Endocrine System
The endocrine system primarily consists of endocrine glands (such as the pituitary gland, thyroid gland, parathyroid glands, pancreatic islets, adrenal glands, and gonads) and endocrine tissues and cells distributed throughout the brain (particularly the hypothalamus), adipose tissue, cardiovascular system, respiratory tract, digestive tract, and urogenital system.
Hormones
Definition and Classification of Hormones
Hormones, produced by endocrine glands or endocrine tissue cells, act on specific target organs to elicit a series of physiological effects and are regulated through feedback mechanisms. Based on chemical structure, hormones can be classified into four main categories:
Peptide and Protein Hormones
These hormones consist of amino acid residues forming a primary molecular structure. This group includes small neuropeptides such as gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), and somatostatin, as well as larger protein hormones like insulin, parathyroid hormone, growth hormone, and prolactin. They are initially synthesized as precursors through gene transcription and translation and subsequently processed or cleaved into bioactive forms. For example, proinsulin contains an insulin molecule and a connecting (C) peptide, which are hydrolyzed in the Golgi apparatus to form functional insulin.
Amine and Amino Acid-Derivative Hormones
These hormones are derived from amino acids. Amine hormones include substances such as adrenaline, noradrenaline, serotonin (5-hydroxytryptamine), and melatonin. Amino acid-derivative hormones, such as thyroid hormones, also fall under this category. Adrenaline, noradrenaline, and dopamine are synthesized from tyrosine, serotonin is derived through the hydroxylation and decarboxylation of tryptophan, and thyroid hormones (T4 and a smaller proportion of T3) are formed when activated iodine ions react with tyrosine residues on thyroglobulin molecules under the action of thyroid peroxidase.
Steroid Hormones
Steroid hormones are lipophilic hormones derived from cholesterol as their precursor. Synthesized in tissues such as the adrenal cortex, testes, ovaries, placenta, liver, adipose tissue, brain, and skin, these hormones include glucocorticoids (e.g., cortisol), mineralocorticoids (e.g., aldosterone), androgens (e.g., testosterone and dihydrotestosterone), estrogens (e.g., estradiol), and progestogens (e.g., progesterone). Additionally, vitamin D3 (cholecalciferol) is synthesized in the skin from 7-dehydrocholesterol under ultraviolet exposure and suitable temperature conditions. It is hydroxylated in the liver by 25-hydroxylase to form 25-hydroxyvitamin D3 [25-(OH)D3], also known as 25-hydroxycholecalciferol, and is subsequently hydroxylated by renal 1α-hydroxylase to generate the bioactive 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], also known as calcitriol.
Fatty Acid-Derivative Hormones
These hormones are primarily derived from arachidonic acid and include biologically active substances such as prostaglandins, thromboxanes, and leukotrienes.
Hormone Synthesis, Release, and Secretion
Hormone Synthesis and Release
Hormones are synthesized and released in real-time based on the body's needs, exerting effects extracellularly. Hormone synthesis and release are regulated by various signals and generally occur through two mechanisms:
- The first mechanism is primarily observed in peptide hormones, which are synthesized via classical gene expression pathways, stored in vesicles, and released extracellularly by exocytosis upon receiving release signals.
- The second mechanism is characteristic of steroid hormones and fatty acid derivatives, which are synthesized from precursor substances by enzymatic processes and directly released extracellularly without vesicle storage.
Most hormones are not stored in large quantities, with thyroid hormones being a notable exception. They are stored in follicles after synthesis, sufficient to meet physiological demands for up to two months.
Hormone Secretion
Modes of Hormone Secretion
Hormone secretion occurs through several modes, including:
- Endocrine secretion (classical endocrine): Hormones are transported to target tissues through the bloodstream.
- Paracrine secretion: Hormones act on neighboring cells within the same tissue. For example, testosterone synthesized by testicular interstitial cells can be secreted into the blood or act locally on spermatogenic cells to regulate spermatogenesis.
- Autocrine secretion: Hormones act on the very cells that secrete them. For instance, insulin-like growth factor-1 (IGF-1) produced by mammary epithelial cells acts on the same cells.
- Intracrine secretion: Hormones synthesized within a cell act directly on the synthesizing cell itself, such as estrogens synthesized in mammary tissue regulating local cellular metabolism.
