Lean Body Mass

What Lean Body Mass Measures

Lean body mass is often talked about as if it were just muscle, but it's a wider category. A 2024 review in the Journal of the Endocrine Society defines it as the estimated mass of all the non-fat components in the body, and puts skeletal muscle at roughly half of that total in a reference adult.^[1] A 2025 review puts the rest of that total at other lean tissue, including viscera, skin, and bone.^[4] Skeletal muscle itself makes up about 40% of total body weight in an average adult and contains most of the body's protein, but "lean body mass" is the broader figure, not a synonym for muscle alone.^[2]

How It's Measured

Lean body mass is calculated as the flip side of body fat: whatever weight a DEXA scan or bioelectrical impedance device doesn't classify as fat mass gets counted as lean mass. That means lean body mass carries the same measurement tradeoffs as body fat percentage, since it's derived from the same scan or device rather than measured on its own.

Why People Track Lean Body Mass

Because lean body mass and fat mass are calculated as a pair, tracking both gives a fuller picture of what's changing when total weight moves, or stays the same. Weight can hold flat while lean and fat mass shift in opposite directions underneath it.

Two areas of research come up often around lean mass: how it responds to resistance training, and how it changes with age. A 2021 meta-analysis of 26 observational studies in postmenopausal and elderly women found resistance training programs produced small-to-moderate but statistically significant increases in lean body mass across the pooled studies.^[3] On the aging side, a 2025 review describes a decline in type II muscle fibers and in muscle size, strength, and power with age, while also noting that resistance training can be taken up at any age with measurable benefit.^[4]

How Go Go Gaia Tracks Lean Body Mass

Go Go Gaia doesn't calculate lean body mass itself. If you use a smart scale or other device that writes to Apple Health, Go Go Gaia syncs that reading automatically through HealthKit, then charts it as a trend alongside your cycle phase and other biometrics.

What the Research Shows

The 2021 meta-analysis on resistance training and lean mass reported an effect size of SMD 0.44 across the pooled studies of postmenopausal and elderly women, described by the authors as small-to-moderate but statistically significant, and it held regardless of participants' age, how long the training program ran, or how often they trained.^[3] The same analysis found no reliable effect on fat mass, underlining that lean mass and fat mass don't necessarily move together.^[3]

The 2025 review on fat-free mass and metabolic health notes that resistance training can be undertaken at any age, from children through older adults, with measurable benefit to muscle mass, alongside the well-documented age-related decline in fast-twitch muscle fiber count and overall muscle size.^[4]

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

What does lean body mass actually include?

Everything that isn't fat: skeletal muscle, bone, organs, and body water. Skeletal muscle makes up roughly half of that total in a reference adult body, with water, bone, and organ tissue making up the rest. It's a broader category than just muscle.

Does resistance training change lean body mass?

A 2021 meta-analysis of resistance training programs in postmenopausal and elderly women found small-to-moderate but statistically significant increases in lean body mass across the studies reviewed. A separate 2025 review notes that resistance training can be undertaken at any age with measurable benefits to muscle mass and strength.

How does Go Go Gaia track lean body mass?

Go Go Gaia doesn't calculate lean body mass itself. If you use a smart scale or other device that writes to Apple Health, Go Go Gaia syncs that reading through HealthKit and charts it as a trend alongside your cycle phase and other biometrics.