Classification of obesity

BMI is further evaluated in terms of fat distribution via the waist–hip ratio and total cardiovascular risk factors.

[2][3] In children, a healthy weight varies with sex and age, and obesity determination is in relation to a historical normal group.

[4] Obesity is a medical condition in which excess body fat has accumulated to the extent that it has an adverse effect on health.

[13] A formula combining BMI, age, and gender can be used to estimate a person's body fat percentage to an accuracy of 4%.

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

The surgical literature breaks down class III obesity into further categories, though the exact values are still disputed.

[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.

Individuals involved in heavy physical labor or sports may have high BMI values despite having little fat.

[22] The preferred obesity metric in scholarly circles is the body fat percentage (BF%)[citation needed] - 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%.

[35][36] 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.

A silhouette is extracted from these images and a 3D model created, from which detailed linear and volume measurements can be calculated.

The BVI algorithm also accounts for demographic factors, such as age and gender, and is based on weight distribution across different parts of the body.

[44] The first is the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer.

[44] 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.

Changing diet and decreasing physical activity are believed to be the two most important in causing the recent increase in the rate of obesity.

Activities from self-propelled transport, to school physical education, and organized sports has been declining in many countries.

Obesity and BMI
A front and side view of a morbidly obese male torso. Stretch marks of the skin are visible along with gynecomastia.
An obese male with a body mass index of 53 kg/m 2 : weight 182 kg (400 lb), height 185 cm (6 ft 1 in)
Two computed tomography images of a horizontal section of the mid abdominal region; one from a normal weight individual the other from an obese person. In both the bony structures and organs appear similar. The primary difference is that in the normal weight person there is little subcutaneous fat and the obese person shows substantially subcutaneous fat.
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. Some obese people would present on abdominal CT with visceral fat and negligible subcutaneous fat instead. [ 40 ]