Glossary: Health & Research Terms Defined

This page defines terms you'll encounter across Proco's content and in consumer health writing more broadly. It is reference material, not medical advice.

Health and research writing uses a lot of specialised vocabulary. Most of it has precise meanings that get distorted in consumer translation. This glossary defines the terms we use most often, with links to the deeper coverage where it exists.

Terms are grouped by category. Use Ctrl/Cmd+F to find a specific term.

Research methodology

Randomised controlled trial (RCT). A study in which participants are randomly assigned to receive an intervention or a control (typically a placebo or alternative treatment). Random assignment is the gold-standard method for establishing causal effects. See How to read a clinical trial.

Meta-analysis. A statistical synthesis of multiple individual studies on the same question, weighted by quality and sample size. Provides more precise effect estimates than any single trial.

Systematic review. A structured review of all available studies on a question using predefined inclusion criteria and risk-of-bias assessment. Often paired with a meta-analysis.

Cohort study. A prospective observational study that follows a group of people over time, measuring exposures and outcomes. Cannot establish causation but is informative for long-term outcomes.

P-value. The probability of observing the data (or more extreme data) if the null hypothesis were true. A p-value below 0.05 is conventionally "statistically significant" but says nothing about effect size or clinical importance.

Effect size. The magnitude of an effect, independent of statistical significance. Cohen's d, hazard ratio, relative risk, and mean difference are common effect size measures.

Confidence interval. A range within which a study's estimate of an effect is statistically expected to lie with a given level of certainty (typically 95%). Wide intervals indicate substantial uncertainty.

GRADE. A structured framework for grading evidence quality based on study design, risk of bias, consistency, directness, and precision. Used in systematic reviews to rate evidence as high, moderate, low, or very low quality.

CONSORT. Consolidated Standards of Reporting Trials. A 25-item checklist for reporting RCTs that strengthens transparency and quality assessment.

PRISMA. Preferred Reporting Items for Systematic Reviews and Meta-Analyses. The corresponding reporting standard for systematic reviews.

Cochrane review. A systematic review published under Cochrane's strict methodology. Considered among the highest-quality evidence syntheses in healthcare.

Surrogate endpoint. A measurable biomarker used as a proxy for a clinical outcome that is harder to measure directly. Surrogate endpoints don't always predict clinical outcomes well.

Pre-registration. Publicly committing to a study's design, primary outcomes, and analysis plan before data collection. Reduces the risk of post-hoc data analysis distorting findings.

Funder effect. The tendency for industry-funded studies to report findings more favourable to their funders' products than independent studies of the same questions. Replicated across many fields.

Sleep

Polysomnography (PSG). An overnight multi-channel recording of brain activity (EEG), eye movements (EOG), muscle tone (EMG), heart rate, breathing, and oxygen saturation. The gold standard for sleep disorder diagnosis. See Sleep apnea diagnosis.

Apnea-Hypopnea Index (AHI). Number of apneas (complete breathing cessations) plus hypopneas (significant reductions in airflow) per hour of sleep. The primary diagnostic metric for sleep apnea. <5 normal; 5-15 mild; 15-30 moderate; >30 severe.

NREM. Non-Rapid Eye Movement sleep. Includes stages N1 (light), N2 (most of sleep), and N3 (deep / slow-wave sleep). See Sleep stages explained.

REM. Rapid Eye Movement sleep. Characterised by fast brain activity, rapid eye movements, and muscle paralysis. The primary stage for vivid dreaming.

Sleep architecture. The distribution of sleep stages across the night. Cycles through NREM and REM approximately every 90 minutes.

Sleep efficiency. Percent of time in bed actually spent asleep. Healthy adults typically achieve >85%.

Sleep latency. Time from going to bed to falling asleep. Typically 15-25 minutes in healthy adults.

Circadian rhythm. The ~24-hour biological clock regulating sleep timing, alertness, hormone release, and many other physiological processes.

Obstructive sleep apnea (OSA). The most common form of sleep apnea, caused by upper-airway collapse during sleep. Requires clinical diagnosis.

Home Sleep Apnea Test (HSAT). A portable diagnostic device worn at home for one night, measuring airflow, oxygen, and breathing effort. Useful for moderate-to-severe OSA screening; misses ~15-20% of cases that PSG would identify.

Cardiovascular & performance

VO2 max. Maximum rate at which the body can take in and use oxygen, measured in ml/kg/min. The most-used measure of cardiorespiratory fitness. See VO2 max: lab tests vs watch estimates.

