Glossary of Terms & Definitions
Listed Alphabetically - "M"

Macrocytic anemia: Low red blood cell count, characterized by the presence in the blood of larger than normal red blood cells.

Macrophage: Phagocytic cell derived from a monocyte which may be fixed or wandering.

Macula: A small area of the retina where vision is the keenest. The macula is located in the center of the retina and provides central vision. Activities that require central vision include driving, reading and other activities that require sharp, straight-ahead vision.

Magnetic Resonance Imaging (MRI): A diagnostic procedure that uses a special imaging technique with a powerful magnet and a computer to provide clear images of soft tissues. Tissues that are well-visualized using MRI include the brain and spinal cord, abdomen, and joints.

Malabsorption syndrome: A disease or condition that results in poor absorption of nutrients from food.

Malaise: Discomfort, uneasiness and indisposition often indicative of malaria.

Malaria: An infectious disease caused by parasitic microorganisms called plasmodia. Malaria can be spread among humans through the sting of certain types of mosquitos (Anopheles) or by a contaminated needle or transfusion. Malaria is a major health problem in the tropics and subtropics, affecting over 200 million people world wide.

Malignant: Referring to diseases that tend to become worse and cause death;  cancerous

Malnutrition: A state of bad or poor nutrition that may be due to inadequate food intake, imbalance of nutrients, malabsorption of nutrients, improper distribution of nutrients increased nutrient requirements, increased nutrient loses or over-nutrition.

Megaloblastic anemia: Low red blood cell count, characterized by the presence in the blood of large, immature, nucleated cells (megaloblasts) that are forerunners of red blood cells. Red blood cells, when mature, have no nucleus.

Melanin: A dark brown pigment found in the skin.

Membrane potential: The electrical potential difference across a membrane. The membrane potential is a result of the concentration differences between potassium and sodium across cell membranes which are maintained by ion pumps. A large proportion of the body's resting energy expenditure is devoted to maintaining the membrane potential, which is critical for nerve impulse transmission, muscle contraction, heart function, and the transport of nutrients and metabolites in and out of cells.

Menstruation: The cyclic loss of blood by a woman, from her uterus (womb) when she is not pregnant. Menstruation generally occurs every 4 weeks after a woman has reached sexual maturity and prior to menopause.

Meta-analysis: A mathematical or statistical analysis, used to pool the results of all studies investigating a particular effect (e.g., the effect of folic acid supplementation on homocysteine levels) and provide an overall estimate of that effect.

Metabolism: Physical and chemical processes within the body involving energy production and utilization.

Metabolite: A compound derived from the metabolism of another compound is said to be a metabolite of that compound.

Metastasize: To spread from one part of the body to another. Cancer is said to metastasize when it spreads from the primary site of origin to a distant anatomical site.

Methionine: A sulfur containing amino acid, required for protein synthesis and other vital metabolic processes. It can be obtained through the diet in protein or synthesized from homocysteine.

Methylation: A biochemical reaction resulting in the addition of a methyl group (-CH3) to another molecule.

Migraine headache: A type of headache thought to be related to abnormal sensitivity of blood vessels (arteries) in the brain to various triggers resulting in rapid changes in the artery size due to spasm (constriction). Other arteries in the brain and scalp then open (dilate), and throbbing pain is perceived in the head. The tendency toward migraine appears to involve serotonin, a neurotransmitter that can trigger the release of vasoactive substances in the blood vessels.

Minerals: Nutritionally significant elements. Elements are composed of only one kind of atom. Minerals are inorganic, i.e., they do not contain carbon as do vitamins and other organic compounds.

Mitochondria: Energy-producing structures within cells. Mitochondria possess two sets of membranes, a smooth continuous outer membrane, and an inner membrane arranged in folds. Among other critical functions, mitochondria convert nutrients into energy via the electron transport chain.

mm Hg: Millimeters of mercury. The unit of measure for blood pressure.

Mole: The fundamental unit for measuring chemical compounds (abbreviated mol). One mole equals the molecular weight of a compound in grams. The number of molecules in a mole is equal to 6.02 x 1023 (Avogadro's number).

Multifactorial: Refers to a disorder or condition that has a number of different causes.

Multiple sclerosis (MS): An autoimmune disorder, which results in the demyelinization of nerves. In MS, the myelin shealth that allows for efficient transmission of nerve impulses is damaged, resulting in progressive neurological symptoms such as, numbness, tingling, loss of control of certain bodily functions, and paralysis.

Mutation: A change in a gene, in other words, a change in the sequence of base pairs in the DNA that makes up a gene. Mutations in a gene may or may not result in an altered gene product.

Myelin: The fatty substance that covers myelinated nerves. Myelin is a layered tissue surrounding the axons or nerve fibers. This sheath acts as a conduit in an electrical system, allowing rapid and efficient transmission of nerve impulses. Myelination refers to the process in which nerves acquire a myelin sheath. 

Myocardial infarction (MI): Commonly known as a "heart attack", a myocardial infarction refers to changes that occur in the heart muscle due to an interruption in its blood supply. An MI is often the result of a clot that lodges in a coronary artery, resulting in deprivation of oxygen to a portion of the heart muscle (ischemia), and ultimately the death (necrosis) of a portion of the heart muscle, if the oxygen supply is not restored within a few minutes.

Myocarditis: An inflammation of the heart muscle.

Myoglobin: A heme-containing pigment in muscle cells that binds and stores oxygen.