Adenosine — A metabolic byproduct that accumulates during wakefulness, creating sleep pressure. Caffeine works by blocking adenosine receptors.

Adipokine — A signaling molecule secreted by adipose (fat) tissue that regulates appetite, metabolism, and inflammation (e.g., leptin, adiponectin).

AMY1 — Gene encoding salivary amylase. High copy number correlates with efficient starch digestion and greater carbohydrate tolerance.

Anabolism — The metabolic process of building complex molecules from simpler ones (e.g., muscle protein synthesis, glycogen storage). Requires energy input.

Autophagy — A cellular "self-cleaning" process in which damaged proteins and organelles are degraded and recycled. Upregulated during fasting and exercise.

Basal Metabolic Rate (BMR) — The energy expended by the body at complete rest to maintain vital functions (breathing, circulation, organ function).

Beta-Oxidation — The biochemical process by which fatty acids are broken down in the mitochondria to generate ATP (cellular energy).

BDNF (Brain-Derived Neurotrophic Factor) — A protein that supports neuron survival, growth, and synaptic plasticity. Elevated by aerobic exercise. Associated with memory, learning, and mood.

Catabolism — The metabolic process of breaking down complex molecules into simpler ones, releasing energy (e.g., muscle protein breakdown, glycogenolysis).

Circadian Rhythm — The body's intrinsic approximately 24-hour biological clock governing sleep-wake cycles, hormone secretion, metabolism, and organ function.

CNS Fatigue (Central Nervous System Fatigue) — Systemic neural fatigue from high-intensity exercise that impairs coordination, motivation, and performance for 24–72 hours post-session.

Cortisol — A glucocorticoid hormone secreted by the adrenal cortex in response to stress. Mobilizes energy; chronically elevated levels promote fat storage and muscle catabolism.

DIAAS (Digestible Indispensable Amino Acid Score) — A measure of protein quality based on digestibility and amino acid profile relative to human requirements. Replaces the older PDCAAS metric.

EPOC (Excess Post-Exercise Oxygen Consumption) — Elevated oxygen uptake and caloric expenditure persisting after exercise as the body restores homeostasis. Magnitude determined primarily by exercise intensity.

Estrogen — Primary female sex hormone; produced by the ovaries (and to a lesser extent adipose tissue). Influences fat distribution, bone density, cardiovascular health, and mood.

Ghrelin — The primary hunger hormone, secreted by the stomach. Rises before meals and during caloric restriction; signals appetite to the hypothalamus.

GLP-1 (Glucagon-Like Peptide-1) — An incretin hormone secreted by the gut after eating. Stimulates insulin release, suppresses glucagon, slows gastric emptying, and reduces appetite.

GLUT-4 — A glucose transporter protein in muscle and fat cells. Moves to the cell membrane in response to insulin or muscle contraction to facilitate glucose uptake.

Glycemic Index (GI) — A ranking of carbohydrate-containing foods by how quickly they raise blood glucose relative to pure glucose (GI = 100). A population-level average; individual responses vary significantly.

Glycogen — The storage form of glucose in the body. Stored primarily in skeletal muscle (~400g) and liver (~100g). Primary fuel for moderate-to-high-intensity exercise.

HbA1c (Glycated Haemoglobin) — A blood marker reflecting average blood glucose over the previous 2–3 months. Standard diagnostic marker for diabetes and pre-diabetes.

HPA Axis (Hypothalamic-Pituitary-Adrenal Axis) — The neuroendocrine system governing the body's stress response and cortisol secretion.

HRV (Heart Rate Variability) — The beat-to-beat variation in heart rate. Higher HRV indicates greater autonomic nervous system flexibility and recovery capacity.

Hyperinsulinemia — Chronically elevated circulating insulin, typically associated with insulin resistance. A key driver of visceral fat accumulation and metabolic syndrome.

Hypertrophy — The increase in muscle cell size (and therefore muscle volume) in response to resistance training and adequate nutrition.