- Neuroendocrine secretion: Neurohormones are secreted by neurons, transported through axonal flow to nerve terminals, and released. For example, arginine vasopressin (AVP)/antidiuretic hormone (ADH) is synthesized in the supraoptic and paraventricular nuclei of the hypothalamus, then transported via the hypothalamic-pituitary neural tract to the posterior pituitary for secretion into the bloodstream.
- Solinocrine secretion: Found in tubular organs like the gastrointestinal tract, bronchi, and urogenital tract, where hormones such as gastrin act directly on the epithelial cells of the lining to regulate local functions.
Characteristics of Hormone Secretion
Hormone secretion often exhibits rhythmic patterns and is influenced by external factors, such as seasons, light exposure, feeding, and stress. A classic example is the monthly secretion cycle of gonadotropins and sex hormones in females, which corresponds to the time required for follicular maturation and ovulation.
Certain hormones, such as melatonin, growth hormone (GH), and cortisol, follow a circadian rhythm in their secretion. For instance, in healthy adults, blood cortisol levels peak around 8 a.m. and reach their lowest levels at midnight. Disruptions in sleep patterns can significantly affect hormone secretion. Loss of circadian rhythms in hormone secretion can indicate endocrine disorders, such as the absence of cortisol secretion rhythms observed in Cushing's syndrome.
Additionally, many hormones exhibit pulsatile secretion. For example, gonadotropin-releasing hormone (GnRH) from the hypothalamus is secreted in pulses every 1 to 2 hours. Pituitary hormone secretion, such as luteinizing hormone (LH) and follicle-stimulating hormone (FSH), depends on the pulsatile release of GnRH. In contrast, continuous GnRH secretion inhibits LH and FSH secretion.
Both rhythmic and pulsatile hormone secretion can influence the measured concentration of hormones in the bloodstream when metabolic clearance rates are constant. Optimal sampling times and frequencies for blood hormone measurements need to consider these secretion patterns. For instance, 24-hour urine measurements can reflect the total daily secretion of a hormone but cannot provide insight into rhythmic variations. Plasma cortisol levels measured at 8 a.m. and midnight reflect the peak and trough of cortisol secretion, respectively, while 24-hour urinary free cortisol reflects total cortisol levels. Growth hormone displays marked pulsatile secretion, but IGF-1 levels are relatively stable and can provide a reliable indicator of growth hormone secretion.
Non-uniformity in hormone components can also affect measurement results. Hormone components may include hormone precursors, active hormone variants, active monomers, dimers, or multimers, as well as hormone degradation fragments. It is essential to account for the range of components assessed and their clinical significance when interpreting measurement results.
Hormone Transport and Metabolism
Hormones in the bloodstream exist in either free or bound states. Free hormones exert direct physiological effects, while bound hormones serve as a reservoir for transport and storage. Hormone transport carriers are primarily proteins. Peptide hormones generally bind less frequently to plasma proteins, whereas amine hormones, amino acid derivatives, and steroid hormones primarily bind plasma proteins, resulting in longer half-lives.
The major plasma proteins include albumin, which has low specificity but high binding capacity, and specific hormone transport proteins, such as thyroxine-binding globulin (TBG), sex hormone-binding globulin (SHBG), corticosteroid-binding globulin (CBG), IGF-binding protein (IGFBP), and GH-binding protein (GHBP). These transport proteins create a hormone reservoir that maintains stable hormone levels in the bloodstream.
Some hormones, such as growth hormone (GH) and insulin, are biologically active upon entering the bloodstream. Others require activation to exert their effects. For example, testosterone is converted into dihydrotestosterone (DHT) by 5α-reductase in male urogenital tissues and the liver, and vitamin D3 undergoes hepatic and renal hydroxylation to become biologically active.
Peptide hormones generally have short half-lives, lasting about 3–7 minutes. For instance, parathyroid hormone (PTH) has a half-life of only about 2 minutes, making intraoperative measurements useful for determining whether a parathyroid adenoma has been completely excised.
The half-life of steroid hormones and amino acid derivative hormones varies depending on their type and molecular structure, ranging from several hours to, in rare cases, several weeks. Thyroxine (T4) has a half-life of 7 days, requiring more than a month to reach a new steady state during hypothyroidism replacement therapy. Hormones undergo metabolic degradation primarily in the liver, kidneys, and peripheral tissues, and their metabolism affects half-life. For example, 25-(OH)D3 has a half-life of approximately 2–3 weeks, whereas the half-life of 1,25-(OH)2D3 is significantly shorter, about 6–8 hours. Impaired liver or kidney function may affect hormone inactivation. For example, estrogen degradation is markedly slowed in severe liver dysfunction, leading to feminization symptoms in affected male patients.
