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Leptin is a peptide hormone produced primarily by adipocytes, especially in white adipose tissue. Its circulating concentration generally reflects the amount of stored body fat, although it also changes with short-term energy availability. Leptin acts as …
The limbic system is a traditional neuroanatomical concept referring to a group of interconnected cortical and subcortical brain structures associated with functions such as memory, emotion, motivation, and the coordination of behavior with internal physiological states. …
A limiting amino acid is the indispensable amino acid supplied in the lowest proportion relative to the amount specified in a reference amino acid pattern. When the amino acid composition of a dietary protein is compared …
Linoleic acid is an essential polyunsaturated fatty acid containing 18 carbon atoms and two cis double bonds. It is written in fatty-acid shorthand as 18:2 n-6, or 18:2 omega-6. The omega-6 designation means that the first …
A lipid-soluble antioxidant is an antioxidant that preferentially dissolves in or associates with lipid-rich environments rather than aqueous compartments. In biological systems, these environments include cell membranes and circulating lipoproteins. This distribution allows lipid-soluble antioxidants to …
Lipids are a broad and chemically diverse group of biological molecules characterized by hydrophobic or amphipathic properties. Some lipids are predominantly hydrophobic, while others are amphipathic, meaning that they contain both water-attracting and water-repelling regions. Unlike …
Lipolysis is the enzymatic breakdown of triacylglycerols, also called triglycerides, into fatty acids and glycerol. It occurs in many tissues but is especially important in adipose tissue, where large amounts of energy are stored as triglycerides …
Listeria monocytogenes is a species of Gram-positive, rod-shaped bacterium that can cause the foodborne disease listeriosis. It is facultatively anaerobic, meaning it can grow with or without oxygen, and occurs widely in environments such as soil, …
The lower esophageal sphincter, or LES, is a specialized high-pressure zone at the junction between the esophagus and stomach. Its main function is to help prevent the backward movement, or reflux, of stomach contents into the …
The lymphatic system is a network of lymphatic vessels, lymph nodes, and associated lymphoid tissues that helps maintain fluid balance, transport material from tissues, and support immune function. Fluid that leaves blood capillaries enters the spaces …
The M-line is a protein-rich structure at the center of a sarcomere, the repeating contractile unit of striated skeletal and cardiac muscle. It lies in the middle of the A band, within the central region occupied …
A MET, or metabolic equivalent of task, is a unit used to express the energy cost or intensity of a physical activity relative to a standardized resting value. One MET represents the conventional energy expenditure of …
Macronutrients are nutrients required by the body in relatively large amounts. In human nutrition, the three main energy-yielding macronutrients are carbohydrates, fats, and proteins. They provide energy and also supply chemical building blocks needed for normal …
Malnutrition is a state resulting from deficient, excessive, or imbalanced intake or utilization of energy or nutrients. In its broad public-health meaning, malnutrition includes undernutrition, micronutrient deficiencies or excesses, overweight, obesity, and diet-related noncommunicable diseases. Malnutrition …
Mastication is the process of chewing food in preparation for swallowing. It involves coordinated movements of the jaw, teeth, tongue, and muscles of the face and mouth. These movements break food into smaller particles and repeatedly …
The menstrual cycle is the recurring sequence of hormonal, ovarian, and uterine changes that prepares the reproductive system for a possible pregnancy. A cycle is counted from the first day of menstrual bleeding to the first …
Metabolism is the complete set of chemical reactions that occur within living cells and organisms. These reactions transform nutrients and other molecules, provide energy for biological processes, and produce materials needed to build and maintain cells …
Micronutrients are essential vitamins and minerals that the body needs in relatively small amounts. The term includes vitamins such as vitamins A, C, D, E, K, and the B vitamins, as well as minerals such as …
Microvilli are microscopic, finger-like projections of the plasma membrane that extend from the surface of many types of cells. They are especially prominent on absorptive epithelial cells, such as enterocytes lining the small intestine. By increasing …
Mitochondria are membrane-bound organelles in eukaryotic cells that carry out major parts of aerobic energy metabolism. The singular form is mitochondrion. Their best-known function is producing adenosine triphosphate, or ATP, through oxidative phosphorylation, converting energy released …
Mixed micelles are small, water-compatible aggregates that form in the small intestine from bile salts together with products of dietary fat digestion and other lipids. They commonly contain bile salts, fatty acids, monoacylglycerols, phospholipids or their …
Mobility generally refers to the ability to move the body or a joint through a useful range of motion. In exercise and sport contexts, the term is often used to emphasize active, controlled movement through an …
A moment arm is the perpendicular distance between the line of action of a force and the axis about which that force produces rotation. In human biomechanics, a muscle’s moment arm is therefore the shortest perpendicular …
