Knowledge Base

What does that number mean?

Try:

Science only, with its sources. No opinions on this page; those live in the blog.

What you will find here, and what you won’t.

Every entry on this page rests on published science, names its sources at the foot, and shows the date it was last reviewed. Where researchers disagree, we show both sides and the evidence each has, and we don’t pick a winner: not between keto and plant-based, not between the energy-balance and the carbohydrate-insulin accounts of weight, not between supplements. Nothing here tells you what your own numbers mean for your health; that is a conversation for Eir in the app, with your data, and for your doctor. Our founder and the people who work on Vitality Tracker have opinions on all of it. They publish them in the blog, labeled as opinion, and nowhere on this page.

The reference

What Vitality Tracker measures, calculates and asks of you.

Every biomarker, every score, every lab test and every sensor, by the health domain it belongs to, with how much a consumer device can be trusted on it and how to measure it well.

Body Composition

24 entries

What you are made of, and where it sits.

Body fatBF%Visceral fat indexVFIHow to use a bioimpedance scale

Nutrition & Energy Balance

16 entries

What goes in, what goes out, and when.

The TDEE engineTDEEProtein, fat and carbohydrateEnergy expenditure, device

Activities & Lifestyle

15 entries

Movement, effort, training and the interventions you run on yourself.

Daily stepsPerceived exertion: the Borg CR10 scaleRPEProtocols: log the things you do to yourself

Metabolic Health

35 entries

Glucose, its averages and its long-run exposure.

HbA1cHbA1cTime in rangeTIRMetabolic age

Cardio-Respiratory Health

29 entries

The overnight vital signs, blood pressure and the arteries, and fitness.

Overnight heart rate variabilityHRVOvernight resting heart rateRHRBlood pressureBP

Stress, Rest & Recovery

18 entries

Sleep, how you feel, and how much you can take on.

Sleep Health IndexSHITotal sleepTSTCapacity: how much you can take on today

Circadian Health

5 entries

Timing: of sleep, of meals, of light.

Sleep Regularity IndexSRIEating window: first and last mealTREChronotype and circadian alignmentMCTQ

Scores and engines

Vitality Score Sleep Health IndexSHIThe TDEE engineTDEECapacity: how much you can take on today Metabolic age Pulse pressure, mean arterial pressure and fractional pulse pressurePPPulse wave velocity and vascular agePWVSymptom Radar and Symptom Trend

Bloodwork

HbA1cHbA1cLipid profileTCApolipoprotein BApoBFull blood count with differentialFBCLiver functionLFTRenal function and electrolyteseGFRCRP, high-sensitivityCRPFerritin TSHTSHFasting insulin Vitamin D (25-OH)25(OH)DLipoprotein(a)Lp(a)PhenoAgePhenoAgeHOMA-IRHOMA-IR

Glucose and CGM

What a CGM measuresCGMWhat a healthy adult's sensor shows Why a month Time in rangeTIRTime in tight rangeTITRCoefficient of variationCVGlucose management indicatorGMIPeak rise Two-hour incremental area under the curve2-h iAUCDip after Reading a CGM without fooling yourself

Best practices

How to use a bioimpedance scale Why your weight and body fat wobble Why VT asks about bowel movements How your data reaches VT, and why you sometimes have to sync first Blood pressure: how and how often How to measure your waist Why your watch's calories are not a food budget Why log meals even if you are not trying to lose weight Use the fasting timer even if you don't fastTREProtocols: log the things you do to yourself Sick days, holidays and unusual periods Same time, same way

Last Week: Longevity

What the field said last week.

Every episode, paper and article covered in Last Week: Longevity, indexed by topic, by expert and by claim, with a link to the original every time. We report who said what; we don’t endorse it. Members can see where the experts agree and where they don’t.

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The first issue goes out at the end of 2026; this section fills from then.

Topics

Topics, explained from the ground up.

Longer reads on a subject rather than a measure: what the evidence supports, what it doesn’t, and what is still open.

Topic article

Exercise science

Aerobic fitness and VO₂ max, zones and the Zone 2 evidence, resistance training and muscle with age, load and recovery.

In preparation

Topic article

Nutrition fundamentals

Energy balance, protein, carbohydrate and fat, fiber, what the diet trials found and did not, timing, alcohol.

In preparation

Topic article

Sleep

Carried over from the Science hub’s sleep article, extended with the device and Night Log material.

In preparation

Topic article

Stress and recovery

HRV and the nervous system, perceived stress, the protocols and their evidence.

In preparation

Topic article

Metabolic health

Glucose regulation, insulin resistance, the CGM month, HbA1c and its estimates, fatty liver and the lab picture.

In preparation

Topic article

Supplements and the evidence

How to read a supplement claim, the few with strong evidence, the many without, interactions.

In preparation

Topic article

Measuring yourself

The accuracy classes explained with the studies behind them, N-of-1 experiments, signal from noise in your own record.

In preparation

Vitality Tracker describes and tracks physiological measures recorded by your own consumer devices against your own history, for general wellness and for discussion with a health professional. It does not diagnose, predict or monitor any disease.