Cardiopulmonary exercise testing (CPET). Direct laboratory measurement of VO2 max using gas analysis during incremental exercise. The gold standard.

Heart rate variability (HRV). Variation in time between consecutive heartbeats. Reflects autonomic nervous system balance. Higher HRV typically reflects better recovery and parasympathetic dominance.

RMSSD. Root Mean Square of Successive Differences. The most common HRV calculation; reflects short-term, primarily parasympathetic variation.

SDNN. Standard Deviation of Normal-to-Normal intervals. Captures longer-term HRV variation. Used differently from RMSSD.

MET (metabolic equivalent). Unit of energy expenditure. 1 MET = resting metabolic rate (approximately 3.5 ml/kg/min of oxygen consumption). A 5 MET activity uses 5× resting energy.

Lactate threshold. Exercise intensity at which blood lactate begins to accumulate faster than it can be cleared. A key training metric for endurance athletes.

Zone 2. Training at intensity where lactate is still being cleared, typically ~60-70% of maximum heart rate. Foundation of endurance training programs.

Anaerobic threshold. Exercise intensity above which the body relies increasingly on anaerobic metabolism. Closely related to lactate threshold.

HIIT. High-Intensity Interval Training. Brief high-intensity efforts alternating with recovery. See HIIT vs steady-state cardio.

Cardiorespiratory fitness (CRF). Broader term for the body's capacity to deliver oxygen and use it during sustained exercise. VO2 max is a primary measure.

Longevity & aging

Healthspan. Years of life lived in good health, free from major disease or disability. See Healthspan vs lifespan.

Lifespan. Total years of life from birth to death.

Biological age. An estimate of physiological aging that may differ from chronological (calendar) age. Measured through epigenetic clocks, biomarker panels, and other methods. See Epigenetic clocks.

Epigenetic clock. An algorithm that estimates biological age from DNA methylation patterns. Major variants include Horvath, Hannum, PhenoAge, GrimAge, and DunedinPACE.

DNA methylation. A chemical modification (methyl group attachment) to DNA that affects gene expression without changing the underlying sequence. Methylation patterns change in predictable ways with age.

Hallmarks of aging. A framework identifying twelve cellular and molecular processes that together produce aging. See The hallmarks of aging.

Cellular senescence. A state in which cells have stopped dividing but have not been cleared from tissue. Senescent cells secrete inflammatory factors that affect neighbouring tissues.

Autophagy. A cellular recycling process that breaks down damaged proteins and organelles. Activity declines with age.

mTOR. Mechanistic Target of Rapamycin. A central nutrient-sensing protein complex that regulates cellular growth and metabolism. mTOR suppression is associated with longevity benefits in animal models.

AMPK. AMP-activated protein kinase. A cellular energy sensor that activates under low energy conditions. Promotes catabolic (energy-producing) processes.

Caloric restriction (CR). Sustained reduction in calorie intake while maintaining nutritional adequacy. See Caloric restriction research: 90 years of studies.

Senolytic. A compound that selectively kills senescent cells. Active area of pharmaceutical research.

Nutrition

Macronutrient. Protein, carbohydrate, or fat. The nutrients required in large quantities.

Micronutrient. Vitamin or mineral. Required in small quantities.

RDA (Recommended Dietary Allowance). The average daily intake of a nutrient sufficient to meet the requirements of 97-98% of healthy individuals. Established by national bodies (Institute of Medicine in the US; EFSA in the EU).

AI (Adequate Intake). Used when RDA cannot be determined; reflects observed mean intake in healthy populations.

UL (Tolerable Upper Intake Level). Highest daily intake unlikely to cause adverse health effects in the general population. Important for supplement safety.

Bioavailability. The proportion of an ingested nutrient that reaches the systemic circulation. Varies dramatically by form (magnesium oxide vs glycinate, vitamin K1 vs K2).

EFSA. European Food Safety Authority. The EU body that evaluates health claims for food and supplements.

NOVA classification. A four-tier framework categorising foods by degree of industrial processing. Group 4 (ultra-processed) is the contested category. See Ultra-processed food research.

Glycemic index (GI). A ranking of carbohydrate foods by their effect on blood glucose. Higher GI = faster glucose rise.

FODMAP. Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols. Short-chain carbohydrates that can trigger digestive symptoms in some people.

Probiotic. Live microorganisms intended to provide a health benefit when consumed. Evidence varies enormously by strain and indication.

