Overweight and Obesity
Obesity is a mental condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy.[1][2] Body mass index (BMI), which compares weight and height, is used to define a person as overweight (pre-obese) when their BMI is between 25 kg/m2 and 30 kg/m2 and obese when it is greater than 30 kg/m2.[3]
Obesity is associated with many diseases, particularly heart disease, type 2 diabetes, breathing difficulties during sleep, certain types of cancer, and osteoarthritis.[2] Obesity is most commonly caused by a combination of excessive dietary calories, lack of physical activity, and genetic susceptibility, though a limited number of cases are due solely to genetics, medical reasons or psychiatric illness.
The primary treatment for obesity is dieting and physical exercise. If this fails, anti-obesity drugs may be taken to reduce appetite or inhibit fat absorption. In severe cases, surgery is performed or an intragastric balloon is placed to reduce stomach volume and or bowel length, leading to earlier satiation and reduced ability to absorb nutrients from food.[4][5]
Obesity is a leading preventable cause of death worldwide, with increasing prevalence in adults and children, and authorities view it as one of the most serious public health problems of the 21st century.[6] Obesity is stigmatized in the modern Western world, though it has been perceived as a symbol of wealth and fertility at other times in history, and still is in many parts of Africa.[2][7]
Classification
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse affect on health.[8] It is defined by body mass index (BMI) and further evaluated in terms of fat distribution via the waist–hip ratio and total cardiovascular risk factors.[9][10] BMI is highly related to both percentage body fat and total body fat.[1]
BMI
Body mass index or BMI is a simple and widely used method for estimating body fat mass.[12] BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet.[13] 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.[14] A formula combining BMI, age and gender can be used to estimate a person's body fat percentage to an accuracy of 4%.[15]
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: BMI = kilograms / meters2
US/Customary and imperial: BMI = lb * 703 / in2
where lb is the subject's weight in pounds and in is the subject's height in inches.
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.[16]
Some modifications to the WHO definitions have been made by particular bodies. The surgical literature breaks down "class III" obesity into further categories.[17]
- Any BMI > 40 is severe obesity
- A BMI of 40.0–49.9 is morbid obesity
- A BMI of >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[18] while China uses a BMI of greater than 28.[19]
Waist circumference and waist–hip ratio
The waist circumference (>102 cm in men and >88 cm in women) and 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 both used as measures of central obesity.[20]
In those with a BMI under 35, intra-abdominal body fat is related to negative health outcomes independent of total body fat.[21] Intra-abdominal or visceral fat has a particularly strong correlation with cardiovascular disease.[20] In a study of 15,000 people, waist circumference also correlated better with metabolic syndrome than BMI.[22] Women with abdominal obesity have a cardiovascular risk similar to that of men.[23] 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.[24]
Body fat percentage
Body fat percentage is total body fat expressed as a percentage of total body weight. It is generally agreed that men with more than 25% body fat and women with more than 33% body fat are obese.[25] Body fat percentage can be estimated from a person's BMI by the following formula:
Bodyfat% = (1.2 * BMI) + (0.23 * age) − 5.4 − (10.8 * gender)
where gender is 0 if female and 1 if male
This formula takes into account the fact that body fat percentage is 10 percentage points greater in women than in men for a given BMI. It recognizes that a person's percentage body fat increases as they age even if their weight remains constant. The results of this formula have an accuracy of 4%.[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).[21] 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
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.[31] 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.[32]
Childhood obesity has reached epidemic proportions in 21st century with rising rates in both the developed and developing world. 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.[33]
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.[34]
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.[35]
Treatments used in children are primarily lifestyle interventions and behavioral techniques. Medications are not FDA approved for use in this age group.[33]