Being underweight in and of itself isn’t a cause for concern.
Why isn’t the same said for being overweight then? Why don’t overweight people automatically keel over after their BMI rises to above 25?
Most girls I know from European countries are just underweight BMI 17-18 and they are healthier than UK girls, eat their 5 a day and such like. aren’t suffering from malnutrition, major fatigue, etc ..
Using the overweight argument, it means on the degree of under/over. The lower you are on BMI the more your risk factors go up for certain diseases/conditions
Some people just are light and would register as BMI underweight, but BMI is just an arbitrary number and you would look at the reasons why someone is that way.
BMI really isn’t a useful tool for under 25’s i.e. most of this forum, especially for the petite girls and their bone density isn’t as high.
It also fails to address the square cube law and therefore isn’t accurate for short/tall people.
TL;dr version – it’s just A tool for assessing health, and you’d look as to why someone is underweight rather than focus on their relative weight. Some people are just underweight and are perfectly healthy like some people are just naturally meant to be overweight.