People who seek medical treatment for obesity or an eating disorder do so with the hope their health plan will pay for part of it. But whether it’s covered often comes down to a measure invented almost 200 years ago by a Belgian mathematician as part of his quest to use statistics to define the “average man.” That work, done in the 1830s by Adolphe Quetelet, appealed to life insurance companies, which created “ideal” weight tables after the turn of the century. By the 1970s and 1980s, the measurement, now dubbed body mass index, was adopted to screen for and track obesity. Now it’s everywhere,…