A monoacylglycerol (MAG) is a glycerolipid consisting of a glycerol molecule with one fatty acyl group attached by an ester bond. Because glycerol has three carbon positions, the fatty acyl group can be attached at different …
A motor neuron, or motoneuron, is a nerve cell involved in controlling muscle activity. In the somatic motor system, lower motor neurons located in the spinal cord or brainstem send axons to skeletal muscle and provide …
A multi-joint exercise is an exercise in which movement occurs at two or more joints during performance. These exercises typically require coordinated action from several muscles or muscle groups. In resistance training, common examples include squats, …
Muscle atrophy is a reduction in the size or mass of muscle tissue. In skeletal muscle, it commonly involves shrinkage of individual muscle fibers, seen as a decrease in their cross-sectional area and loss of contractile …
A muscle spindle is a specialized sensory receptor located within skeletal muscle that detects muscle length and changes in length, including the speed of stretch. Muscle spindles provide the nervous system with information used for proprioception, …
A muscle–tendon unit, or MTU, is the functional mechanical system formed by skeletal muscle and the tendinous tissues connected to it. It includes the force-generating muscle fibers together with force-transmitting structures such as tendons and, where …
Muscular endurance is the ability of a muscle or group of muscles to repeatedly produce force or sustain force over time before task-limiting fatigue occurs. It is usually assessed during submaximal muscular work, meaning the resistance …
Muscular failure in resistance training refers to the point during a set when a person can no longer complete another repetition against the given resistance despite attempting to do so. A more precise term is momentary …
Myelin is a specialized, multilayered membrane that wraps around selected axons in the nervous system. It is produced by glial cells, specifically oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. …
Myosin refers to a large superfamily of actin-associated molecular motor proteins. Myosins use energy from adenosine triphosphate, or ATP, to generate mechanical force or movement relative to actin filaments, which are part of the cell's cytoskeleton. …
NADPH, or reduced nicotinamide adenine dinucleotide phosphate, is a coenzyme that carries reducing power between biochemical reactions. It is the reduced form of NADP+ and can donate electrons to other molecules during oxidation-reduction reactions. NADPH is …
NAD⁺ and NADH are the oxidized and reduced forms of nicotinamide adenine dinucleotide, a coenzyme that carries electrons in cellular oxidation-reduction reactions. NAD consists of two nucleotide units, one containing adenine and the other nicotinamide. Its …
Neurodevelopment is the process by which the nervous system forms, organizes, and matures. It begins during embryonic development and continues after birth as the nervous system undergoes structural and functional change. Neurodevelopment includes the production and …
The neuromuscular junction, or NMJ, is the specialized chemical synapse where a motor neuron communicates with a skeletal muscle fiber. It consists of the motor neuron's presynaptic nerve terminal, the narrow synaptic cleft between the cells, …
A nicotinic acetylcholine receptor, or nAChR, is a membrane protein that acts as both a receptor for the neurotransmitter acetylcholine and a ligand-gated ion channel. When acetylcholine binds, the receptor changes shape and opens a pore …
Noncovalent interactions are attractive or repulsive interactions between atoms or molecules that do not involve formation of a covalent bond between the interacting groups. In biological molecules, they help stabilize three-dimensional structures and allow molecules to …
A nondigestible carbohydrate is a carbohydrate that resists digestion and absorption in the human small intestine and therefore reaches the large intestine to some extent. Rather than being fully broken down into absorbable sugars by human …
Nonheme iron is dietary iron that is not incorporated into a heme molecule. It is one of the two major forms of dietary iron, the other being heme iron. Nonheme iron is the form present in …
Nutrient density describes the amount of one or more nutrients in a food or diet relative to a defined reference amount. In a traditional nutritional definition, it refers to nutrient content relative to the energy provided …
Nutrition is the process by which the body obtains and uses nutrients from food and beverages to support growth, development, maintenance, and normal physiological function. The term also refers to the scientific study of these processes …
The Nutrition Facts label is a standardized nutrition information panel required on most packaged foods and beverages sold in the United States. It is designed to help consumers understand the nutritional composition of a food and …
An oral food challenge, or OFC, is a medically supervised test in which a person eats a suspected allergenic food in controlled, gradually increasing amounts while being observed for signs of an allergic reaction. It is …
Osmoregulation is the physiological control of the osmotic concentration of body fluids. In humans, it helps keep plasma osmolality within a narrow range despite changes in water intake and water loss. This is important because changes …
Osmosis is the net movement of water across a selectively permeable membrane in response to a difference in solute concentration across that membrane. When the membrane allows water to cross more readily than the relevant solute, …
Oxidation is a chemical process in which a substance loses electrons or in which the oxidation state of one or more of its atoms increases. Historically, oxidation referred to reactions involving oxygen, and many oxidation reactions …
