Obesity is a medical 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. 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.

Obesity is associated with many diseases, particularly heart disease, type 2 diabetes, breathing difficulties during sleep, certain types of cancer, and osteoarthritis. 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.

Silhouettes and waist circumferences representing normal, overweight, and obese.

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. 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.

BMI

Body mass index or BMI is a simple and widely used method for estimating body fat mass. BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet. 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. 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 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

The most commonly used definitions, established by the World Health Organization (WHO) in 1997 and published in 2000, provide the values:

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. 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%.

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). 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.

Management

The main treatment for obesity consists of dieting and physical exercise. Diet programs may produce weight loss over the short term, but keeping this weight off can be a problem and often requires making exercise and a lower calorie diet a permanent part of a person's lifestyle. Success rates of long-term weight loss maintenance are low and range from 2–20%. In a more structured setting, however, 67% of people who lost greater than 10% of their body mass maintained or continued to lose weight one year later. An average maintained weight loss of more than 3 kg (6.6 lb) or 3% of total body mass could be sustained for five years.[147] Some studies have found significant benefits in mortality in certain populations. In a prospective study of obese women with weight related diseases, intentional weight loss of any amount was associated with a 20% reduction in mortality. In obese women without obesity related illnesses a weight loss of greater than 9 kg ( 20 lb ) was associated with a 25% reduction in mortality. A recent review however concluded that ”benefits of weight loss on all cause mortality for the overweight and obese is meagre.” Benefits of weight loss for certain subgroups however is well supported by evidence such as in people with type 2 diabetes, women, and those with severe obesity.

The most effective, but also most risky treatment for obesity is bariatric surgery. Due to its cost and risk of complications, researchers are fervently searching for new obesity treatments.

Medication

Only two anti-obesity medications are currently approved by the FDA for long term use. One is orlistat (Xenical), which reduces intestinal fat absorption by inhibiting pancreatic lipase; the other is sibutramine (Meridia), which acts in the brain to inhibit deactivation of the neurotransmitters norepinephrine, serotonin, and dopamine (very similar to some anti-depressants), therefore decreasing appetite. Rimonabant (Acomplia), a third drug, works via a specific blockade of the endocannabinoid system. It has been developed from the knowledge that cannabis smokers often experience hunger, which is often referred to as "the munchies".

Weight loss with these drugs is modest; over the longer term, average weight loss on orlistat is 2.9 kg (6.4 lb), sibutramine is 4.2 kg (9.3 lb) and rimonabant is 4.7 kg (10.4 lb). Orlistat and rimonabant lead to a reduced incidence of diabetes, and all three drugs have some effect on cholesterol. There is however little data on how these drugs affect the longer-term complications or outcomes of obesity.