IGF-1 (Insulin-Like Growth Factor 1) — A growth hormone downstream effector. Promotes cell proliferation and survival; stimulated by insulin and dietary protein.

Insulin — A hormone secreted by pancreatic beta cells in response to elevated blood glucose. Promotes glucose uptake into cells, glycogen synthesis, and fat storage.

Insulin Resistance — A condition in which cells fail to respond adequately to insulin signaling, requiring progressively higher insulin concentrations to achieve normal glucose disposal.

Ketones (Ketone Bodies) — Alternative fuel molecules (beta-hydroxybutyrate, acetoacetate, acetone) produced by the liver from fatty acids during carbohydrate restriction or prolonged fasting. Primary fuel in nutritional ketosis.

Leptin — A hormone secreted by adipose tissue that signals energy sufficiency and satiety to the hypothalamus. Levels fall during caloric restriction, driving increased hunger.

Leucine — An essential branched-chain amino acid (BCAA) that acts as the primary molecular trigger for muscle protein synthesis (MPS). Found highest in dairy, meat, and eggs.

LISS (Low-Intensity Steady-State Cardio) — Sustained cardiovascular exercise at 50–65% of maximum heart rate. Primarily uses fat as fuel; minimal CNS stress; can be performed daily.

Metabolic Adaptation — The physiological response to sustained caloric restriction involving reduced BMR, decreased NEAT, and hormonal shifts that collectively resist further fat loss.

Microbiome — The collective community of microorganisms (bacteria, fungi, viruses) residing primarily in the large intestine. Influences digestion, immunity, neurotransmitter production, and metabolic function.

MPS (Muscle Protein Synthesis) — The biological process of assembling new muscle protein from amino acids. Stimulated by resistance exercise and dietary leucine.

NEAT (Non-Exercise Activity Thermogenesis) — Energy expended through all physical movement that is not deliberate exercise — walking, standing, fidgeting, household activities. Can account for 15–50% of TDEE.

NF-κB (Nuclear Factor Kappa B) — A transcription factor that regulates genes involved in inflammation. Chronically activated in obesity, poor diet, and sedentary lifestyle; suppressed by exercise and anti-inflammatory phytonutrients.

PCOS (Polycystic Ovary Syndrome) — The most common endocrine disorder in reproductive-age women, characterized by hyperandrogenism, ovulatory dysfunction, and typically insulin resistance.

Phosphocreatine — The immediate ATP reservoir in muscle cells, replenished by creatine. Depleted within 8–10 seconds of maximal effort; replenished over 2–3 minutes of rest.

Progressive Overload — The systematic, gradual increase in training stimulus over time to continue driving adaptation. The fundamental principle of long-term strength and hypertrophy development.

Sarcopenia — Age-related loss of skeletal muscle mass and strength. Begins in the 30s at ~1%/year without resistance training; accelerates after age 60.

SCFAs (Short-Chain Fatty Acids) — Metabolic byproducts of bacterial fermentation of dietary fiber in the colon (butyrate, propionate, acetate). Support gut barrier integrity, reduce inflammation, and stimulate satiety hormones.

Set-Point Theory — The hypothesis that the body has a defended weight range that it actively resists deviation from through hormonal and metabolic adaptations.

TDEE (Total Daily Energy Expenditure) — The total calories burned in a 24-hour period across all activities (BMR + TEF + EAT + NEAT).

TEF (Thermic Effect of Food) — The energy cost of digesting, absorbing, and metabolizing food. Protein has the highest TEF (20–35%); fat the lowest (2–3%).

Testosterone — The primary male sex hormone (also present in women at lower levels). Promotes muscle protein synthesis, fat metabolism, bone density, libido, and mood.

VO₂ max — The maximum rate of oxygen consumption during intense exercise. The gold standard measure of cardiorespiratory fitness. Higher VO₂ max correlates strongly with longevity.

Visceral Fat — Fat deposited around internal abdominal organs (liver, pancreas, intestines). Metabolically active; associated with elevated cardiovascular and metabolic disease risk. Distinct from subcutaneous fat (under the skin).