Body Composition API: Weight, Body Fat, and Lean Mass Data

Body composition means weight, body fat percentage, BMI, and lean (fat-free) mass. The single most important thing to know before you build: most fitness wearables do not measure any of this. A wrist band or watch cannot read body fat, lean mass, or BMI — those come from a smart scale (bioimpedance), a body scanner, or the user typing values in. Weight itself is almost always scale-measured or entered by hand, and BMI is computed from weight and height, not sensed. You read the results from the on-device stores (Apple HealthKit bodyMass, bodyFatPercentage, bodyMassIndex, leanBodyMass; Android Health Connect WeightRecord, BodyFatRecord) or from a scale vendor's cloud API (Withings, Garmin) via OAuth. Best pick: integrate a smart-scale source like Withings for a full breakdown, or read whatever a paired scale wrote into HealthKit or Health Connect for a hardware-agnostic path. If you only need weight and BMI, the on-device store plus manual entry is enough.

Where you can get body-composition data

This is the sharpest "the phone can't do it" case in health data. Neither a phone nor a typical wrist wearable produces body fat or lean mass — you need dedicated hardware or manual entry. Field names, units, and record types change; treat everything below as "as of 2026, verify" against each provider's live data dictionary.

Measured or estimated?

Body composition is a mix of measured, computed, and estimated values, and being explicit about which is which matters here:

• Weight: measured (on a scale) or manually entered. This is the reliable part.
• BMI: computed, not measured — it is weight divided by height squared. Treat it as a derived number, not a reading.
• Body fat percentage and lean mass: estimated via bioelectrical impedance analysis (BIA). A scale sends a small current through the body and infers fat versus fat-free mass from the resistance. BIA is sensitive to hydration, recent food or exercise, time of day, and whether the electrodes are foot-to-foot or hand-to-foot. It is an estimate, not a lab-grade measurement — DEXA and hydrostatic weighing are the reference methods. Never quote a body-fat accuracy percentage; none is sourced, and the numbers move with conditions.

Frame these as general wellness signals for your users, not clinical body-composition results. A single BIA reading can swing simply because someone drank water or worked out beforehand, so trends over consistent conditions are more useful than any one value.

Which should you pick?

• Full body-composition breakdown (fat, muscle, water, bone): integrate a smart-scale source. Withings is the common pick for a rich bioimpedance field set — verify current fields and units in its docs.
• Hardware-agnostic path: read whatever a paired scale wrote into HealthKit (bodyFatPercentage, leanBodyMass) or Health Connect (BodyFatRecord). You inherit the scale's data without integrating each vendor, and on-device keeps values on the user's phone until you upload them.
• Weight and BMI only: the on-device store plus manual entry is enough — you do not need a cloud scale integration at all.
• Many scale brands at once: an aggregator so you get one normalized schema instead of N vendor integrations.

Before you ship

Re-verify each provider's current field names, units, and OAuth scopes against its live data dictionary — do not hardcode scale models or field lists that were not taken from the vendor's own docs. Keep the framing honest with users: weight is measured, BMI is computed, and body fat and lean mass are BIA estimates affected by hydration and timing. Above all, set expectations up front that a wearable alone cannot produce these numbers — a smart scale or manual entry is required.