Body Composition in Apple Health: Weight, Body Fat and Lean Mass

Most of what My Bio Age reads from Apple Health comes from your heart, your sleep and your movement. Those signals show how your body copes with effort and rest, but not what it is made of: a steady resting heart rate can't tell you whether you have gained muscle or lost it. Body composition readings add some of that context, as long as you treat them as estimates and read them over weeks.

What Apple Health can store

Apple's HealthKit documentation lists separate data types for body mass (your weight), body fat percentage and lean body mass, the part of your weight that isn't fat. Height has its own type too. Weight also sits inside a familiar fitness number: Apple records VO₂max in millilitres of oxygen per kilogram per minute, as our VO₂max guide explains.

Apple Watch doesn't measure body composition. Apple's guide to Health data explains that you can enter body measurements by hand, and that weight scales and other connected devices store their readings in Health once set up. Body fat and lean mass usually arrive from a smart scale's app, while weight may also come from another app or manual entry. If the numbers look out of step, check which source supplied the latest reading.

How reliable are home measurements?

Many scales estimate body fat with bioelectrical impedance analysis (BIA). They pass a small electrical signal through your body, use the resistance to estimate body water, and convert that into fat-free mass with a prediction equation. A 2017 review of measurement techniques notes that this assumes lean tissue holds a fairly constant share of water, and that equations built for one population need caution in another.

An ESPEN working group review describes when BIA works well in research and clinical use: validated equations matched to the people being measured, careful measurement procedures, and no major fluid or electrolyte imbalance. It also points to a lack of standardised methods. Home scales rely on their makers' own equations and are used in far less controlled conditions, so treat their numbers as rough estimates. You can't choose your scale's equation, but you can make your own readings more comparable:

• Use the same scale. Devices may use different equations, so switching can shift the number without any change in you.
• Keep conditions similar. Measure at a similar time of day, for example soon after waking and before eating, drinking or exercising.
• Allow for hydration. Because the estimate depends on body water, a hard session or an illness can nudge it.
• Read trends over weeks. Compare several weeks of readings rather than reacting to one.

How My Bio Age uses body composition

My Bio Age can read your weight, body fat percentage and lean body mass from Apple Health. Body composition is optional context, not one of the 12 weighted inputs to the weekly MyBioAge Index™, and the app doesn't score your body fat against a reference range.

• The weekly MyBioAge Index™. When Apple Health holds both your lean body mass and your body mass, the Index uses your latest readings to adjust how much emphasis its heart-rate inputs receive: resting heart rate, walking heart rate average and HRV. The adjustment depends on how much of your weight is lean mass. Body fat percentage alone doesn't change the Index.
• Data coverage. The checks that decide whether the Index has enough data treat body composition as optional, so a missing reading never holds your Index back.
• AI Coach. You can choose Body Composition in the AI Coach and ask a Status, Why or Improve question about your body composition readings, such as lean body mass, answered on your iPhone.

There is no body composition card in the dashboard's Overview section.

Numbers in context

For adults, the usual starting points are simpler. NIDDK explains that body mass index (BMI) and waist size help show whether your weight is in a healthy range, and links carrying more fat around the waist with higher health risks. The CDC describes BMI as a screening measure that doesn't measure body fat directly or tell fat, muscle and bone apart, which is where a body composition estimate can add context.

Keep day-to-day changes in proportion: one weigh-in is a single point in a noisy series. If you want to change your weight, NIDDK notes that a healthy way of eating you can keep up for the long term may help. If your weight changes without an obvious reason, or it worries you, talk to a clinician.

Supporting lean mass

The NHS physical activity guidelines recommend muscle-strengthening work for the legs, hips, back, core, chest, shoulders and arms on two or more days each week, alongside regular aerobic activity. The same guidelines suggest checking with your GP before you start if you've been inactive for a while or have a health condition. Lifting weights, resistance bands, body-weight exercises, yoga and heavy gardening all count. If you are new to it, the NHS also has a set of gentle home strength exercises. Judge any change in a lean mass estimate by the trend across several weeks of readings, not by a single weigh-in.

Putting it together

Body composition won't tell you your biological age, but it helps explain the rest of your data. In My Bio Age it works quietly in the background rather than as a score. To see where it fits, read how Apple Health metrics become the MyBioAge Index™ and what functional age means. Our Methods & Sources page explains the approach in more depth.

Download My Bio Age free on the App Store to include the body composition readings you already keep in Apple Health in your wider weekly picture. The iPhone app is completely free, with no in-app purchases, subscription or account required. Your health data stays on your iPhone, and optional AI Coach explanations are generated fully on-device.

My Bio Age provides general wellness information, not medical advice. Home body composition readings are estimates, not a diagnosis, and a change in any number is a prompt to look at your routine, not a reason to change your treatment or medication.