A low BMI on its own can cause serious health complications. You can die of heart failure due to a low BMI even if you're maintaining at the time.

 

I think the first most important thing is that your body gets the nutrients it needs to function. But there is a certain underweight, like below 13-14 bmi maybe, that is just dangerous in itself.

Anyway, what I can highly recommend is to have your blood checked if you are worried. Because that's the first sign if something is wrong. I usually have it checked every 6 months because I'm paranoid when it comes to electrolyte and vitamin levels.

 

I felt pretty okay at a bmi of 18 and maintained it, was very athletic and active. However more dangerous to relapse and get lower also thought about food 24/7 and was typically bingeing/restricting.

 

It really depends on how long. If you're restricting severely for a long amount of time, then your body will start to break down. But if you only restrict severely for a few days, you should be fine and your BMI would be more worrisome.

 

Severe restriction is definitely a big risk factor for health complications and/or early death. While low BMI's have there own set of health consequences just from being underweight, I am a firm advocate that severe restriction is dangerous at any weight. I was bedridden at a normal weight from restricting way too low my during my initial bout with AN, because I starved from a nearly obese BMI.

 

It depends on how much body fat you have, ethnicity, if you're small boned/proportions, things like that. Person A could be way too skinny and be malnourished at a BMI of 18. Person B could be healthy forever at a BMI of 18. The content of the calories is also important. 1200 calories per day of leafy greens, rice, quinoa, beans, chicken, dark chocolate, olive oil etc is so much better than 3000 calories binge of candy and Oreos one day and then fasting for 3 days.

I had all normal blood tests at 5'8 and 110 pounds, except for mild anemia so the dr just told me to take an iron supplement. I'm a vegetarian so the anemia would have been normal even in a non-underweight person.

 

In my opinion both can be dangerous, the thing is that it is often more BMI focused to determine if a person is unwell - it is relevant and does cause health issues but more needs to be taken into account. I hope its different overseas but from what I've heard its similar.