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Obesity classification is a ranking of obesity, the medical condition in which excess body fat has accumulated to the extent that it has an adverse effect on health. [1] The World Health Organization (WHO) classifies obesity by body mass index (BMI). 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. [1] Relative weight [ clarification needed ] and body mass index (BMI) are nearly identical and are reasonable estimates of body fatness as measured by percentage body fat. [5] However, BMI does not account for the wide variation in body fat distribution, and may not correspond to the same degree of fatness or associated health risk in different individuals and populations. [6] [7] [3] 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 with the same health risks of obesity. [8] [9] BMI can be used to predict the risk of metabolic abnormalities like diabetes. [10]
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.
Category [16] | BMI (kg/m2) |
---|---|
Underweight | < 18.5 |
Normal weight | 18.5 – 24.9 |
Overweight | 25.0 – 29.9 |
Obese (Class I) | 30.0 – 34.9 |
Obese (Class II) | 35.0 – 39.9 |
Obese (Class III) | ≥ 40.0 |
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. [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]
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 defines it as any 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 is 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. Sumo wrestlers may be categorized by BMI as "severely obese" or "very severely obese" but many Sumo wrestlers are not categorized as obese when body fat percentage is used instead (having <25% body fat). [24] Some Sumo wrestlers were found to have no more body fat than a non-Sumo comparison group, with high BMI values resulting from their high amounts of lean body mass. [24] 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, [10] 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 Index (BVI) is a novel method for calculating body fat distribution. Human body volumes and body volume ratios and their links to body compositions have been researched for several years. [33] [34] Later the term Body Volume Index (BVI) instead of ratio was introduced by Select Research and Mayo Clinic; BVI has been developed as an alternative to BMI. [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. BVI uses 3D technology to analyse overall body shape, identifying where fat is distributed across the body. [37]
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 person's 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 and hip circumferences, waist to hip ratio, total body fat, visceral fat and BVI number.
The BVI Number has been designed by Mayo Clinic as an alternative to the BMI number. 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. The BVI is expressed as a single numerical value on a scale from 1 to 20. A higher BVI number indicates a higher calculated risk for cardio-metabolic complications and other health risks associated with body fat distribution. A BVI score below 13 is considered lower-risk, while scores above 13 are associated with increased risk, accounting for the tendency of risk to increase with age. 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 metabolic disease which includes cardiovascular disease [38] and diabetes type II. [39]
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, [41] but the use of these values have been disputed. [42]
As by anthropometric methods, body fat percentage can be estimated from a person's BMI by the following formula: [43]
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. [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. [45] The other is bioelectrical impedance analysis which uses electrical resistance. Bioelectrical impedance has not been shown to provide an advantage over BMI. [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. [46]
The healthy BMI range varies with the age and sex of the child. Obesity in children and adolescents is defined as a BMI greater than the 95th percentile. [47] The reference data that these percentiles are based on is from 1963 to 1994 and thus has not been affected by the recent increases in rates of obesity. [48]
Childhood obesity has reached epidemic proportions in the 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 the 1980s to over 30% in the 1990s, while during this same time period rates increased from 4 to 14% in Brazilian children. [49]
As with obesity in adults many different factors contribute to the rising rates of childhood obesity. 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. [50]
Because childhood obesity often persists into adulthood, and is associated with numerous chronic illnesses, it is important that children who are obese be tested for hypertension, diabetes, hyperlipidemia, and fatty liver. [51]
Treatments used in children are primarily lifestyle interventions and behavioral techniques. Medications are not FDA approved for use in this age group. [49]
Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms (kg) and height in metres (m).
Metabolic syndrome is a clustering of at least three of the following five medical conditions: abdominal obesity, high blood pressure, high blood sugar, high serum triglycerides, and low serum high-density lipoprotein (HDL).
Abdominal obesity, also known as central obesity and truncal obesity, is the human condition of an excessive concentration of visceral fat around the stomach and abdomen to such an extent that it is likely to harm its bearer's health. Abdominal obesity has been strongly linked to cardiovascular disease, Alzheimer's disease, and other metabolic and vascular diseases.
Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it can potentially have negative effects on health. People are classified as obese when their body mass index (BMI)—a person's weight divided by the square of the person's height—is over 30 kg/m2; the range 25–30 kg/m2 is defined as overweight. Some East Asian countries use lower values to calculate obesity. Obesity is a major cause of disability and is correlated with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis.
