DEXA body composition calculator

Why percentages mislead and indices do not

Body fat percentage has an awkward property: it moves when your lean mass moves, even if the amount of fat you carry has not changed at all. Gain four kilograms of muscle and your body fat percentage drops without a gram of fat leaving.

Dividing fat mass by height squared removes that problem. Fat mass index and lean mass index are the body composition equivalents of BMI, except that they separate the two things BMI cannot tell apart, which is the whole reason to get scanned.

That is also why tracking works better with indices. If both indices are printed on each scan, you can see whether a two kilogram weight change was fat, lean tissue or both, which is the question a scale can never answer.

Appendicular lean mass, the number with real cutoffs

Appendicular lean mass is the lean tissue in your arms and legs, which tracks skeletal muscle far better than total lean mass does, since total lean mass includes organs and residual water. Divided by height squared it becomes appendicular lean mass index, and this is the one measure on your report with published cutoffs.

The 2019 revised European consensus on sarcopenia (EWGSOP2) uses an index below 7.0 kg/m2 in men and below 5.5 kg/m2 in women as low muscle quantity. The earlier Baumgartner 1998 criteria, still widely cited, used 7.26 kg/m2 in men and 5.45 kg/m2 in women.

One important caveat: EWGSOP2 treats low muscle strength, not low mass, as the primary criterion for sarcopenia, with mass used to confirm it. A low index with normal grip strength and normal walking speed is a finding to watch, not a diagnosis.

• EWGSOP2 2019 low muscle quantity: appendicular lean mass index under 7.0 kg/m2 in men, under 5.5 kg/m2 in women.
• EWGSOP2 also gives absolute cutoffs: appendicular lean mass under 20 kg in men, under 15 kg in women.
• Baumgartner 1998, from the Rosetta Study reference population: under 7.26 kg/m2 in men and under 5.45 kg/m2 in women.
• Strength thresholds in EWGSOP2: grip strength under 27 kg in men, under 16 kg in women.

How to make repeat scans comparable

DEXA is precise and easily fooled. Hydration status shifts lean mass readings, since lean tissue is mostly water, and a scan after a heavy carbohydrate load or a hard training week can show lean mass gains that are glycogen and water.

Scan on the same machine, fasted, at the same time of day, without training in the previous 24 hours and without unusual fluid intake. Machines from different manufacturers do not agree closely enough for a switch to be treated as a change in you.

Then leave enough time between scans. Three to six months is a reasonable interval for tracking a program. Anything closer measures noise, and it is the honest answer to why we do not scan monthly.

What we do with this at Baseline

Body composition is one of the few places where the goal of a therapy can be stated precisely. With a GLP-1 therapy the aim is not simply weight loss, it is losing fat mass while defending appendicular lean mass, and only a scan can tell you which happened, which is why a scan is worth considering if you choose to measure.

That is the reason protein intake and resistance training are treated as part of the therapy rather than as advice attached to it. Weight coming down while lean mass comes down with it is a poor outcome dressed up as a good one on the scale.

Where this number is misleading

• Fat mass index and lean mass index have published reference distributions from NHANES, but the classification bands are less standardized than the sarcopenia cutoffs, so this page gives values without pass or fail labels.
• Appendicular lean mass index cutoffs are population thresholds for identifying low muscle mass, not personal targets, and EWGSOP2 requires low strength as the primary criterion for sarcopenia.
• DEXA lean mass is sensitive to hydration and glycogen, so short interval changes are often not tissue.
• Results are not comparable across machines from different manufacturers, or reliably across software versions.
• Visceral fat estimates from DEXA are derived rather than measured directly, and are less accurate than MRI or CT.

Common questions

What is fat mass index on a DEXA report?

Fat mass in kilograms divided by height in meters squared. Unlike body fat percentage it does not change when your lean mass changes, which makes it the better number for tracking fat over time.

What is a low appendicular lean mass index?

The 2019 EWGSOP2 consensus uses below 7.0 kg/m2 in men and below 5.5 kg/m2 in women as low muscle quantity. The older Baumgartner criteria used 7.26 and 5.45 kg/m2. EWGSOP2 treats low strength as the primary criterion, with mass used to confirm.

How often should I get a DEXA scan?

Every three to six months is enough to track a program. Shorter intervals mostly measure hydration and glycogen, particularly in lean mass.

Why did my body fat percentage stay flat while I looked leaner?

Because the percentage has lean mass in its denominator. If you lost fat and gained muscle at similar rates, the percentage barely moves while fat mass index falls and lean mass index rises. That is exactly why indices are worth calculating.

Sources and disclosures

• Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2). Age and Ageing, 2019
• Baumgartner RN, Koehler KM, Gallagher D, et al. Epidemiology of sarcopenia among the elderly in New Mexico. American Journal of Epidemiology, 1998
• Kelly TL, Wilson KE, Heymsfield SB. Dual energy X-ray absorptiometry body composition reference values from NHANES. PLOS One, 2009
• Schutz Y, Kyle UUG, Pichard C. Fat-free mass index and fat mass index percentiles in Caucasians aged 18 to 98 years. International Journal of Obesity, 2002
• These calculators are for education. They do not diagnose, they do not replace testing, and they are not a prescription. Baseline is a wellness consultancy, and any prescription is written by an independent licensed provider in our network who has reviewed your history, your goals and any lab work you choose to share.