Glossary

Plain-language definitions of scientific terms used across the site. Terms with a dotted underline show definitions on hover.

glossary.englishNote

Study Types

Meta-analysisA study that combines results from multiple independent studies to draw stronger conclusions. Considered the highest level of evidence.

Systematic reviewA comprehensive summary of all available research on a specific question, using strict methods to find and evaluate studies.

RCTRandomized Controlled Trial. Participants are randomly assigned to a treatment or placebo group, considered the gold standard for testing interventions.

Cohort studyA study that follows a group of people over time to see how certain factors affect health outcomes.

Crossover trialA study where each participant receives both the treatment and placebo at different times, serving as their own control.

Observational studyResearch where the investigators observe participants without assigning treatments. Can show associations but not prove causation.

In vitroResearch conducted in a lab setting (test tubes, cell cultures) rather than in living organisms. Early-stage evidence.

In vivoResearch conducted in living organisms (animals or humans). More relevant than lab studies but animal results don't always translate to humans.

Longitudinal studyA study that tracks the same participants over an extended period, sometimes years or decades.

PlaceboAn inactive treatment (like a sugar pill) given to a control group for comparison with the real intervention.

Double-blindA study design where neither participants nor researchers know who receives the treatment or placebo, reducing bias.

Dose-responseThe relationship between the amount of a substance taken and the size of its effect. A clear dose-response relationship strengthens evidence.

Intention-to-treatAn analysis method that includes all participants as originally assigned, even if they dropped out or didn't follow the protocol. Considered more reliable.

Statistics

Sample sizeThe number of participants in a study. Larger sample sizes generally produce more reliable results.

P-valueA measure of how likely the results occurred by chance. Below 0.05 is generally considered statistically significant.

Effect sizeHow large the measured impact of an intervention is. A statistically significant result can still have a small, practically meaningless effect.

Confidence intervalA range of values that likely contains the true effect. Narrower intervals mean more precise estimates.

Statistical significanceA result unlikely to have occurred by chance alone (typically p < 0.05). Does not necessarily mean the effect is large or clinically meaningful.

Odds ratioA measure of association between an exposure and an outcome. An odds ratio above 1 suggests increased risk, below 1 suggests decreased risk.

Relative riskHow much more (or less) likely an outcome is in one group compared to another. A relative risk of 2 means twice as likely.

Absolute riskThe actual probability of an event occurring in a group, expressed as a percentage. More meaningful than relative risk for individual decisions.

Number needed to treatHow many people need to take a treatment for one person to benefit. Lower numbers mean more effective treatments.

Publication biasThe tendency for studies with positive results to be published more often than those with negative or null results, skewing the evidence base.

HeterogeneityThe degree of variation between study results in a meta-analysis. High heterogeneity suggests the studies may not be measuring the same thing.

Biology & Neuroscience

BioavailabilityThe proportion of a substance that enters your bloodstream and can have an active effect. Higher bioavailability means more of what you take is actually used by your body.

Half-lifeThe time it takes for half of a substance to be eliminated from your body. Determines how long an effect lasts and how often you need to take it.

Blood-brain barrierA selective membrane that protects the brain by controlling which substances can pass from the bloodstream into brain tissue. Some supplements cross it, many don't.

NeurotransmitterA chemical messenger that transmits signals between nerve cells. Examples: dopamine, serotonin, GABA, acetylcholine.

GABAGamma-aminobutyric acid. The brain's main inhibitory neurotransmitter. Promotes calm, relaxation, and sleep.

DopamineA neurotransmitter involved in motivation, reward, focus, and pleasure. Many supplements and habits affect dopamine levels.

SerotoninA neurotransmitter that regulates mood, sleep, and appetite. Low serotonin is associated with depression and anxiety.

NorepinephrineA neurotransmitter and hormone involved in alertness, attention, and the fight-or-flight response. Released during cold exposure and exercise.

AcetylcholineA neurotransmitter critical for memory, learning, and muscle contraction. Alpha-GPC and citicoline increase its availability.

CortisolThe primary stress hormone. Healthy levels follow a daily rhythm (high in morning, low at night). Chronic elevation is harmful.

TestosteroneThe primary male sex hormone, also present in women. Important for muscle mass, bone density, energy, and mood. Declines with age.

