After my Bod Pod, I got interested in how low is too low of a body fat percentage. I found this study which looks at body fat and all cause mortality (Ann Intern Med. 2016;164(8):532-541. Relationship Among Body Fat Percentage, Body Mass Index, and All-Cause Mortality: A Cohort Study. Raj Padwal, MD, MSc; William D. Leslie, MD, MSc; Lisa M. Lix, PhD; Sumit R. Majumdar, MD, MPH). Here’s a (fully-adjusted) figure which shows all cause mortality hazard ratio vs body fat percentage:
I looks like from this curve that being between 15-30% body fat is advantageous. Unfortunately, there’s no analysis of body fat percentages below 15% but the direction of the curve looks positive.
Note: This study had some serious limitations since it only looked at people who were referred for a DEXA scan. It looked at almost ten times the number of women than men due to the prevalence of hip fractures in women.