## INTRODUCTION

• absolute $V˙O2$ the amount of oxygen consumed over a given time period; expressed as L · min−1.

• acceptable macronutrient distribution ranges ranges of intakes for carbohydrate, fat, and protein that provide adequate amounts of vitamins and minerals and reduce the risk of developing diet-related chronic diseases such as type 2 diabetes and heart disease.

• acclimation the change that occurs in response to repeated environmental stresses and results in the improved function of an existing homeostatic system. In general, acclimation is commonly used to refer to a rapid physiological adaptation that occurs within days to a few weeks.

• acclimatization a gradual, long-term adaptation of an organism (e.g., humans) to a change in the environment (e.g., heat exposure). Acclimatization results in the improved function of an existing homeostatic system. Although acclimatization and acclimation are similar terms, acclimatization is often used to describe a gradual physiological adaptation that occurs within months to years of exposure to the environmental stress.

• Acetyl-CoA a two carbon moledule formed from pyruvate produced by glycolysis or from the oxidation of fatty acids or amino acids. Acetyl-CoA can enter the citric acid cycle and undergoes oxidation resulting in the release of electrons into the electron transport chain.

• acidosis an abnormal increase in blood hydrogen ion concentration (i.e., arterial pH below 7.35).

• acids compounds capable of giving up hydrogen ions into solution.

• acromegaly a condition caused by hypersecretion of growth hormone from the pituitary gland; characterized by enlargement of the extremities, such as the jaw, nose, and fingers.

• actin a structural protein of muscle that works with myosin in permitting muscular contraction.

• action potential the all-or-none electrical event in the neuron or muscle cell in which the polarity of the cell membrane is rapidly reversed and then reestablished.

• activation energy energy required to initiate a chemical reaction. The term activation energy is sometimes referred to as the energy of activation.

• adaptation the term adaptation refers to a change in the structure and function of a cell or organ system that results in an improved ability to maintain homeostasis during stressful conditions.

• adenosine diphosphate (ADP) a molecule that combines with inorganic phosphate to form ATP.

• adenosine triphosphate (ATP) the high-energy phosphate compound synthesized and used by cells to release energy for cellular work.

• adenylate cyclases enzyme found in cell membranes that catalyzes the conversion of ATP to cyclic AMP.

• adequate intakes (AI) recommendations for nutrient intake when insufficient information is available to set an RDA standard.

• adiponectin is a protein hormone that modulates a number of metabolic processes, including glucose regulation and fatty acid oxidation.

• adrenal cortex the outer portion of the adrenal gland. Synthesizes and secretes corticosteroid hormones, such as cortisol, aldosterone, and androgens.