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I guess it depends on what you mean by "works", but to my untrained eye BMI doesn't work especially well for individuals in general. It was designed to measure large populations using routinely collected statistical data, not to inform individual lifestyle and medical decisions. These days we have much more effective methods to evaluate metabolic and cardiovascular health instead of using a metric that's a proxy of a proxy, but calculating BMI and applying a (largely arbitrary) label is dirt-cheap by comparison to all of those; it requires no special expertise, expensive equipment, time-consuming procedure, or thoughtful analysis.