Body composition terms, defined.

Body mass (weight)

Body mass is total weight: everything on the scale, with nothing said about what it is made of.

Level Whole body

Measured by Any scale.

It is the one number every instrument agrees on, and the one that answers the least. Two people can lose the same ten pounds and lose different things.

Fat mass

Fat mass is the total weight of chemical fat (triglyceride) in the body, wherever it is stored.

Level Molecular

Measured by DEXA, which assigns each pixel to fat, lean soft tissue or bone by how it attenuates two X-ray energies; the four-compartment model; estimated by a body-composition scale.

Fat mass is not adipose tissue. Adipose is the tissue, and it is not pure fat; part of it is water and protein. Losing adipose therefore takes some water and protein with it, and those count as lean on a scan.

Fat-free mass (FFM)

Fat-free mass is everything that is not fat: water, protein, bone mineral, glycogen and the small amount of essential lipid.

Level Molecular

Measured by By subtraction, weight minus fat mass. A body-composition scale estimates it from electrical resistance; a DEXA scan reports it as lean soft tissue plus bone mineral.

Mostly water. Fat-free mass moves with hydration, glycogen, and the size of organs, none of which is muscle. It is the quantity that is comparable across instruments, which is why our charts use it.

Lean soft tissue (LST)

Lean soft tissue is fat-free mass with the bone mineral taken out: the water, protein and other soft material that is neither fat nor bone.

Level Molecular

Measured by DEXA only. It is the residual after fat and bone are assigned, and it includes organs, blood, gut contents and every drop of body water.

This is the number most DEXA reports print under the word lean. It differs from fat-free mass by the bone mineral content, typically several pounds.

Lean mass

Lean mass has no fixed definition: on one DEXA report it means lean soft tissue, on another it means lean soft tissue plus bone, and on a body-composition scale it means fat-free mass.

Level Ambiguous

Measured by Whatever produced the report. Check its convention before comparing two numbers.

The expert standard treats the term as undefined until the convention is known. Two reports can both say lean and differ by the weight of your skeleton. And one scale maker goes further: Withings labels weight minus fat minus bone, by its own published equation, muscle mass. That number is lean soft tissue, water and organs included, and at about 85 percent of body weight it cannot be skeletal muscle.

Lean body mass (LBM)

Lean body mass is an older term that historically meant fat-free mass plus a small allowance for essential fat, and is now discouraged because it names no defined component.

Level Deprecated

Measured by Not a measured quantity.

Apple Health still labels the scale's fat-free mass reading Lean Body Mass. Read it as fat-free mass.

Appendicular lean mass (ALM) and ALMI

Appendicular lean mass is the lean soft tissue of the arms and legs; ALMI divides it by height squared to give a size-adjusted index in kg/m².

Level Molecular, regional

Measured by DEXA regional analysis. Not all reports carry it.

It is the best DEXA proxy for skeletal muscle because most of the body's muscle is in the limbs, and it is the number the sarcopenia definitions set their cutoffs on. It is still lean soft tissue, not muscle.

Skeletal muscle mass (SMM)

Skeletal muscle mass is the weight of the muscle organ itself: the contractile tissue that moves the skeleton, with its own water and fat inside it.

Level Tissue and organ

Measured by MRI or CT, which draw a boundary around the muscle and measure its volume; D3-creatine dilution, which measures the creatine pool inside muscle. No consumer device and no DEXA measures it.

On the reference-body figures, skeletal muscle is roughly half of fat-free mass in men and about 40 percent in women, and individuals vary widely around that. A fixed conversion from lean to muscle asserts a precision no one has.

Muscle quality

Muscle quality is either the fat fraction inside the muscle, measured by MRI, or the force a muscle produces per unit of its mass.

Level Tissue and functional

Measured by MRI fat-fraction imaging; strength divided by measured mass.

Quality can improve while volume falls. A muscle that loses fat infiltration during weight loss can be a better muscle in a smaller package.

Bone mineral content (BMC) and density (BMD)

Bone mineral content is the weight of mineral in the skeleton; bone mineral density is that mineral spread over the bone's projected area, reported as g/cm² and as a T-score against young-adult norms.

Level Molecular

Measured by DEXA. Every whole-body scan carries it, whether or not the report leads with it.

Bone is the third compartment and the one weight-loss conversations skip. It is also the reason lean soft tissue and fat-free mass are different numbers on the same scan.

Body fat percentage

Body fat percentage is fat mass divided by total body mass, which makes it a ratio of two measurements rather than a measurement of its own.

Level Derived

Measured by Computed from whichever instrument produced the fat mass.

It can fall while fat mass holds, if lean rises, and rise while fat falls, if lean falls faster. The masses are the signal; the percentage is a summary of them.

Fat-free mass index (FFMI)

FFMI is fat-free mass divided by height squared, in kg/m²: a size-adjusted index of everything that is not fat.

Level Derived

Measured by Computed from any fat-free mass reading and your height.

A screening number. It reads low in tall people and cannot tell muscle from water, so a low FFMI is a reason to look at ALMI, never a finding on its own.

Weight-only scale

Measures mass. Infers nothing. The most precise instrument on this page and the least informative.

Body-composition scale (bioimpedance, BIA)

Runs a small current through you and measures the resistance. Water conducts and fat does not, so resistance is a proxy for total body water; an equation fitted on a reference population turns water into fat-free mass, and fat is what is left. Two stacked inferences, which is why hydration, a meal, a workout and skin temperature all move the readout. Foot-to-foot scales measure the legs and extrapolate the trunk.

DEXA scan

Passes two X-ray energies through you and reads how much each is attenuated. Pixels containing bone are identified by the mineral signature; in the rest, the ratio of the two attenuations separates fat from lean soft tissue. Fat and bone are the measured components; lean soft tissue is the residual. A change in hydration shows up in lean soft tissue, and only slightly in the fat estimate: a dehydrated person gets a genuinely lower lean number, not a wrong fat number. Manufacturer, model and software version change the absolute values, so compare scans on the same machine.

MRI

Reads the proton signal and separates fat from water by their chemical shift, voxel by voxel. Segmenting the image gives muscle volume, and the fat fraction inside the muscle comes free. It is the best available measurement of the muscle organ, and it is still a volume, not a count of contractile protein.

CT

Measures X-ray attenuation per voxel in Hounsfield units. A single slice gives muscle cross-sectional area and a density that stands in for fat infiltration. Radiation and slice selection are the costs.

Four-compartment model

Combines body density, total body water by isotope dilution, and bone mineral by DEXA, so fat can be solved without assuming a constant hydration. The reference the others are validated against.

D3-creatine dilution

You swallow a labelled creatine dose; the enrichment that appears in urine tells how large the creatine pool is, and the pool lives almost entirely inside skeletal muscle. It measures muscle mass in a way none of the imaging does, and in a cohort of older men it predicted falls and mobility limitation where DEXA lean mass did not.