from the 2017–2018 NHANES survey indicate that an estimated 43% of noninstitutionalized U.S. adults aged 20–74 are obese (including 9% who are severely obese) and an additional 31% are overweight. Only 26% were either normal weight or are underweight. In 1983, the body fat percentages of American Olympians averaged 14–22% for women and 6–13% for men. Body fat guidelines Essential fat is the level at which physical and physiological health would be negatively affected, and below which death is certain. Above that level, controversy exists as to whether a particular body fat percentage is better for one's health. Athletic performance might be affected by body fat: A study by the University of Arizona indicated that the ideal body fat percentage for athletic performance is 12–18% for women and 6–15% for men. 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. Measurement techniques Underwater weighing Irrespective of the location from which they are obtained, the fat cells in humans are composed almost entirely of pure triglycerides with an average density of about 0.9 kilograms per litre. Most modern body composition laboratories today use the value of 1.1 kilograms per litre for the density of the "fat free mass", a theoretical tissue composed of 72% water (density = 0.993), 21% protein (density = 1.340) and 7% mineral (density = 3.000) by weight. With a well engineered weighing system, body density can be determined with great accuracy by completely submerging a person in water and calculating the volume of the displaced water from the weight of the displaced water. A correction is made for the buoyancy of air in the lungs and other gases in the body spaces. If there were no errors whatsoever in measuring body density, the uncertainty in fat estimation would be about ± 3.8% of the body weight, primarily because of normal variability in body constituents. Whole-body air displacement plethysmography Whole-body air displacement plethysmography (ADP) is a recognised and scientifically validated densitometric method to measure human body fat percentage. ADP uses the same principles as the gold-standard method