Essential fat is the level at which physical and physiological health would be negatively affected, and below which death is certain. Controversy exists as to whether a particular body fat percentage is better for one's health; athletic performance may also be affected. The leanest athletes typically compete at levels of about 6-13% for men or 14–20% for women. Bodybuilders may compete at essential body fat range, in fact certified personal trainers will suggest them to keep that extremely low level of body fat only for the contest time. However, it is unclear that such levels are ever actually attained since (a) the means to measure such levels are, as noted below, lacking in principle and inaccurate, and (b) 4–6% is generally considered a physiological minimum for human males.