Athletes and athletic individuals are constantly told they are overweight/obese according to BMI. We know it's outdated and to ignore it yet people are denied medical procedures and employment into the forces/service based on it. It needs replacing with something that isn't inaccurate for thousands.
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Personally being repeatedly told I was overweight triggered my eating disorder as a teen. I want to stop this from happening to the next generation of young people. BMI doesn't account for muscle mass and increases weight stigma so actively discourages athleticism. It also encourages the loss of muscle mass and inactivity.
In the army, BMI is a deciding factor at the point of entry. You need muscle mass to lift a casualty, especially as a woman. BMI is actively discouraging muscle mass.