Mechanisms of Hormone Action
Hormones exert their effects by binding to their respective receptors. Based on the receptor location, hormones are classified as intracellular (nuclear or cytoplasmic) receptor hormones and membrane receptor hormones.
Intracellular (Nuclear or Cytoplasmic) Receptor Hormones
Steroid hormones, 1,25-(OH)2D3, retinoic acid, and thyroid hormones, among others, can cross the target cell membrane through diffusion, active uptake, or translocation. Within the cell, they bind to specific receptors located in the nucleus or cytoplasm to form hormone-receptor complexes. After binding, the receptor undergoes a conformational change, resulting in the formation of an "active complex," which interacts with the DNA of target genes to activate or inhibit gene transcription. This leads to the production of the corresponding biological effects.
Hormone-receptor complexes dissociate once receptor affinity decreases, after which the hormone is inactivated, and the receptor becomes available for recycling. These hormones often have natural or synthetic agonists and antagonists. Agonists enhance hormone effects by extending the hormone's half-life or increasing the affinity of hormone-receptor complexes, while antagonists reduce or block hormone action.
Membrane Receptor Hormones
Peptide hormones, neurotransmitters, growth factors, and prostaglandins, which are hydrophilic, cannot pass freely through the lipid-soluble cell membranes. Instead, they bind to specific receptors on target cell membranes, activating the receptors to generate intermediate compounds known as "second messengers" that regulate cellular functions. Hormone-receptor interactions at the membrane level are rapid and reversible. Despite the low concentration of membrane receptor hormones in the blood, high-affinity receptors are capable of selectively "capturing" specific hormones from the blood or extracellular fluid.
Receptors commonly have two binding sites: high-affinity, low-capacity sites and low-affinity, high-capacity sites. The binding between a receptor and its corresponding hormone is highly specific, with the highest binding affinity observed for the specific hormone or its analogs. Non-target cells may also have receptors in varying numbers—for example, lymphocytes, gonadal cells, and brain cells contain small amounts of insulin receptors, although their function in these cells remains unclear. Additionally, some steroid hormones also have membrane receptors present in their effector cells.
Membrane receptors consist of three segments: the extracellular domain, responsible for hormone recognition; the transmembrane domain; and the intracellular domain, which initiates intracellular signaling pathways. These intracellular signaling pathways activate or modify intracellular signal proteins through covalent modifications. Based on their structure, membrane receptors can generally be classified into four categories:
- G protein-coupled receptors (GPCRs);
- Receptors with intrinsic kinase activity (receptor kinases, RKs);
- Receptors associated with kinases (receptor-linked kinases, RLKs);
- Ligand-gated ion channel receptors (RLGICs).
Hormones binding to these receptors occurs rapidly and reversibly and can transmit signals through one or multiple pathways with cascade amplification. Various molecular pathways mediate the physiological effects of membrane receptor hormones. These include the following major pathways:
- Second messenger-mediated signaling pathways, such as the adenylate cyclase-cAMP-protein kinase A pathway, the phosphatidylinositol-Ca2+ signaling pathway, the DAG-protein kinase C pathway, the cGMP-protein kinase G pathway, and the nitric oxide synthase-nitric oxide pathway.
- Receptor tyrosine kinase-signal transduction protein-mitogen-activated protein kinase (MAPK) signaling pathways.
- Cytokine-STAT signaling pathways.
- Second messenger-protein kinase-DNA (gene expression) signaling pathways.
The rate of receptor synthesis or degradation determines the number of membrane receptors and regulates hormone activity. Besides receptor numbers, factors such as the hormone-receptor binding affinity, signal transduction across the membrane, and subsequent cascade amplification also play critical roles in regulating hormone activity.
Under normal conditions, receptors are diffusely distributed on membranes. After binding to hormones, hormone-receptor complexes are internalized through membrane invaginations (caveolae) to enter the cytoplasm, forming endosomes. Endosomes may fuse with lysosomes to undergo digestion and degradation or enter receptor recycling pathways. Hormones, meanwhile, are typically degraded and inactivated within this process.
