In one sentence
The scale's index for the fat packed around your organs, as opposed to the fat under your skin, on a scale of the maker's own choosing.
Why it matters
Of all the places fat sits, the fat around the organs is the one most strongly associated with the metabolic measures in your lab results, which is why every scale now tries to report it. The honest position is that a scale cannot see it. The reference methods are MRI and CT; what a scale produces is an index from the same impedance reading as everything else, calibrated by the maker against one of those methods in one group of people.
What the validation shows
In 120 adults measured against MRI, a bioimpedance "visceral fat level" tracked total abdominal fat closely (correlation 0.92 to 0.94) but visceral fat itself only loosely (0.64 to 0.65), no better than a tape measure around the waist, and the authors concluded that neither the device nor the waist was a useful proxy for visceral fat as such (Browning 2010). In 48 people losing a great deal of weight after surgery, bioimpedance under-read the CT visceral fat index, more so at higher values, but the percentage change over six months tracked CT (correlation 0.71), so the index followed the direction of a large real change (Lee 2021). Read it as "belly fat, roughly", and as a trend.
What moves it
Everything that moves body fat on the scale, plus one thing of its own: the current crosses the trunk, so a full bowel reads as more visceral fat. That is why VT asks about bowel movements (Why VT asks about bowel movements).
How to measure it well
The scale's routine, bowel question answered, and the waist measured with a tape every few weeks, which is the better home measure of the same thing.
How Vitality Tracker uses it
Stored with the name of the scale that produced it and never compared across scales, because each maker's index has its own range (one widely sold brand uses 1 to 59 and describes 1 to 12 as its healthy range; others use other scales). Charted as a within-device trend, paired on the labs dashboard with the hepatic steatosis index, and never scored or banded. The waist-to-height ratio, not this index, is VT's measure of central fat.
How far to trust the number
Estimate, and a loose one: the device agrees with itself well and with the thing it claims to measure poorly. Direction over months, on one scale, is the usable part.
Sources
- Browning LM, Mugridge O, Chatfield MD, et al. Validity of a new abdominal bioelectrical impedance device to measure abdominal and visceral fat: comparison with MRI. Obesity 2010;18:2385–91. doi:10.1038/oby.2010.71
- Lee JK, Park YS, Kim K, Oh TJ, Chang W. Comparison of bioelectrical impedance analysis and computed tomography on body composition changes including visceral fat after bariatric surgery in Asian patients with obesity. Obes Surg 2021;31:4243–50. doi:10.1007/s11695-021-05569-6.