Getting back to the question about whether the target can be too low, here's an interesting analysis: http://www.ncbi.nlm.nih.gov/pubmed/20216568
After the researchers corrected for confounding factors like smoking, poverty etc, they concluded:
After the researchers corrected for confounding factors like smoking, poverty etc, they concluded:
Overall, we could not conclude that the underweight were less healthy than individuals in the other BMI categories.