1.3 Liver function tests

Liver function tests assess enzymes, proteins, and other markers linked to hepatic metabolism and integrity. They can suggest altered synthesis, injury, or impaired processing of nutrients and toxins. Interpretation depends on the full clinical context.

8.2 Metabolic panels

Metabolic panels measure groups of substances such as electrolytes, glucose, kidney markers, and indicators of acid-base status. They provide a broad overview of internal balance. Abnormalities may point to endocrine, renal, hepatic, or nutritional problems.

8.3 Enzyme assays

Enzyme assays measure the activity of specific metabolic enzymes. They are especially useful in diagnosing inherited enzyme deficiencies. Results may be obtained from blood, tissue, or cultured cells.

8.4 Genetic testing

Genetic testing identifies inherited variants that affect metabolic pathways. It can confirm a suspected diagnosis, guide family counseling, and support treatment selection. Molecular diagnosis is increasingly important in rare metabolic disease.

8.5 Metabolic screening

Metabolic screening aims to detect disorders before serious symptoms develop. Newborn screening is a common example, allowing early treatment of certain inherited conditions. Screening programs vary by region and by available laboratory methods.

9 Treatment and management

9.1 Dietary therapy

Dietary therapy modifies nutrient intake to improve metabolic control. It may involve restriction of certain substrates, adjustment of calorie intake, or supplementation with needed nutrients. Diet is often a core part of long-term management.

9.1.1 Caloric modification

Caloric modification changes total energy intake to match health goals and metabolic needs. It may be used to reduce excess weight, prevent undernutrition, or improve glucose control. The approach is usually individualized.

9.1.2 Macronutrient adjustment

Macronutrient adjustment alters the balance of carbohydrates, fats, and proteins in the diet. This can help manage blood sugar, lipid levels, or specific inherited disorders. Careful planning is important because excessive restriction may create deficiencies.

9.2 Pharmacologic therapy

Medications can alter hormone action, nutrient handling, or downstream metabolic effects. Drug choice depends on the disorder being treated and the target pathway involved.

Insulin and related agents are used to improve glucose uptake and reduce blood sugar. They are central in the treatment of diabetes and some acute metabolic states. Their dosing must be tailored to the patient’s needs.

9.2.2 Lipid-lowering drugs

Lipid-lowering drugs reduce cholesterol or triglyceride levels and help correct dyslipidemia. They may act by decreasing synthesis, absorption, or circulating concentrations of lipids. These agents are commonly used alongside diet and exercise measures.

9.3 Enzyme replacement and supplementation

Some metabolic disorders benefit from enzyme replacement, vitamin therapy, or other targeted supplementation. Such treatments can compensate for missing activity or support residual pathway function. They are most effective when matched to the underlying defect.

9.4 Lifestyle modification

Lifestyle measures influence metabolic health across many conditions. Regular activity, sleep, and sustained dietary habits all affect fuel balance and hormone sensitivity.

9.4.1 Exercise

Exercise increases energy expenditure and improves metabolic flexibility. It enhances glucose uptake by muscle and supports cardiovascular and body composition benefits. The best form and intensity depend on individual ability and medical status.

9.4.2 Weight management

Weight management aims to achieve and maintain a healthy body composition. It may include dietary planning, exercise, behavioral strategies, and sometimes medication. The goal is to improve metabolic outcomes while supporting long-term sustainability.

10 Research and future directions

10.1 Systems biology

Systems biology studies metabolism as an interconnected network rather than as isolated pathways. It integrates biochemical, genetic, and physiological data to understand how cells and organs coordinate fuel use. This approach helps explain complex responses that single-pathway models may miss.

10.2 Metabolomics

Metabolomics is the large-scale study of small molecules in cells, tissues, and body fluids. It provides a snapshot of metabolic state and can reveal pathway changes associated with disease or treatment. The field has become important in biomarker discovery and mechanistic research.

10.3 Personalized medicine

Personalized medicine applies individual clinical, genetic, and biochemical information to metabolic care. It may improve diagnosis, risk prediction, and treatment choice. This approach is especially valuable in conditions with variable responses to diet or medication.

10.4 Emerging therapies

Emerging therapies include targeted drugs, gene-based approaches, microbiome-related interventions, and advanced nutritional strategies. Many are being investigated for inherited metabolic disease, diabetes, obesity, and lipid disorders. Continued research aims to increase precision while reducing side effects.