BMI of 23 is where, even in the US, risks start rising sharply for things like metabolic disorder and heart disease. The fact that sooooo few people are slender in the US who aren't diseased, addicted, or mentally disordered distorts the health studies.  The number of people who are slim and healthy are getting close to outnumbered by people who are slim as a side effect from other terrible life choices.  In countries where slim is the norm, the risks of being fat are more obvious.  But scientist speculate "it's genetics" instead of the obvious.  So apparently, Chinese people in Singapore and Hong Kong have different "genetics" than people in Beijing, never mind out in the countryside or in less major cities. Anyway, unless you're super-swole or elderly and sedentary (and at risk of sarcopenia), risks begin to mount above a BMI of 22.  If you're old and the only exercise you get is moving your body around a little, some body fat is protective of excessive muscle loss.  But the better solution is to exercise.