Regulation of Hormones
Feedback Regulation of Hormones
Feedback regulation is the primary mechanism controlling hormone activity. The physiological concentration of circulating hormones is maintained by a balance between hormone secretion and clearance. Hormone secretion is tightly regulated by circulating hormone concentrations to maintain an optimal level for target cell activity. Common endocrine regulatory axes include the following:
Hypothalamic-Pituitary-Target Gland Axis
A feedback relationship exists between the hypothalamus, pituitary gland, and target glands (thyroid, adrenal cortex, and gonads). This regulation can occur through negative or positive feedback. Negative feedback occurs when hormones secreted by the hypothalamus and pituitary stimulate target glands, and excessive target gland hormone levels inhibit further secretion of hypothalamic and pituitary hormones to maintain target hormone levels within a normal range. Pituitary hormones can also directly inhibit hypothalamic hormone secretion via negative feedback to ensure proper secretion levels. Positive feedback operates in the opposite manner, where increased levels of target gland hormones stimulate further secretion of hypothalamic and pituitary hormones. One example of positive feedback is the regulation of gonadal hormones and hypothalamic-pituitary hormones during the menstrual cycle. In the follicular phase, FSH and LH stimulate the ovaries to secrete estrogen. As estrogen levels rise and approach the threshold for ovulation, the elevated estrogen enhances the secretion of FSH and LH from the hypothalamus and pituitary, resulting in a surge that triggers ovulation.
Renin-Angiotensin-Aldosterone System (RAAS)
RAAS represents a key system involved in regulating blood pressure, blood volume, and water-electrolyte balance. Other hormones, such as atrial natriuretic peptide (ANP) and antidiuretic hormone (ADH, also known as vasopressin), are part of this regulatory axis. Multiple tissues, including the kidneys, heart, ovaries, and testes, synthesize and secrete angiotensin II and angiotensin-converting enzyme. These locally acting systems significantly influence tissue function and remodeling.
Energy Metabolism Regulation System
Adipocytes synthesize and secrete hundreds of hormones, such as leptin and adiponectin, forming negative feedback loops with the body's nutritional state and weight. This system regulates energy intake, expenditure, and the metabolism of carbohydrates and fats. Additionally, this system integrates into the neuroendocrine-nutritional regulatory network.
Parathyroid Hormone-Calcitonin-1,25-Dihydroxyvitamin D3 System
This system plays a critical role in regulating bone metabolism and maintaining the stability of calcium, phosphorus, and magnesium levels in blood and extracellular fluids. The regulation of bone metabolism primarily involves three hormones (PTH, calcitonin, and 1,25-(OH)2D3) and three major organs (bone, kidney, and intestine). However, bone development, growth, maturation, and metabolism are also influenced by many other hormones, including growth hormone (GH), thyroid hormones, sex steroids, glucocorticoids, insulin, and IGF-1. Additionally, complex local regulatory mechanisms in bone tissue involve various factors like osteoprotegerin (OPG), receptor activator of NF-κB (RANK), and RANK ligand (RANKL), which couple bone resorption and formation to maintain normal bone metabolism.
Modes of Hormonal Interactions
The regulation of physiological processes, including endocrine and metabolic activities, relies on the interactions among multiple hormones. These interactions follow several modes:
Integrative Actions
Hormones selectively regulate different physiological processes to achieve a common physiological outcome. For instance, during hypoglycemia, insulin secretion decreases, while glucagon and glucocorticoid secretion increases, collectively resulting in elevated blood glucose levels.
Synergistic Actions
Multiple hormones may exert similar effects to achieve a common physiological goal. For example, during hypoglycemia, hormones such as adrenaline, glucagon, glucocorticoids, and growth hormone work together through different mechanisms to raise blood glucose levels, especially during severe hypoglycemia.
Antagonistic Actions
Certain hormones oppose the actions of others. For example, insulin lowers blood glucose levels and counteracts the hyperglycemic effects of hormones like adrenaline, glucagon, glucocorticoids, and growth hormone.
Interactions Between the Endocrine System and Other Systems
Neuroendocrine-Immune Regulatory Network
The nervous, endocrine, and immune systems share signaling molecules and receptors and exhibit similar signal transduction processes. These systems are widely and intimately interconnected, forming a network of mutual dependence and regulation that collectively governs vital physiological processes. These three systems jointly perceive internal and external environmental changes, process, store, and integrate information, and coordinate responses based on the body's overall needs.
