Article also says that people with health issues that will kill them or shorten lifespan (lung disease, malnourishment, heavy alcohol/tobacco/drug use, mental illness) make individuals underweight. Being underweight is the symptom, not the cause.
It's hard to prove correlation for causality.
A BMI<18 is pretty extreme. 6ft tall male <135lbs? I could see my ribs and had random people feeding me when I was 165 (I'm 6ft tall and relatively athletic, I was in great shape). The lowest I hit was 148, and that was in the midst of a serious health issue last month. I can't imagine being 135 or less.
It's hard to prove correlation for causality.
A BMI<18 is pretty extreme. 6ft tall male <135lbs? I could see my ribs and had random people feeding me when I was 165 (I'm 6ft tall and relatively athletic, I was in great shape). The lowest I hit was 148, and that was in the midst of a serious health issue last month. I can't imagine being 135 or less.