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Calcitriol is the principal hormonal form of vitamin D, chemically known as 1,25-dihydroxyvitamin D. It is produced after vitamin D undergoes two hydroxylation steps. Vitamin D is first converted mainly in the liver to 25-hydroxyvitamin D, …
Calcium homeostasis is the set of physiological processes that keeps the concentration of calcium in extracellular fluid, especially ionized calcium in blood, within a narrow range. This regulation is essential because calcium is required for functions …
Capillarization refers to the extent of the capillary network supplying a tissue. In skeletal muscle physiology, it describes the number and distribution of capillaries in relation to muscle fibers. These microscopic blood vessels provide an important …
Carbohydrate counting estimates the carbohydrate in food portions, but equal counts do not guarantee equal glucose responses across foods or people.
Carbohydrate loading, also called glycogen loading, is a pre-exercise nutrition strategy designed to increase the amount of glycogen stored in skeletal muscle before prolonged endurance exercise. Glycogen is the stored form of glucose in muscle and …
A carbohydrate-electrolyte drink is a water-based beverage containing carbohydrates together with dissolved electrolytes. These drinks are commonly formulated for use during or after exercise, when fluid, carbohydrate, and electrolytes may need to be replaced together. Sodium …
Carbohydrates are a broad class of organic compounds that includes monosaccharides, oligosaccharides, polysaccharides, and compounds derived from them. Monosaccharides are basic carbohydrate units and include molecules such as glucose. They can be joined through glycosidic bonds …
Cardiac muscle is the specialized muscle tissue that forms the myocardium, the contractile middle layer of the heart wall. It is one of the three major types of muscle tissue, alongside skeletal and smooth muscle. Its …
Cardiometabolic is an adjective describing the interconnected metabolic and cardiovascular aspects of health, risk, and disease. It is commonly used in terms such as cardiometabolic health, cardiometabolic risk, and cardiometabolic disease. The concept reflects links between …
Cardiomyocytes, also called cardiac myocytes, are the specialized muscle cells of the heart. They form the contractile component of the myocardium and generate the force that pumps blood. Most working cardiomyocytes are elongated, striated cells containing …
Cardiorespiratory endurance is the capacity to sustain rhythmic, large-muscle exercise through the integrated transport and use of oxygen. It is a component of physical fitness and is closely related to concepts such as cardiorespiratory fitness, aerobic …
Cardiorespiratory fitness describes the body's capacity to deliver and use oxygen during exercise, rather than the amount of activity a person reports.
Cardiovascular drift is the gradual change in cardiovascular responses that occurs during prolonged, steady-intensity exercise. It is characterized mainly by a progressive rise in heart rate together with a decline in stroke volume, even though the …
Cariogenicity is the capacity of a substance, food, or dietary exposure to promote the development of dental caries. In nutrition and dentistry, the term is commonly used to describe the caries-promoting potential of foods and carbohydrates. …
Carotenoids are a large family of naturally occurring pigments characterized by an extended system of conjugated double bonds. They are produced by plants, algae, and many microorganisms and contribute yellow, orange, and red coloration to many …
The carotid bodies are small, paired sensory organs located near the bifurcation of the common carotid arteries in the neck. They are the principal peripheral chemoreceptor organs for monitoring the chemical composition of arterial blood and …
Catabolism is the part of metabolism that breaks down larger or more complex molecules into smaller products. Catabolic pathways include the degradation of carbohydrates, fats, and amino acids. These reactions often release free energy, some of …
Catechins are a group of plant compounds in the flavan-3-ol branch of the flavonoid family. Flavonoids are themselves part of the broader group called polyphenols. The plural name describes related molecules, whereas “catechin” can name one …
Catecholamines form a family of signaling molecules made from tyrosine, with effects that depend on the molecule, receptor and tissue involved.
Celiac disease is a chronic immune-mediated disorder in which eating gluten triggers an abnormal immune response that damages the lining of the small intestine in genetically susceptible people. Gluten is a group of proteins found in …
Central chemoreceptors are chemosensitive cells and neural circuits within the brain that help regulate breathing by detecting changes related to carbon dioxide and acidity. They are located primarily within the brainstem, particularly the medulla oblongata. The …
The central nervous system, or CNS, is the part of the nervous system composed of the brain and spinal cord. It serves as a major center for receiving and integrating sensory information, generating motor commands, and …
Cereal fiber comes from grains and includes components with different properties, so equal fiber amounts need not behave identically during digestion.
