In one sentence
Your waist divided by your height, the simplest measure of central fat there is, and the one behind the rule people remember: keep your waist under half your height.
Why it matters
Dividing by height removes the obvious objection to a raw waist, that tall people have bigger middles, and the result works across ages, sexes and populations in a way BMI does not. The 0.5 boundary comes from two reviews: one found that the ratio separated adults with and without raised blood pressure, blood fats and glucose better than BMI or waist alone in 31 studies (Ashwell 2012); an earlier review of 78 studies across 14 countries found 0.5 to be a sensible single boundary across them (Browning 2010). It is a published reference point, not a line Vitality Tracker draws for you.
What moves it
The waist; height is fixed. Everything in the waist entry applies, including the site.
How Vitality Tracker uses it
It is the central-fat input to the Vitality Score's body-composition area and, through the cube root of the same ratio, one of the two shape terms in your metabolic age, where it is the term that moves the result: about a third of a year per centimeter of waist at a fixed weight.
How far to trust the number
As good as the waist that went into it, and no better.
Sources
- Ashwell M, Gunn P, Gibson S. Obes Rev 2012;13:275–86. doi:10.1111/j.1467-789X.2011.00952.x
- Browning LM, Hsieh SD, Ashwell M. A systematic review of waist-to-height ratio as a screening tool for the prediction of cardiovascular disease and diabetes: 0.5 could be a suitable global boundary value. Nutr Res Rev 2010;23:247–69. doi:10.1017/S0954422410000144.