In one sentence
The scale's estimate of the weight of mineral in your skeleton, a few kilograms in an adult, which is not a measurement of your bones.
Why it matters, and why not here
Bone health matters a great deal later in life, but a bathroom scale cannot see it. The figure is produced from the same impedance reading as everything else, by assuming a typical proportion of mineral in lean tissue, and it changes mostly when your water does. The real measure is bone mineral density from a DEXA scan, a medical test your doctor orders and interprets, and the scale's "bone mass" has no relationship to it that you should act on.
What moves it
Hydration and the scale's algorithm. Real bone mineral changes over years, not days.
How Vitality Tracker uses it
Stored from the full scale reading and displayed; nothing scores on it, and VT never turns it into a density or a bone-health statement. If you have a DEXA bone density report, that lives with your lab results and is for your doctor to read.
How far to trust the number
Estimate, and the least informative line on the scale screen. Leave it alone.
Sources
- Kyle UG, et al. Bioelectrical impedance analysis, part I: review of principles and methods. Clin Nutr 2004;23:1226–43. doi:10.1016/j.clnu.2004.06.004.