Body composition metrics
Weight, BMI, body fat percentage, lean body mass and body temperature mostly come from a smart scale or manual entry rather than the Apple Watch — Vita displays them and uses them as context for nutrition and lab-report interpretation.
Body weightkg
Field: weight_body_mass
- What it measures
- Written to Apple Health by a smart scale or other connected scale, or entered by hand. The Apple Watch cannot weigh you, so this field depends entirely on owning a scale that syncs.
- What Vita does with it
- Basal energy estimates, protein targets in nutrition guidance, and lab-report context.
- Easily misread
- Day-to-day swings are mostly water, not fat — a weekly trend says more than any single reading.
Body mass index (BMI)
Field: body_mass_index
- What it measures
- Weight divided by height squared, computed automatically by Apple Health.
- What Vita does with it
- Displayed in the Health tab.
- Easily misread
- BMI does not distinguish muscle from fat and overstates muscular people. Lean body mass and body fat percentage are more informative.
Body fat percentage%
Field: body_fat_percentage
- What it measures
- Fat as a share of body weight. Consumer scales estimate it by bioelectrical impedance.
- What Vita does with it
- Displayed, and context for lab-report interpretation.
- Easily misread
- BIA's absolute accuracy is limited and shifts with hydration, food and exercise — the trend on one scale is usable, absolute values across devices are not comparable.
Lean body masskg
Field: lean_body_mass
- What it measures
- Body weight with fat subtracted — muscle, bone, water and organs. A scale derives it from weight and body fat percentage rather than measuring it directly.
- What Vita does with it
- Displayed, and a better basis for protein targets than total body weight.
Body temperature°C
Field: body_temperature
- What it measures
- Written by an external thermometer or entered by hand — a completely different metric from sleeping wrist temperature.
- What Vita does with it
- Displayed in the Health tab.
Blood glucosemmol/L
Field: blood_glucose
- What it measures
- Written to Apple Health by a glucose meter or a CGM. The Apple Watch does not measure glucose.
- What Vita does with it
- Displayed, and context when interpreting an uploaded lab report; not an input to any score.
- Easily misread
- 🚨 A clinical measure. Vita displays it and does not interpret it, grade it, or offer anything medication-related — consult a clinician.
Limitations
- Most of this group is not something an Apple Watch can measure, so many users have nothing here — which does not affect recovery, stress or the sleep score.
- Body Age does not use weight or body fat: the strongest evidence is for cardiorespiratory fitness, and wearable body-composition accuracy is not good enough to convert into years.
References
Apple. HealthKit — HKQuantityTypeIdentifier reference. Apple Developer Documentation, accessed 2026.
What it supports：The official definition, unit and aggregation style of each metric — the source for how every field on this page is described.
Related metrics
Every metric on this page is a reference index for observing your own trends. It is not medical advice and cannot replace a professional diagnosis. If you have health concerns, consult a qualified clinician.
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