What is the most useful health number to track if you only track one?

Waist circumference, for most people. It correlates better with metabolic risk than weight or BMI, requires only a tape measure, and picks up the visceral fat that matters most — which is particularly relevant for Indians, who tend to carry more of it at a given BMI than other populations. Measure at the navel, at the end of a normal exhale, at the same time of day. Broad risk thresholds commonly cited for South Asians are lower than international ones: around 90cm for men and 80cm for women. It changes slowly, which makes it a better signal than daily weight.