Causes

At an individual level, a combination of excessive caloric intake, lack of physical activity, and genetic susceptibility is thought to explain most cases of obesity, with a limited number of cases due solely to genetics, medical reasons, or psychiatric illness.[72] In contrast at a societal level increasing rates of obesity are felt to be due to an easily accessible and palatable diet,[73] increased reliance on cars, and mechanized manufacturing.[74][75]
A 2006 review identified ten other possible contributors to the recent increase of obesity: (1) insufficient sleep,

(2) endocrine disruptors (environmental pollutants that interfere with lipid metabolism),

(3) decreased variability in ambient temperature,

(4) decreased rates of smoking, because smoking suppresses appetite,

(5) increased use of medications that can cause weight gain (e.g., atypical antipsychotics),

(6) proportional increases in ethnic and age groups that tend to be heavier,

(7) pregnancy at a later age (which may cause susceptibility to obesity in children),

(8) epigenetic risk factors passed on generationally,

(9) natural selection for higher BMI, and

(10) assortative mating leading to increased concentration of obesity risk factors (this would not necessarily increase the number of obese people, but would increase the average population weight).

There is substantial but not conclusive evidence for these mechanisms, and the authors specify that they are probably less influential than the ones discussed in the previous paragraph (but still important).

Effects on health

Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis.[2] As a result, obesity has been found to reduce life expectancy.[2]
Mortality
Obesity is one of the leading preventable causes of death worldwide.[6][35][36] Large scale American and European studies have found that mortality risk varies with BMI; the lowest risk is found at a BMI of 22.5–25 kg/m2[37] in non smokers and at a BMI of 24–27 kg/m2 in current smokers and increases with changes in either direction.[38][39] Obesity increases the risk of death in current and former smokers as well as in those who have never smoked.[39] A BMI of over 32 has been associated with a doubled mortality rate among women over a 16-year period[40] and obesity is estimated to cause an excess 111,909 to 365,000 death per year in the United States.[2][36] Obesity on average reduces life expectancy by six to seven years.[2][41] A BMI of 30–35 reduces life expectancy by two to four years[37] while severe obesity (BMI > 40) reduces life expectancy by 20 years for men and five years for women.[25]
Morbidity
See main article: Obesity associated morbidity
Obesity increases the risk of many physical and mental conditions. These comorbidities are reflected predominantly in metabolic syndrome.[2] Metabolic syndrome being a combination of medical disorders, which includes diabetes mellitus type 2, high blood pressure, high blood cholesterol, and high triglyceride levels.[42]
Complications are either directly caused by obesity or indirectly related through mechanisms sharing a common cause such as a poor diet or a sedentary lifestyle. The strength of the link between obesity and specific conditions varies. One of the strongest is the link with type 2 diabetes. Excess body fat underlies 64% of cases of diabetes in men and 77% of cases in women.[43]
Health consequences can be categorized by the effects of increased fat mass (osteoarthritis, obstructive sleep apnea, social stigmatization) or by the increased number of fat cells (diabetes, cancer, cardiovascular disease, non-alcoholic fatty liver disease).[2][44] Increases in body fat alter the body's response to insulin, potentially leading to insulin resistance. Increased fat also creates a proinflammatory state, increasing the risk of thrombosis.[44]

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 or to put it another way the 5% with the greatest BMI.[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] Treatments used in children are primarily lifestyle interventions and behavioral techniques. Medications are not FDA approved for use in this age group.[33]

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 Classification
< 18.5 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: 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]

Body Fat

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.[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]