Body Mass Index - Limitations and Shortcomings

Limitations and Shortcomings

The medical establishment has acknowledged major shortcomings of BMI. Because the BMI formula depends only upon weight and height, its assumptions about the distribution between lean mass and adipose tissue are inexact. BMI generally overestimates adiposity on those with more lean body mass (e.g., athletes) and underestimates excess adiposity on those with less lean body mass. 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 BMI-defined obesity was present in 21% of men and 31% of women. Using body fat percentages (BF%), however, BF%-defined obesity was found in 50% of men and 62% of women. While BMI-defined obesity showed high specificity (95% of men and 99% of women presenting BMI-defined obesity also presented BF%-defined obesity), BMI showed poor sensitivity (BMI only identified 36% of the men and 49% of the women who presented BF%-defined obesity).

Mathematician Keith Devlin and a restaurant industry association The Center for Consumer Freedom argue that the error in the BMI is significant and so pervasive that it is not generally useful in evaluation of health. University of Chicago political science professor Eric Oliver says BMI is a convenient but inaccurate measure of weight, forced onto the populace, and should be revised.

A study published by JAMA in 2005 showed that "overweight" people had a similar relative risk of mortality to "normal" weight people as defined by BMI, while "underweight" and "obese" people had a higher death rate.

In an analysis of 40 studies involving 250,000 people, patients with coronary artery disease with "normal" BMIs were at higher risk of death from cardiovascular disease than people whose BMIs put them in the "overweight" range (BMI 25–29.9). In the "overweight", or intermediate, range of BMI (25–29.9), the study found that BMI failed to discriminate between bodyfat percentage and lean mass. The study concluded that "the accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. …These results may help to explain the unexpected better survival in overweight/mild obese patients."

A 2010 study that followed 11,000 subjects for up to eight years concluded that BMI is not a good measure for the risk of heart attack, stroke or death. A better measure was found to be the waist-to-height ratio. However, a 2011 study that followed 60,000 participants for up to 13 years found that waist-hip ratio was a better predictor of ischaemic heart disease mortality.

BMI is particularly inaccurate for people who are fit or athletic, as the higher muscle mass tends to put them in the "overweight" category by BMI, even though their body fat percentages frequently fall in the 10-15% category, which is below that of a more sedentary person of average build who has a "healthy" BMI number. 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 index. However, recent studies of American football linemen who undergo intensive weight training to increase their muscle mass show that they frequently suffer many of the same problems as people ordinarily considered obese, notably sleep apnea.

BMI also does not account for body frame size; A person may have a small frame and be carrying too much excess fat, but their BMI reflects that they are "healthy". Conversely, a large framed individual may be quite healthy with a fairly low body fat percentage, but be classified as "overweight" by BMI. 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 standard is to use frame size in conjunction with ideal height/weight charts and add roughly 10% for a large frame or subtract roughly 10% for a smaller frame.

For example, a chart may say the ideal weight for a man 5'10" (178 cm) is 165 pounds (75 kg). But if that man has a slender build (small frame), he may be overweight at 165 pounds (75 kg) and should reduce by 10%, to roughly 150 pounds (68 kg). In the reverse, the man with a larger frame and more solid build can be quite healthy at 180 pounds (82 kg). If one teeters on the edge of small/medium or medium/large, a dose of common sense should be used in calculating their ideal weight. However, falling into your ideal weight range for height and build is still not as accurate in determining health risk factors as waist/height ratio and actual body fat percentage.

A further limitation of BMI relates to loss of height through aging. In this situation, BMI will increase without any corresponding increase in weight.

The exponent of 2 in the denominator of the formula for BMI is arbitrary. It is meant to reduce variability in the BMI associated only with a difference in size, rather than with differences in weight relative to one's ideal weight. If taller people were simply scaled-up versions of shorter people, the appropriate exponent would be 3, as weight would increase with the cube of height. However, on average, taller people have a slimmer build relative to their height than do shorter people, and the exponent which matches the variation best is between 2 and 3. An analysis based on data gathered in the USA suggested an exponent of 2.6 would yield the best fit for children aged 2 to 19 years old. The exponent 2 is used instead by convention and for simplicity.

As a possible alternative to BMI, the concepts fat-free mass index (FFMI) and fat mass index (FMI) were introduced in the early 1990s.