If someone has a healthy body fat percentage, but is technically underweight according to BMI, would this mean the person had insufficient muscle mass. How can you differentiate between a healthy body fat percentage in someone with a good level of muscle mass and an unhealthy level of muscle mass. Otherwise someone could be deemed as healthy, but have unhealthily low levels of muscle mass. For example, a 5ft2 female loses with only 70lb of lean muscle and 20lb of fat. She would have what appears to be a healthy 22% body fat level even though she only weighs 90lb, yet her lean body mass appears far too low (unless she is very small boned indeed). I don't see how this can be healthy. In fact she could just keep losing weight and still have a healthy body fat percentage due to all the muscle loss. So is there a minimum lean muscle mass amount people should have to be healthy? TIA