In one sentence
One number from 0 to 100 that sums up your metabolic health from five areas, blood sugar, body composition, fitness, arteries and the last seven days of lifestyle, each of which you can open to see what fed it and how fresh it is.
Why it is built this way
Each area's share follows how much of the outcome evidence it carries and how much of it is yours to move. Blood sugar, 35 percent: in 10,232 adults aged 45 to 79 in Norfolk, each percentage point of HbA1c was associated with 24 percent (men) and 28 percent (women) higher all-cause mortality over six to eight years, continuously across the range, including in people without diabetes (Khaw 2004), and the three glucose figures together cover both the slow exposure and the day-to-day picture. Body composition, 20 percent: across 31 studies of more than 300,000 adults, the waist-to-height ratio discriminated the cardiometabolic outcomes studied better than waist circumference or body mass index in both sexes (Ashwell 2012), which is why central adiposity carries 40 percent of the area beside body fat's 60. Fitness, 20 percent: in 122,007 adults followed for a median of 8.4 years after a treadmill test, all-cause mortality fell step by step with fitness and had no observed upper limit of benefit; the least fit quarter had five times the adjusted hazard of the elite performers (Mandsager 2018). Arteries, 15 percent: in one million adults from 61 prospective studies, each 20 mm Hg of usual systolic pressure was associated with a twofold difference in death from ischemic heart disease and more than twofold from stroke at ages 40 to 69, with no threshold down to 115/75 (Lewington 2002), and in 17 studies of 15,877 people each 1 m/s of aortic pulse wave velocity was associated with 14 to 15 percent higher adjusted rates of cardiovascular events and death (Vlachopoulos 2010). Lifestyle, 10 percent, the only area that answers to this week: how long you slept, adjusted by your sleep quality score, steps (in 15 cohorts of 47,471 adults, the second, third and fourth quartiles of daily steps had adjusted hazards of 0.60, 0.55 and 0.47 for all-cause mortality against the lowest, flattening at 6,000 to 8,000 steps over 60 and 8,000 to 10,000 under; Paluch 2022), and alcohol (in 599,912 current drinkers from 83 studies, mortality was lowest at or below 100 g a week, about ten standard drinks, and rose with every band above it; Wood 2018).
What you see
The score with its confidence, High, Medium or Low; each area's own score; what fed each area and when it was measured; and which measurements are about to age out. The headline needs two of the three core areas (blood sugar, body composition, fitness) and your height, sex and date of birth, so it can appear from your first day. An area with no data is left out and its share is spread across the areas you have, and each area's own score is still shown, so a score built from three areas says so.
What moves it
Slowly, for the most part, because its inputs are slow: HbA1c over three months, body composition over weeks, VO₂ max over months. Two things move it quickly. Each day you mark as sick counts as a score of 20 in the lifestyle area for the seven days that include it, and glucose from sick days and the three days after is not carried forward. And a measurement that returns after aging out can step the score, because the area it feeds comes back with it; the step is the measurement arriving, not your health changing. Every input has a shelf life, fresh for a while, still used after that, then dropped until you measure again:
| Measurement | Fresh for | Still used for up to | | --- | --- | --- | | HbA1c from a laboratory | 30 days | 90 days | | Fasting glucose, sensor daily average | 7 days | 14 days | | Blood pressure | 7 days | 14 days | | Body fat, waist | 14 days | 30 days | | VO₂ max | 30 days | 60 days | | Vascular age | 30 days | 90 days |
An HbA1c estimated from a glucose sensor lasts as long as a laboratory result only when the sensor covered at least 70 percent of the 90 days behind it; with less coverage it ages faster. If your weight and habits have held steady, VT can keep a laboratory HbA1c for up to 180 days and a waist for up to 90 or 180 days, and while it does, the confidence shows Low and VT asks you to measure again.
How to measure it well
The score is only as good as its inputs, and each has its own entry. The baseline lab panel gives the blood sugar area its anchor; a waist every two to four weeks and a scale most mornings keep body composition fresh; a seven-day blood-pressure block, then a weekly reading, keeps the arteries area fresh; VO₂ max comes from your device or a test; a wearable brings sleep and steps; alcohol is yours to log, in standard drinks, half units allowed.
How Vitality Tracker uses it
The headline on Today, the number a Playbook can target, and the first line of the pre-visit report, which lists each area with its inputs and their dates. When an HbA1c, waist or blood pressure has run past its window, or is being kept on the steady-habits rule, VT asks you to measure again; other inputs drop out when they expire. The Trajectory Layer will carry the score and its areas forward as a direction over months. Each area's score is kept day by day beside the measurements it came from, so you can open any day and see what the number was made of.
How far to trust the number
Calculated, from measured, estimated, laboratory and self-reported inputs, which is why it never appears without a confidence. High needs at least four areas measured, all fresh, and 20 or more days of data; Medium at least three areas, no more than one past its fresh period, and 7 or more days; Low is anything less, or a measurement kept past its usual shelf life. Fitness is the one area scored against population norms for your age and sex; the rest are scored against published reference points. Read the areas, not the total, when you want to know what to do; read the total over months, and treat a move of a few points on the day a fresh input replaced an old one as the input, not you. The weights and curves are the engine's current settings and are tuned over time; a change is dated in the method note.
Sources
- Khaw KT, et al. Association of hemoglobin A1c with cardiovascular disease and mortality in adults: the European Prospective Investigation into Cancer in Norfolk. Ann Intern Med 2004;141:413–20. doi:10.7326/0003-4819-141-6-200409210-00006
- Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev 2012;13:275–86. doi:10.1111/j.1467-789X.2011.00952.x
- Mandsager K, et al. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Netw Open 2018;1:e183605. doi:10.1001/jamanetworkopen.2018.3605
- Lewington S, et al. Age-specific relevance of usual blood pressure to vascular mortality: a meta-analysis of individual data for one million adults in 61 prospective studies. Lancet 2002;360:1903–13. doi:10.1016/S0140-6736(02)11911-811911-8>)
- Vlachopoulos C, Aznaouridis K, Stefanadis C. Prediction of cardiovascular events and all-cause mortality with arterial stiffness: a systematic review and meta-analysis. J Am Coll Cardiol 2010;55:1318–27. doi:10.1016/j.jacc.2009.10.061
- Paluch AE, et al. Daily steps and all-cause mortality: a meta-analysis of 15 international cohorts. Lancet Public Health 2022;7:e219–28. doi:10.1016/S2468-2667(21)00302-900302-9>)
- Wood AM, et al. Risk thresholds for alcohol consumption: combined analysis of individual-participant data for 599 912 current drinkers in 83 prospective studies. Lancet 2018;391:1513–23. doi:10.1016/S0140-6736(18)30134-X30134-X>)
- Vitality Tracker, How it works (weights, shelf life, confidence); the engine catalogue of 15 October 2026 (the sub-weights, the sick-day rule).