Prebiotic. A non-digestible food component that selectively stimulates the growth of beneficial gut bacteria.

Postbiotic. Bacterial metabolites with potential health effects, including short-chain fatty acids.

Supplements & ingredients

Form. The chemical version of a nutrient (e.g. magnesium glycinate vs magnesium oxide; vitamin K1 phylloquinone vs vitamin K2 menaquinone). Different forms have different bioavailability and uses.

Chelated. A nutrient bound to an organic molecule (typically an amino acid) to improve absorption. Magnesium glycinate is chelated; magnesium oxide is not.

Synergistic combination. A combination of ingredients where the combined effect exceeds the sum of individual effects. Often claimed in marketing; less often demonstrated in research.

Bioactive. Having biological effects beyond basic nutrition. Polyphenols, omega-3 fatty acids, and creatine are examples.

Pharmaceutical-grade. A vague marketing term sometimes used to imply higher quality. The term has no formal definition for supplements outside specific manufacturing standards.

Third-party tested. Independently verified for ingredient identity, purity, and potency. NSF Certified for Sport, USP Verified, and ConsumerLab are common third-party programs.

DSHEA. Dietary Supplement Health and Education Act of 1994. The US law governing supplement regulation. See How supplements are regulated: EU vs US vs UK.

Structure/function claim. A claim about how a supplement affects normal body structure or function (e.g. "supports immune function") permitted under US DSHEA without FDA pre-approval.

Disease claim. A claim that a product prevents, treats, cures, or mitigates a disease. Not permitted on supplement labels in the US, EU, or UK.

AI in health

SaMD. Software as a Medical Device. Software intended for medical purposes (diagnosis, treatment, prevention). Subject to FDA, EU, and UK medical device regulation.

510(k) clearance. The most common FDA clearance pathway, demonstrating "substantial equivalence" to an existing cleared device. Used for most cleared AI medical devices. See FDA-cleared AI medical devices.

De novo classification. FDA clearance pathway for novel devices with no predicate. More rigorous than 510(k).

PMA (Premarket Approval). The most rigorous FDA clearance pathway, requiring clinical trial evidence of safety and effectiveness.

Medical Devices Regulation (MDR) 2017/745. EU regulation governing medical devices, including AI medical software. Generally more rigorous than US 510(k) for novel devices.

Symptom checker. An AI tool that takes user-reported symptoms and returns a differential diagnosis and triage recommendation. See AI symptom checkers: validation studies.

Differential diagnosis. A ranked list of conditions consistent with a patient's symptoms.

Triage. Categorising patients by urgency of needed care.

Wearables & measurement

PPG (Photoplethysmography). Optical measurement of blood volume changes, used by most wearables to derive heart rate and HRV. Works by shining light into the skin.

ECG (Electrocardiogram). Electrical recording of heart rhythm. Single-lead consumer ECGs (Apple Watch, KardiaMobile) are cleared for limited indications.

Accelerometer. A sensor measuring movement in three axes. Used for step counts, sleep movement, and activity classification.

SpO2. Blood oxygen saturation percentage. Measured by pulse oximetry. Consumer wearable SpO2 has approximately 4-5% mean absolute error compared with fingertip pulse oximetry.

Sleep stage estimation. Wearable inference of sleep stages (light, deep, REM) from heart rate and movement patterns. Accuracy for total sleep time is reasonable (within ~30 min of PSG); specific stage accuracy is limited (50-60% agreement). See Wearables: what they can and can't measure.

Recovery score. A composite metric (Whoop, Oura, Garmin) combining HRV, resting heart rate, sleep, and other inputs to estimate training readiness. Useful for trend tracking; not a precise measurement.

Regulatory & compliance

EFSA. European Food Safety Authority. Evaluates health claims for food and supplements in the EU.

Health Claims Regulation (EC 1924/2006). EU regulation controlling what health claims can be made on food and supplement labels.

ASAI Code. Code of Standards for Advertising and Marketing Communications in Ireland. Governs health claims in Irish advertising.

FSA. Food Standards Agency. The UK food regulator.

MHRA. Medicines and Healthcare products Regulatory Agency. The UK medical device regulator.

FDA. Food and Drug Administration. The US regulator of food, drugs, supplements, and medical devices.

GDPR. General Data Protection Regulation. EU data protection law applying to processing of EU residents' personal data.

Acronyms quick reference

Missing a term?

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