Neuroendocrine-Nutritional Regulatory Network
This regulatory network integrates the central nervous system, neuroendocrine system (e.g., the hypothalamus), and effector organs and tissues (e.g., adipose tissue and the gastrointestinal tract) to regulate energy metabolism. It mediates physiological and biochemical responses to food and nutrients, manifesting as sensations of hunger or satiety and responses to appetite. Advances in this field include the discovery of various hypothalamic hormones involved in appetite regulation, such as neuropeptide Y (which enhances appetite) and pro-opiomelanocortin (POMC, which suppresses appetite). Hormones like leptin (secreted by adipocytes) and ghrelin (secreted by stomach mucosal cells) influence the hypothalamus to regulate the secretion of appetite-controlling hormones. Insulin, glucagon, and adrenaline also contribute to energy metabolism regulation. These hormones collectively target feeding centers in the hypothalamus and effector tissues (e.g., adipose tissue and the gastrointestinal tract) to regulate food intake, energy metabolism, and fat storage. Glucagon-like peptide-1 (GLP-1) receptor agonists, acting within the neuroendocrine-nutritional regulatory network, are currently used in the treatment of diabetes and obesity.
Metabolism
Metabolism is the fundamental process underlying human life activities. Through metabolism, the body exchanges and transforms substances with the external environment and breaks down, utilizes, and renews ingested materials. This provides the essential substances and energy required for survival, activity, growth, development, reproduction, and maintaining homeostasis. Metabolism consists of two primary processes: anabolism and catabolism.
Anabolism refers to the process whereby nutrients absorbed from the external environment undergo a series of chemical reactions to synthesize macromolecules and are converted to components of the body. These resultant macromolecules are stored in the body in forms such as glycogen, proteins, fats, and other compounds. This process is energy-consuming. Catabolism involves the degradation of macromolecules such as glycogen, proteins, and fats into smaller molecules, typically accompanied by the generation and release of energy.
Intermediate metabolism refers to the series of chemical reactions involved in both anabolic and catabolic processes after nutrients enter the body. Metabolic diseases may arise when one of these processes is disrupted. Nutritional diseases and metabolic diseases are closely related. For example, vitamin D deficiency often results in calcium-phosphorus metabolic abnormalities, while insufficient energy intake leads to malnutrition, and excessive energy intake causes obesity and diabetes.
Nutrients
To sustain life, growth, and development and to perform various life activities, humans must obtain nutrients from external sources such as food. Nutrients provide energy, build body tissues, and regulate physiological functions. These nutrients are classified into seven major categories: carbohydrates, fats, proteins, vitamins, minerals, dietary fiber, and water.
Macronutrients
Carbohydrates, proteins, and fats are categorized as macronutrients. After ingestion, macronutrients undergo digestion, absorption, and metabolism to produce glucose, amino acids, fatty acids, and glycerol. These smaller molecules participate in a series of chemical reactions to synthesize macromolecules that are converted into body components such as glycogen, proteins, fats, and other compounds for storage.
Certain amino acids that are essential for maintaining normal bodily functions cannot be synthesized by the body and must be obtained externally; these are called essential amino acids. Carbohydrates, proteins, and fats within the body can convert into one another as energy sources.
Minerals
Minerals, also known as inorganic salts, perform several physiological functions. They contribute to the structure of the body (e.g., bones and teeth), maintain cell membrane permeability, regulate osmotic pressure and acid-base balance, support the normal excitability of nerves and muscles, and participate in or mediate the activity of enzymes, hormones, and specific proteins.
For instance, calcium ions can act as second messengers to regulate lipase and adenylate cyclase activity and participate in converting fibrinogen to fibrin during blood clotting. Minerals cannot be synthesized in the body and must be obtained from food and water. Their distribution in body tissues and organs is uneven, and interactions such as synergy or antagonism exist among different mineral elements.
Certain minerals are required in small quantities; their physiological requirements often have a narrow range between adequacy and toxicity, making excessive intake potentially harmful. Minerals are classified into macroelements and microelements. Macroelements such as calcium, magnesium, sodium, chloride, phosphorus, and potassium are present in relatively large amounts in the body. Microelements are present in trace quantities (e.g., iron, fluorine, zinc, copper, nickel, selenium, vanadium, tin, manganese, iodine, molybdenum, chromium, cobalt, and silicon), but at least 14 of them perform specific physiological functions and are considered essential trace elements for the human body.