InsulinA hormone that regulates blood sugar by signaling cells to absorb glucose. Insulin resistance is a precursor to type 2 diabetes.

Insulin sensitivityHow responsive your cells are to insulin. Higher sensitivity means your body manages blood sugar more efficiently.

MelatoninA hormone produced by the pineal gland that signals your body it's time to sleep. Suppressed by bright light at night.

Circadian rhythmYour internal 24-hour biological clock that regulates sleep, hormones, body temperature, and metabolism.

AdenosineA molecule that builds up during wakefulness and creates sleep pressure. Caffeine works by blocking adenosine receptors.

NAD+Nicotinamide adenine dinucleotide. A coenzyme essential for cellular energy production. Levels decline with age.

ATPAdenosine triphosphate. The primary energy currency of cells. Creatine helps regenerate ATP during high-intensity activity.

BDNFBrain-Derived Neurotrophic Factor. A protein that supports the growth of new neurons and synapses. Increased by exercise, learning, and certain supplements.

NGFNerve Growth Factor. A protein essential for the survival and maintenance of nerve cells. Lion's mane mushroom stimulates NGF production.

AutophagyThe process by which cells clean up and recycle damaged components. Activated by fasting, exercise, and certain compounds.

MitochondriaThe energy-producing structures inside cells. Mitochondrial health is linked to aging, energy levels, and chronic disease.

NeuroplasticityThe brain's ability to form new neural connections and reorganize itself. Can be enhanced by learning, exercise, and certain supplements.

HPA axisHypothalamic-pituitary-adrenal axis. The body's central stress response system that controls cortisol release.

InflammationThe immune system's response to injury or threat. Acute inflammation is protective; chronic inflammation drives many diseases.

Oxidative stressAn imbalance between free radicals and antioxidants in the body, contributing to cell damage and aging.

Free radicalsUnstable molecules that damage cells and DNA. The body produces them naturally, but excess levels accelerate aging and disease.

AntioxidantA substance that neutralizes free radicals, protecting cells from oxidative damage. Found in vitamins C and E, polyphenols, and many foods.

GlutathioneThe body's most abundant antioxidant, produced in the liver. Protects cells from oxidative damage. NAC is a precursor.

Nitric oxideA molecule that relaxes blood vessels, improving blood flow. Produced naturally; enhanced by exercise, beets, and nasal breathing.

CollagenThe most abundant protein in the body, providing structure to skin, bones, tendons, and joints. Production declines with age.

Gut-brain axisThe bidirectional communication system between the gut microbiome and the brain, affecting mood, cognition, and immune function.

MicrobiomeThe community of trillions of bacteria and other microorganisms living in your digestive tract. Affects immunity, mood, and metabolism.

MTORMechanistic target of rapamycin. A protein that regulates cell growth. Inhibiting mTOR may promote longevity; activated by protein intake and exercise.

AMPKAMP-activated protein kinase. An energy-sensing enzyme activated by exercise and fasting. Promotes fat burning and cellular cleanup.

TelomereProtective caps on the ends of chromosomes that shorten with age. Telomere length is a biomarker of biological aging.

EpigeneticChanges in gene expression that don't alter the DNA sequence itself. Lifestyle factors like diet, exercise, and stress can modify epigenetic markers.

Senescent cellsCells that have stopped dividing but remain in the body, releasing inflammatory signals. Accumulate with age and contribute to aging-related decline.

Nutrition & Biochemistry

Amino acidBuilding blocks of proteins. Some are essential (must come from diet), others are conditionally essential or non-essential. Many supplements are amino acids.

Essential fatty acidFats the body cannot produce and must get from food. Omega-3 (EPA/DHA) and omega-6 are the two main types.

EPAEicosapentaenoic acid. An omega-3 fatty acid with strong anti-inflammatory properties. Found in fish oil.

DHADocosahexaenoic acid. An omega-3 fatty acid that is a major structural component of brain tissue. Found in fish oil.

PolyphenolPlant compounds with antioxidant properties found in berries, tea, dark chocolate, and olive oil. Studied for cardiovascular and cognitive benefits.

FlavonoidA subclass of polyphenols found in fruits, vegetables, and tea. Includes quercetin, apigenin, and catechins.

CofactorA substance required for an enzyme or process to function. Example: vitamin C is a cofactor for collagen synthesis.

