In one sentence
Where your body fat sits among people of your age and sex in a large reference population, as a percentile: "leaner than 72 percent of men your age".
Why a percentile and not an age
Body fat rises with age in the population, but slowly and with a wide spread: between 20 and 50 the median man gains about 0.19 points of body fat a year while people of the same age differ from one another by 5 to 7 points (Kelly 2009). That makes "the age at which your body fat is typical" a meaningless number, a single standard deviation of ordinary people spanning about 30 "years", which is why Vitality Tracker does not fold body fat into its metabolic age. A percentile keeps the information and drops the false precision. It is also honest about what the reference is: the curves' own authors note that the median is a description of a population, not a healthy target.
Where the reference comes from
The US National Health and Nutrition Examination Survey measured whole-body DEXA in a nationally representative sample between 1999 and 2004, and the published curves give the median and spread of body fat at every age from 20 to 85, by sex and ethnicity, under an open license (Kelly 2009). VT uses the White reference curve, the one with the largest sample, and says so on the entry; the ethnic-specific curves differ by a point or two.
How Vitality Tracker uses it
Shown beside your metabolic age, from the 14-day mean of your DEXA-corrected body fat, so that composition shows through where the metabolic age cannot carry it. A worked example: a 40-year-old man at 20 percent body fat sits at about the 8th percentile, leaner than 92 percent of men his age; at 35 percent he would be at about the 90th.
How far to trust the number
As good as the body fat behind it. A raw scale value reads leaner than the DEXA reference by several points on most devices, so the percentile is computed from the DEXA-corrected value when a scan exists and labeled as uncorrected when one does not. The reference was measured on one maker's scanners; a scan on another maker's machine reads about a point apart.
Sources
- Kelly TL, Wilson KE, Heymsfield SB. Dual energy X-ray absorptiometry body composition reference values from NHANES. PLoS One 2009;4:e7038. doi:10.1371/journal.pone.0007038
- Imboden MT, et al. Reference standards for body fat measures using GE dual energy X-ray absorptiometry in Caucasian adults. PLoS One 2017;12:e0175110. doi:10.1371/journal.pone.0175110
- Vitality Tracker, Metabolic age method (S-AnthropoAge), October 2026.