Hello Barbell Medicine crew,I saw that in some videos you guys recommend weight gain for strength work if the person is underweight.Do you consider underweight being at a BMI below 20 or somewhere in between 20-23?Because 20-24 is considered normal on most charts.Thanks!
IDK of any hard numbers that BBM has put out, but under 18.5 is underweight by standard BMI classification. Off the top of my head I’d guess that under 20 indicates you could stand to build some muscle in order to reduce risk of developing sarcopenia later on. Beyond that it probably depends on your circumstances and goals. You probably won’t be a good powerlifter with a BMI under 25.
We haven’t published anything regarding BMI and underweight status, you’ll find that we tend to go my the marker of waist circumference, and weight.
Also there are plenty of competitive PLers would would fall in the BMI 25 markers. 
That’s pretty arbitrary.