# Classification of obesity

An obese male with a body mass index of 46 kg/m²: weight 146 kg (322 lb), height 177 cm (5 ft 10 in)

Some associate obesity with anyone who is overweight, which is not necessarily true. Obesity is characterized by body mass which exceeds a certain percentage. The doctor measures the body mass with the body mass index (BMI),meaning that a person’s weight in kilograms (kg) is divided by the person’s height in meters (m) squared to equal the percentage range in which one's BMI should fall. Patients that have a BMI of 30 or greater are generally considered obese, although very muscular people (especially young males) can be exceptions to this definition. Other measurements of fat distribution include the waist–hip ratio and body fat percentage. Normal weight obesity is a condition of having normal body weight, but high body fat percentages and poses the same health risks as obesity.[1][2]

## BMI

Body mass index or BMI is a simple and widely used method for estimating body fat mass.[3] BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet.[4] BMI is an accurate reflection of body fat percentage in the majority of the adult population. It however is less accurate in people such as body builders and pregnant women.[5] A formula combining BMI, age and gender can be used to estimate a person's body fat percentage to an accuracy of 4%.[6] An alternative method, body volume index (BVI), is being developed in an effort to better take into account different body shapes.[7]

BMI Classification
< 18.5 underweight
18.5–24.9 normal weight
25.0–29.9 overweight
30.0–34.9 class I obesity
35.0–39.9 class II obesity
≥ 40.0   class III obesity

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

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.[8]

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.[9]

• Any BMI ≥ 35 or 40 is severe obesity
• A BMI of ≥ 35 or 40–44.9 or 49.9 is morbid obesity
• A BMI of ≥ 45 or 50 is super obese

As Asian populations develop negative health consequences at a lower BMI than Caucasians, some nations have redefined obesity. The Japanese have defined obesity as any BMI greater than 25[10] while China uses a BMI of greater than 28.[11]

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.[12] However, their mean body fat percentage, 14%, is well within what's considered a healthy range.[13]

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%.[14] 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,[15] and possibly superior to BMI as predictors of cardiovascular disease.[16]

## Waist circumference and waist–hip ratio

In the United States, a waist circumference of >102 cm in men and >88 cm in women[17] 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.[18]

In the European Union, waist circumference of ≥ 94 cm in men and ≥ 80 cm in non pregnant women are used as cut offs for central obesity.[19]

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.[19]

In those with a BMI under 35, intra-abdominal body fat is related to negative health outcomes independent of total body fat.[20] Intra-abdominal or visceral fat has a particularly strong correlation with cardiovascular disease.[18] In a study of 15,000 people, waist circumference also correlated better with metabolic syndrome than BMI.[21] Women with abdominal obesity have a cardiovascular risk similar to that of men.[22] 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 an abnormal waist circumferences.[23]

## Body fat percentage

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.[24] However, the finding that metabolic disturbance increases with increasing body fat percentage[25] suggests that focusing exclusively on cut-points of body fat percent may be of limited value.

Body fat percentage can be estimated from a person's BMI by the following formula:

$\text{body fat percentage} = 1.2\times \text{BMI} + 0.23 \times \text{age} -5.4- 10.8 \times \text{gender}$
where gender is 0 if female and 1 if male

This formula takes into account the fact that body fat percentage tends to be 10 percentage points greater in women than in men for a given BMI. It recognizes that a person's percentage body fat tends to increase as they age, even if their weight and BMI remain constant. The results of this formula have been shown to have an accuracy of 4% in one group of individuals.[26]

There are many other methods used to determine body fat percentage. Hydrostatic weighing, one of the most accurate methods of body fat calculation, involves weighting a person underwater. Two other simpler and less accurate methods have been used historically but are now not recommended.[27] The first is the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer.[28] The other is bioelectrical impedance analysis which uses electrical resistance. Bioelectrical impedance has not been shown to provide an advantage over BMI.[29]

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).[20] 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.[30]

## Childhood obesity

Variations in apparent body fat among children

A misconception regarding obesity is that it is only present in adults; there have been more and more cases where children are obese. Children are becoming number one for new cases dealing with this chronic disease. In children that have developed obesity during the early years are at a higher risk when being an adult and being obese. It can start in the early stages of a child’s life and progress to overweight in teen years, due to pop culture putting more focus on video games and the television. Children these days have a lot of stress and unhealthy habits.[31]

Childhood obesity has reached epidemic proportions in 21st century with rising rates in both the developed and developing world.[citation needed] Rates of obesity in Canadian boys have increased from 11% in 1980s to over 30% in 1990s, while during this same time period rates increased from 4 to 14% in Brazilian children.[32]

Children that become obese have a couple of different factors leading them to it. The family environment can be a big contributor. If your child comes from a family of overweight people, he or she may be more likely to put on excess weight, especially in an environment where high-calorie food is always available and physical activity is not encouraged. Some children overeat to cope with problems or to deal with emotions, such as stress, or to fight boredom. Children that are being bullied can be obese because they are eating away their problems.[33]

Depression in obese children is very likely. That is due to having low self-esteem in the child, which can create overwhelming feelings of hopelessness. When children lose hope that their lives will improve, they may become depressed. A depressed child may lose interest in normal activities, sleep more than usual or cry a lot. Some depressed children hide their sadness and appear emotionally flat instead. Either way, depression is as serious in children as in adults. Anyone with obesity can assume that their life expectancy will be lower than others. Some of the kids will carry obesity with them throughout life, and ultimately can cause them to die before their parents.[34]

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