PrecursorA substance the body converts into another. Example: L-tyrosine is a precursor to dopamine; NAC is a precursor to glutathione.

Supplement Terms

AdaptogenA natural substance that helps the body adapt to stress and maintain balance. Examples: ashwagandha, rhodiola.

NootropicA substance that enhances cognitive function, memory, or focus. Can be natural (lion's mane) or synthetic.

ChelatedA mineral bound to an amino acid for better absorption. Chelated magnesium (like glycinate) absorbs better than oxide.

SenolyticA compound that selectively removes damaged, aging (senescent) cells. Studied for anti-aging potential.

HormesisA biological response where a small dose of stress (cold, heat, exercise) triggers beneficial adaptations.

LiposomalA delivery method that encapsulates a substance in tiny fat bubbles (liposomes) for better absorption. Used for vitamin C, glutathione, and others.

Standardized extractA supplement extract processed to contain a specific, consistent percentage of the active compound. Example: KSM-66 ashwagandha.

Enteric coatedA supplement coating that prevents dissolution in the stomach, allowing release in the small intestine. Reduces stomach irritation.

Loading doseA higher initial dose taken for a short period to build up levels in the body faster. Common with creatine (though not necessary).

StackingCombining multiple supplements to achieve complementary or synergistic effects. Example: caffeine + L-theanine for calm focus.

CyclingTaking a supplement for a period then stopping for a period to prevent tolerance or maintain effectiveness. Example: 8 weeks on, 2 weeks off.

ThermogenicA substance that increases body heat production, potentially boosting metabolism and calorie expenditure.

Sleep

Circadian rhythmYour internal 24-hour biological clock that regulates sleep, hormones, body temperature, and metabolism.

Cortisol awakening responseThe natural spike in cortisol that occurs within 30-60 minutes of waking. Important for alertness and energy. Disrupted by irregular sleep.

Sleep onset latencyThe time it takes to fall asleep after getting into bed. Normally 10-20 minutes. Longer times may indicate sleep difficulties.

Deep sleepThe most restorative sleep stage (also called slow-wave sleep). Important for physical recovery, immune function, and memory consolidation.

REM sleepRapid Eye Movement sleep. The stage where most dreaming occurs. Critical for emotional processing, memory, and learning.

Exercise

Progressive overloadGradually increasing the weight, reps, or intensity of exercise over time. The fundamental principle of strength training.

Zone 2A low-intensity exercise zone where you can maintain a conversation. Builds aerobic base, mitochondrial density, and fat oxidation.

HIITHigh-Intensity Interval Training. Short bursts of intense exercise alternated with rest periods. Time-efficient but more demanding than steady-state cardio.

DOMSDelayed Onset Muscle Soreness. Muscle pain that peaks 24-72 hours after exercise. A normal response to new or intense training.

Time under tensionThe total time a muscle is under strain during a set. Longer time under tension may promote more muscle growth.

Compound movementAn exercise that works multiple joints and muscle groups simultaneously. Examples: squat, deadlift, bench press.

DeloadA planned period of reduced training volume or intensity to allow recovery and prevent overtraining.

Health Metrics

PSQIPittsburgh Sleep Quality Index. A standardized questionnaire used in research to measure sleep quality. Lower scores mean better sleep.

VO2 maxMaximum oxygen uptake during exercise. One of the strongest predictors of longevity and cardiovascular health.

HRVHeart Rate Variability. The variation in time between heartbeats. Higher HRV generally indicates better cardiovascular fitness and stress resilience.

BMIBody Mass Index. A simple ratio of weight to height. Useful at population level but doesn't distinguish muscle from fat.

HOMA-IRA calculation used to estimate insulin resistance from fasting glucose and insulin levels. Lower values indicate better insulin sensitivity.

HsCRPHigh-sensitivity C-reactive protein. A blood marker of systemic inflammation. Elevated levels are associated with cardiovascular risk.

HbA1cGlycated hemoglobin. A blood test that reflects average blood sugar levels over the past 2-3 months. Used to diagnose and monitor diabetes.

FerritinA blood protein that stores iron. Low ferritin indicates iron deficiency; very high ferritin can indicate iron overload or inflammation.

25-hydroxyvitamin DThe main circulating form of vitamin D, measured in blood tests to assess vitamin D status. Optimal range is debated but generally 30-60 ng/mL.