NCHS' NHANES 1994 data. Data in the upper left and lower right quadrants suggest the limitations of<unk> . The medical establishment and statistical community have both highlighted the limitations of<unk> . Scaling The exponent in the denominator of the formula for<unk>  is arbitrary. The<unk>  depends upon weight and the square of height. Since<unk>  increases to the third power of linear dimensions, taller individuals with exactly the same body shape and relative composition have a larger<unk> .<unk>  is proportional to the<unk>  and inversely proportional to the square of the height. So, if all body dimensions double, and<unk>  scales naturally with the cube of the height, then<unk>  doubles instead of remaining the same. This results in taller people having a reported<unk>  that is uncharacteristically high, compared to their actual body fat levels. In comparison, the Ponderal<unk>  is based on the natural scaling of<unk>  with the third power of the height. However, many taller people are not just "scaled up" short people but tend to have narrower frames in proportion to their height. Carl Lavie has written that "The B.M.I. tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals." According to mathematician Nick Trefethen, "BMI divides the weight by too large a number for short people and too small a number for tall people. So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter." For US adults, exponent estimates range from 1.92 to 1.96 for males and from 1.45 to 1.95 for females. Physical characteristics The<unk>  overestimates roughly 10% for a large (or tall) frame and underestimates roughly 10% for a smaller frame (short stature). In other words, persons with small frames would be carrying more fat than optimal, but their<unk>  indicates that they are normal. Conversely, large framed (or tall) individuals may be quite healthy, with a fairly low body fat percentage, but be classified as overweight by<unk> . For example, a height/weight chart may say the ideal weight <unk>  21.5) for a man is . But if that man has a slender build (small frame), he may be overweight at and should reduce by 10% to roughly <unk>  19.4). In the reverse, the man with a larger frame and more solid build should increase by 10%, to roughly <unk>  23.7). If one teeters on the edge of small/medium or medium/large, common sense should be used in calculating one's ideal weight. However, falling into one's ideal weight range for height and build is still not as accurate in determining health risk factors as waist-to-height ratio and actual body fat percentage. Accurate frame size calculators use several measurements (wrist circumference, elbow width, neck circumference, and others) to determine what category an individual falls into for a given height. The<unk>  also fails to take into account loss of height through ageing. In this situation,<unk>  will increase without any corresponding increase in weight. Proposed new<unk>  A new formula for computing<unk>  that accounts for the distortions of the traditional<unk>  formula for shorter and taller individuals has been proposed by Nick Trefethen, Professor of numerical analysis at the University of Oxford: : The scaling factor of 1.3 was determined to make the proposed new<unk>  formula align with the traditional<unk>  formula for adults of average height, while the exponent of 2.5 is a compromise between the exponent of 2 in the traditional formula for<unk>  and the exponent of 3 that would be expected for the scaling of weight (which at constant density would theoretically scale with volume, i.e., as the cube of the height) with height; however, in Trefethen's analysis, an exponent of 2.5 was found to fit empirical data more closely with less distortion than either an exponent of 2 or 3. Muscle versus fat Assumptions about the distribution between muscle<unk>  and fat<unk>  are<unk> act.<unk>  generally overestimates adiposity on those with more lean body<unk>  (e.g., athletes) and underestimates excess adiposity on those with less lean body<unk> . A study in June 2008 by Romero-Corral et al. examined 13,601 subjects from the United States' third National Health and Nutrition Examination Survey (NHANES III) and found that<unk> -defined obesity <unk>  ≥ 30) was present in 21% of men and 31% of women. Body fat-defined obesity was found in 50% of men and 62% of women. While<unk> -defined obesity showed high specificity (95% for men and 99% for women),<unk>  showed poor sensitivity (36% for men and 49% for women). In other words, the<unk>  will be mostly correct when determining a person to be obese, but can err quite frequently when determining a person not to be. Despite this undercounting of obesity by<unk> ,<unk>  values in the intermediate<unk>  range of 20–30 were found to be associated with a wide range of body fat percentages. For men with a<unk>  of 25, about 20% have a body fat percentage below 20% and about 10% have body fat percentage above 30%. Body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skinfold measurements or underwater weighing and the limitations of manual measurement have also led to new, alternative methods to measure obesity, such as the body volume indicator. Variation in definitions of categories It is not clear where on the<unk>  scale the threshold for overweight and obese should be set. Because of this, the standards have varied over the past few decades. Between 1980 and 2000 the U.S. Dietary Guidelines have defined overweight at a variety of levels ranging from a<unk>  of 24.9 to 27.1. In 1985 the National Institutes of Health (NIH) consensus conference recommended that overweight<unk>  be set at a<unk>  of 27.8 for men and 27.3 for women. In 1998, an NIH report concluded that a<unk>  over 25 is overweight and a<unk>  over 30 is obese. In the 1990s the World Health Organization (WHO) decided that a<unk>  of 25 to 30 should be considered overweight and a<unk>  over 30 is obese, the standards the NIH set. This became the definitive guide for determining if someone is overweight. The current WHO and NIH ranges of normal weights are proved to be associated with decreased risks of some diseases such as diabetes type II; however using the same range of<unk>  for men and women is considered arbitrary and makes the definition of underweight quite unsuitable for men. One study found that the vast majority of people labelled 'overweight' and 'obese' according to current definitions do not in fact face any meaningful increased risk for early death. In a quantitative analysis of several studies, involving more than 600,000 men and women, the lowest mortality rates were found for people with<unk> s between 23 and 29; most of the 25–30 range considered 'overweight' was not associated with higher risk.
Body mass index
Body adiposity index
Obesity
Lean body mass
Family history 
Body fat percentage
Body composition
Sedentary lifestyle