Vitamins
Vitamins are a group of organic compounds that humans and animals must obtain from food to maintain normal physiological functions. Found naturally in food, they are generally not synthesized in the body or are synthesized only in small amounts. Although the body requires them in minimal amounts, they play vital roles. Despite providing neither energy nor structural components for tissues, vitamins often function as coenzymes or cofactors in enzymatic activities.
Vitamins are essential for growth and development, metabolism, organ function, and delaying aging. They are classified into two types: fat-soluble (e.g., vitamins A, D, E, and K) and water-soluble (e.g., the B vitamins and vitamin C). Vitamins, along with essential trace elements, are collectively referred to as micronutrients. Although micronutrients are consumed in trace amounts, they have critical physiological roles, such as in enzyme catalysis, and deficiencies can lead to nutritional or metabolic diseases.
Dietary Fiber
Dietary fiber facilitates gastrointestinal motility, prevents constipation, aids in the elimination of harmful substances, and helps prevent intestinal tumors. Dietary fiber also contributes to weight control, blood sugar regulation, and lipid reduction.
The body has specific requirements for various nutrients, as well as recommended intake levels and tolerable upper intake limits. Nutritional imbalances occur when one or more essential nutrients are insufficient, excessive, or consumed in inappropriate proportions, or when functional or structural diseases within the body affect nutrient balance. These imbalances may result in nutritional diseases.
Digestion, Absorption, Metabolism, and Excretion of Nutrients
When food enters the gastrointestinal tract, it is broken down into monosaccharides, amino acids, short- and medium-chain fatty acids, and glycerol through the action of digestive fluids and enzymes. These components, along with water, salts, and vitamins, are absorbed into the bloodstream, while neutral fats and most long-chain fatty acids enter the blood circulation via the lymphatic system. These nutrients are transported to the liver and peripheral tissues, where they are utilized for synthesizing substances or providing energy. The body's own substances are also constantly broken down to provide energy or to synthesize new compounds.
The intermediate metabolism of various nutrients is governed by genetic factors and regulated by enzymes, hormones, and the neuroendocrine system. Factors such as the quality and quantity of metabolic substrates, cofactors, body fluid composition, ion concentrations, the chemical environment of the reaction, and the quality and quantity of intermediate and final metabolites all influence the regulation of intermediate metabolism. Substances produced during intermediate metabolism are either stored or reused by the body but are ultimately excreted in the form of water, carbon dioxide, nitrogenous compounds, or other metabolic products via the lungs, kidneys, intestines, or skin and mucosa.
Abnormal intake, synthesis, or excretion of one or more nutrients can result in nutritional diseases. For example, when energy intake exceeds expenditure, obesity may occur. During decompensated liver cirrhosis, impaired albumin synthesis may lead to hypoalbuminemia, and diarrhea may result in hypokalemia. In certain situations, such as fever, hyperthyroidism, tumors, chronic wasting diseases, post-major surgeries, as well as during growth, pregnancy, or lactation, the body's demand for nutrients increases. Insufficient supply in such cases can lead to nutrient deficiencies.
Energy Metabolism
The body's energy supply and consumption exist in a dynamic state of balance. The total daily energy expenditure consists of basal metabolic energy expenditure (which accounts for 50% to 70%) and energy expenditure from physical activity. Basal metabolic energy expenditure varies based on factors such as sex, age, height, and weight. Beyond basal metabolic needs, additional energy is often required for special functional activities, such as growth, development, pregnancy, and lactation. This additional energy expenditure is referred to as "special functional activity energy expenditure." From the age of 60, energy expenditure decreases by approximately 0.7% annually.
The energy demand due to physical activity varies depending on the intensity and duration of activity. For instance, the energy requirements for light, moderate, and heavy physical activities are approximately 30%, 50%, and 100% of the basal metabolic energy expenditure, respectively. During metabolism, the body converts the energy contained in macronutrients from external sources into energy that can be stored and used by the body. The efficiency of this energy conversion varies greatly among individuals.
Metabolic diseases may arise from dysfunction at any stage of the digestion, absorption, metabolism, transformation, or excretion of energy substances. These dysfunctions can result from substrate insufficiency or excess, abnormalities in enzymes, hormones, or other regulatory factors, or structural or functional defects in metabolic tissues. For instance, the malabsorption of carbohydrates may result from gastrointestinal inflammation, structural or functional defects of the digestive tract, or abnormalities in glucose transport proteins. Inherited metabolic disorders, such as phenylketonuria, are caused by deficiencies in enzymes like phenylalanine hydroxylase.