Cerebral edema is an abnormal accumulation of water in brain tissue that increases brain volume. The excess water may accumulate inside brain cells, in the extracellular space between cells, or both, depending on the underlying mechanism. …
Chemiosmosis is the coupling of cellular work to the energetically favorable movement of ions down an electrochemical gradient across a membrane, most notably during ATP synthesis. In many biological energy-conversion systems, the relevant ion is the …
Chronic low-grade inflammation describes sustained modest inflammatory activity, without defining one diagnosis or a single universal laboratory cutoff.
Chylomicrons are large, triglyceride-rich lipoprotein particles produced by absorptive cells of the small intestine, called enterocytes. Their main function is to transport lipids absorbed from food, particularly triglycerides, through the lymph and bloodstream. They also carry …
Chyme is the semifluid mixture of partially digested food and digestive secretions that forms in the stomach during digestion. After swallowed food, called a food bolus, enters the stomach, it is mixed and mechanically broken down …
Chymotrypsin is a digestive enzyme that hydrolyzes peptide bonds within proteins and peptides in the small intestine. It belongs to the serine protease family and is an endopeptidase, meaning that it cleaves peptide bonds within a …
Circuit training is an exercise-training method in which several different exercises or stations are performed sequentially, usually with relatively short rest periods between them. After completing the stations, the sequence may be repeated for additional circuits. …
The clavicle, or collarbone, is an S-shaped long bone that extends horizontally across the upper front of the chest between the sternum and the scapula. Together with the scapula, it forms the pectoral, or shoulder, girdle. …
Clean eating has no agreed scientific definition, so nutritional adequacy and the effects of dietary rules matter more than the label itself.
The coccyx, commonly called the tailbone, is the small, triangular bone at the lowest end of the vertebral column, immediately below the sacrum. It develops from coccygeal vertebral segments, with three to five segments commonly reported …
Colonic fermentation is the anaerobic microbial breakdown of substrates in the large intestine. It occurs mainly when dietary components escape digestion and absorption in the small intestine and reach the colon, where resident microorganisms can use …
Colonocytes are the absorptive epithelial cells found in the lining of the colon, or large intestine. They form part of the single layer of epithelial cells that separates the intestinal contents from underlying tissues. Colonocytes arise …
The term complete protein is commonly used for a dietary protein that supplies all of the indispensable, or essential, amino acids in proportions that meet a human reference amino acid pattern. Humans require nine indispensable amino …
CTGF interacts with cells and extracellular matrix to shape tissue remodeling, with actions that depend on context rather than one simple growth effect.
Consistency in habit research concerns repeated behavior in similar circumstances, allowing recurring cues to become linked with a response.
Contractile tissue is tissue containing cells specialized to generate mechanical force through contraction. In standard human anatomy and physiology, the term most commonly refers to muscle tissue, one of the body's four primary tissue types. Human …
Cortisol helps regulate normal physiology throughout the day, with changing secretion that cannot be fully described by one measurement or stress alone.
Creatine is a nitrogen-containing organic compound involved in rapid cellular energy transfer. The body obtains creatine through both endogenous synthesis and the diet. It is synthesized from amino-acid-derived precursors, principally arginine and glycine, with a methyl …
Criterion-related validity examines whether test scores relate to a relevant external measure, rather than merely producing consistent results.
The cross-bridge cycle is the repeating sequence of molecular interactions between myosin and actin that generates force during muscle contraction. A cross-bridge forms when a myosin head on a thick filament attaches to actin on a …
Cross-contamination is the transfer of harmful microorganisms from a contaminated food, surface, object, or person to another food or food-contact surface. In food safety, the term commonly describes the spread of disease-causing microorganisms such as bacteria …
DXA, or dual-energy X-ray absorptiometry, is an imaging technique that uses low-dose X-rays at two different energy levels to assess bone mineral and estimate body composition. As the X-rays pass through the body, bone and soft …
De novo lipogenesis (DNL) is the metabolic process by which cells synthesize new fatty acids from acetyl-CoA rather than obtaining those fatty acids directly from dietary or stored fat. Acetyl-CoA can be generated from several metabolic …
A dehydrogenase is an enzyme that catalyzes an oxidation-reduction reaction in which a substrate is oxidized and electrons, often transferred together with hydrogen, are passed to an electron acceptor. Dehydrogenases belong to the broad enzyme class …
Dementia describes cognitive or behavioral decline that disrupts daily activities, with different underlying diseases rather than normal aging alone.