The waist is the part of the abdomen between the rib cage and hips. Normally, it is the narrowest part of the torso.
The waist–hip ratio or waist-to-hip ratio (WHR) is the dimensionless ratio of the circumference of the waist to that of the hips. This is calculated as waist measurement divided by hip measurement. For example, a person with a 75 cm waist and 95 cm hips has WHR of about 0.79.
The body fat percentage of an organism is the total mass of its fat divided by its total body mass, multiplied by 100; body fat includes essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions. The percentage of essential body fat for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Storage body fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. A number of methods are available for determining body fat percentage, such as measurement with calipers or through the use of bioelectrical impedance analysis.
Human body shape is a complex phenomenon with sophisticated detail and function. The general shape or figure of a person is defined mainly by the molding of skeletal structures, as well as the distribution of muscles and fat. Skeletal structure grows and changes only up to the point at which a human reaches adulthood and remains essentially the same for the rest of their life. Growth is usually completed between the ages of 13 and 18, at which time the epiphyseal plates of long bones close, allowing no further growth.
Being overweight is having more body fat than is optimally healthy. Being overweight is especially common where food supplies are plentiful and lifestyles are sedentary.
Sarcopenic obesity is a combination of two disease states, sarcopenia and obesity. Sarcopenia is the muscle mass/strength/physical function loss associated with increased age, and obesity is based off a weight to height ratio or body mass index (BMI) that is characterized by high body fat or being overweight.
Female body shape or female figure is the cumulative product of a woman's bone structure along with the distribution of muscle and fat on the body.
Android fat distribution describes the distribution of human adipose tissue mainly around the trunk and upper body, in areas such as the abdomen, chest, shoulder and nape of the neck. This pattern may lead to an "triangle"-shaped body or central obesity, and is more common in males than in females. Thus, the android fat distribution of men is about 48.6%, which is 10.3% higher than that of premenopausal women. In other cases, an ovoid shape forms, which does not differentiate between men and women. Generally, during early adulthood, females tend to have a more peripheral fat distribution such that their fat is evenly distributed over their body. However, it has been found that as females age, bear children and approach menopause, this distribution shifts towards the android pattern of fat distribution, resulting in a 42.1% increase in android body fat distribution in postmenopausal women. This could potentially provide evolutionary advantages such as lowering a woman's center of gravity making her more stable when carrying offspring.
The body adiposity index (BAI) is a method of estimating the amount of body fat in humans. The BAI is calculated without using body weight, unlike the body mass index (BMI). Instead, it uses the size of the hips compared to the person's height.
The waist-to-height ratio is the waist circumference divided by body height, both measured in the same units.
A number of lifestyle factors are known to be important to the development of type 2 diabetes including: obesity, physical activity, diet, stress, and urbanization. Excess body fat underlies 64% of cases of diabetes in men and 77% of cases in women. A number of dietary factors such as sugar sweetened drinks and the type of fat in the diet appear to play a role.
TOFI (thin-outside-fat-inside) is used to describe lean individuals with a disproportionate amount of fat stored in their abdomen. The figure to illustrate this shows two men, both 35 years old, with a BMI of 25 kg/m2. Despite their similar size, the TOFI had 5.86 litres of internal fat, whilst the healthy control had only 1.65 litres.
A Body Shape Index (ABSI) or simply body shape index (BSI) is a metric for assessing the health implications of a given human body height, mass and waist circumference (WC). The inclusion of WC is believed to make the BSI a better indicator of risk of mortality from excess weight than the standard body mass index. ABSI correlates only slightly with height, weight and BMI, indicating that it is independent of other anthropometric variables in predicting mortality.
Normal weight obesity is the condition of having normal body weight, but with a high body fat percentage, leading to some of the same health risks as obesity.
Relative Fat Mass (RFM) is a simple formula for the estimation of overweight or obesity in humans that requires only a calculation based on a ratio of height and waist measurements.
Body roundness index (BRI) is a calculated geometric index used to quantify an aspect of a person's individual body shape. Based on the principle of body eccentricity, it provides a rapid visual and anthropometric tool for health evaluation.
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