A BMI below 18.5 is underweight according to NIH: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm https://www.nhlbi.nih.gov/health/educatio

A BMI below 18.5 is underweight according to NIH: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmi... But comparing the extremes (<18, >30) and using in-between as control probably makes statistical power easier.