Classification of obesity

Body mass index or BMI is a simple and widely used method for estimating body fat mass.[11] BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet.[12] BMI is an accurate reflection of body fat percentage in the majority of the adult population. However it is less accurate in people such as body builders and pregnant women.[13] A formula combining BMI, age, and gender can be used to estimate a person's body fat percentage to an accuracy of 4%.[14] An alternative method, body volume index (BVI) , is being developed by Mayo Clinic in an effort to better take into account different body shapes.[15] BVI measures body fat distribution and calculates the BVI number, based on these results. This BVI number is an indication of weight associated health risk.

BMI is calculated by dividing the subject's mass by the square of his or her height, typically expressed either in metric or US "Customary" units:

Metric: ${\displaystyle \mathrm {BMI} ={\frac {\mathrm {kilograms} }{\mathrm {meters} ^{2}}}}$

US/Customary and imperial: ${\displaystyle \mathrm {BMI} =\mathrm {pounds} {\frac {703}{\mathrm {inches} ^{2}}}}$

The most commonly used definitions, established by the World Health Organization (WHO) in 1997 and published in 2000, provide the values listed in the table at right.[17]

Some modifications to the WHO definitions have been made by particular bodies. The surgical literature breaks down class III obesity into further categories, though the exact values are still disputed.[18]

• Any BMI ≥ 35 is severe obesity
• A BMI of ≥ 40 is morbid obesity
• A BMI of ≥ 50 is super obese

As Asian populations develop negative health consequences at a lower BMI than western populations, some nations have redefined obesity. The Japanese have defined obesity as any BMI greater than 25[19] while China uses a BMI of greater than 28.[20]

The BMI-based definition is easy to use and it is particularly convenient for statistical purposes, since it only depends on two commonly measured quantities, one's height and weight. However, it ignores variations between individuals in amounts of lean body mass, particularly muscle mass. Individuals involved in heavy physical labor or sports may have high BMI values despite having little fat. For example, more than half of all NFL players are classified as "obese" (BMI ≥ 30), and 1 in 4 are classified as "extremely obese" (BMI ≥ 35), according to the BMI metric.[21] However, their mean body fat percentage, 14%, is well within what's considered a healthy range.[22]

The preferred obesity metric in scholarly circles is the body fat percentage (BF%) - the ratio of the total weight of person's fat to his or her body weight, and BMI is viewed merely as a way to approximate BF%.[23] Levels in excess of 32% for women and 25% for men are generally considered to indicate obesity. However, accurate measurement of body fat percentage is much more difficult than measurement of BMI. Several methods of varying accuracy and complexity exist.

Other proposed but less common obesity measures include waist circumference and waist–hip ratio. These measure a common form of obesity known as abdominal or central obesity, characterized by excess deposits of fat in the abdominal region and inside peritoneal cavity. They have been shown to be comparable to BMI in their power to predict the risk of metabolic abnormalities such as type II diabetes,[24] and possibly superior to BMI as predictors of cardiovascular disease.[25]

In the United States a waist circumference of >102 cm (≈40") in men and >88 cm (≈34.5") in women[26] or the waist–hip ratio (the circumference of the waist divided by that of the hips of >0.9 for men and >0.85 for women) are used to define central obesity.[27]

In the European Union waist circumference of ≥ 94 cm(≈37") in men and ≥ 80 cm(≈31.5") in non pregnant women are used as cut offs for central obesity.[28]

A lower cut off of 90 cm has been recommended for South Asian and Chinese men, while a cut off of 85 cm has been recommended for Japanese men.[28]

In those with a BMI under 35, intra-abdominal body fat is related to negative health outcomes independent of total body fat.[29] Intra-abdominal or visceral fat has a particularly strong correlation with cardiovascular disease.[27] In a study of 15,000 people, waist circumference also correlated better with metabolic syndrome than BMI.[30] Women with abdominal obesity have a cardiovascular risk similar to that of men.[31] In people with a BMI over 35, measurement of waist circumference however adds little to the predictive power of BMI as most individuals with this BMI have abnormal waist circumferences.[32]

The Body Volume Indicator (BVI) is a novel method for calculating body fat distribution. Designed by Select Research and Mayo Clinic, BVI has been developed as an alternative to BMI.[33][34] BMI doesn't account for muscle and fat distribution, or gender, or ethnicity; and therefore is not an accurate measure of obesity in many individuals. BVI uses 3D technology to analyse overall body shape, identifying where fat is distributed across the body.[35]

Body Volume means the composition of a person and their body parts; each part of a person’s body (arms, legs, chest, etc.) has its own 3D shape, individual weight and measurement. An increase in the volume of fat and muscle normally corresponds with an increase or decrease of volume in that area of the body. BVI calculates a persons body fat composition from two images taken front and side on. A silhouette is extracted from these images and a 3D model created, from which detailed linear and volume measurements can be calculated. By comparing the 3D models to MRI data, BVI software is able to calculate fat distribution, and in particular visceral fat accumulated near the individual’s organs. The results will provide accurate measurements for waist to hip ratio, BMI, total body fat, visceral fat and BVI number.

The BVI Number has been designed by Mayo Clinic as an alternative to BMI. It provides an indication of health risk based on fat distribution with a particular emphasis on visceral fat; which is located around organs. Visceral fat is metabolically active, with high levels a known risk factor for cardiovascular disease[36] and diabetes type II.[37]

Cross-sections of the torso of a person of normal weight (left) and an obese person (right), taken by CT scan. Note the 3.6 cm (1.4 inches) of subcutaneous fat on the obese person.

Body fat percentage is total body fat expressed as a percentage of total body weight. There is no generally accepted definition of obesity based on total body fat. Most researchers have used >25% in men, and >30% in women, as cut-points to define obesity.[38]

As by anthropometric methods, body fat percentage can be estimated from a person's BMI by the following formula:[39]

${\displaystyle {\text{body fat percentage}}=1.39\times {\text{BMI}}+0.16\times {\text{age}}-10.34\times {\text{gender}}-9}$

where gender (sex) is 0 if female and 1 if male to account for the lower body fat percentage of men.

There are many other methods used to determine body fat percentage. Hydrostatic weighing, one of the most accurate methods of body fat calculation, involves weighing a person underwater. Two other simpler and less accurate methods have been used historically but are now not recommended.[40] The first is the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer.[41] The other is bioelectrical impedance analysis which uses electrical resistance. Bioelectrical impedance has not been shown to provide an advantage over BMI.[42]

Body fat percentage measurement techniques used mainly for research include computed tomography (CT scan), magnetic resonance imaging (MRI), and dual energy X-ray absorptiometry (DEXA).[29] These techniques provide very accurate measurements, but it can be difficult to obtain in the severely obese due to weight limits of most equipment and insufficient diameter of many CT or MRI scanners.[43]