Obesity is a condition in which the natural energy reserve of a mammal (such as a human), which is stored in fat, is expanded far beyond usual levels to the point where it causes health stress. Obesity in wild animals is relatively rare, but it is common in domestic animals like barrows and household pets who may be castrated, overfed and underexercised.
Obesity is a concept that is being continually redefined. In humans, the most common statistical measurement of obesity is the body mass index (BMI).
A person with a BMI over 25.0 kg/m2 is considered overweight; a BMI over 30.0 kg/m2 is considered obese. A further threshold at 40.0 kg/m2 is identified as urgent morbidity risk. The American Institute for Cancer Research considers a BMI between 18.5 and 25 to be an ideal target for a healthy individual (although several sources consider a person with a BMI of less than 20 to be underweight). The BMI was created in the 19th century by the Belgian statistician Adolphe Quetelet. The cut-off points between categories are occasionally redefined, and may differ from country to country. In June 1998 the NIH brought official US category definitions into line with those used by the WHO, moving the American ‘overweight’ threshold from BMI 27 to BMI 25. About 30m Americans moved from “ideal” weight to being 1-10 pounds “overweight” as a result.
The BMI cannot offer a complete diagnosis, in that it ignores fat distribution within the body (see central obesity), and the relative fat-muscle-bone contributions to total body weight. A powerful athlete may be classified as obese by the BMI due to heavy musculature, while a false ‘normal’ may be diagnosed in the case of an elderly person with very low lean mass, which masks excess adiposity. On its own, a BMI score is therefore inadequate as a diagnostic tool. In practice, in most examples of overweight that may be harmful to health, both doctor and patient can see ‘by eye’ that fat is an issue. In these cases, BMI thresholds provide simple targets all patients can understand. Doctors may also use a simple measure of waist circumference (which is a better predictor of complications such insulin resistance due to visceral fat – see Janssen et al, 2004); the skinfold test, in which a pinch of skin is precisely measured to determine the thickness of the subcutaneous fat layer; or bioelectrical impedance analysis, usually only carried out at specialist clinics.
Such clinical data is rarely available in the statistical raw materials required for large public health studies, however – whereas height and weight is commonly recorded. For this essential reason, BMI remains the most commonly-used approach for public health studies, and the most useful for cross-border, longitudinal and other types of comparative analysis.
Obesity is generally a result of a combination of factors:
Although there is no definitive explanation for the recent epidemic of obesity, the evolutionary hypothesis comes closest to providing some understanding of this phenomenon. In times when food was scarce, the ability to take advantage of rare periods of abundance and use such abundance by storing energy efficiently was undoubtedly an evolutionary advantage. This is precisely the opposite of what is required in a sedentary society, where high-energy food is available in abundant quantities in the context of decreased exercise. Although many people may have a genetic propensity towards obesity, it is only with the reduction in physical activity and a move towards high-calorie diets of modern society that it has become widespread. Significant proportions (up to 30%) of the population in wealthy countries are now obese, and seen to be at risk of ill health (see e.g. Dr Joel Fuhrman.)
Eating disorders can lead to obesity, especially binge eating disorder (BED). As the name indicates, patients with this disorder are prone to overeat, often in binges. A proposed mechanism is that the eating serves to reduce anxiety, and some parallels with substance abuse can be drawn. An important additional factor is that BED patients often lack the ability to recognize hunger and satisfaction, something that is normally learnt in childhood. Learning theory suggests that early childhood conceptions may lead to an association between food and a calm mental state.
Some recent research has suggested that some human obesity may be caused by a viral infection. The virus adenovirus vectors AD-36 and AD-37 have been identified as a cause of obesity in animals and as potential stimulants on human preadipocytes (Vangipuram et al 2004). While these viruses occur in humans, there is no clear evidence that their presence leads to in increased risk of obesity.
While it is often quite obvious why a certain individual gets fat, it is far more difficult to understand why the average weight of certain societies have recently been growing. While genetic causes are central to who is obese, they cannot explain why one culture grows fatter than another.
This is most notable in the United States. In the years from just after the Second World War until 1960 the average person’s weight increased, but few were obese. In 1960 almost the entire population was well fed, but not overweight. In the two and a half decades since 1980 the growth in the rate of obesity has accelerated markedly and is increasingly becoming a public health concern.
Researchers from the US Centers of Disease Control and Prevention in Atlanta (Mokdad et al 2004) reported that approximately 400,000 US deaths annually were associated with poor diet and little exercise, and that if the trend continued, this would be 500,000 in 2005, overtaking smoking as the leading cause of death. These statistics are fiercely contested 
Canada and Europe are somewhat behind the United States, with the rest of the world mixed. Some nations like Egypt and Mexico have also suffered from greatly increasing rates of obesity.
There are a number of theories as to the cause of this change since 1980. Most believe it is a combination of various factors.
Interestingly the vast increase in the number of Americans who exercise and diet occurred before the increase in obesity, and some scholars have even argued that these trends actually encouraged obesity. Most diets fail, ending in binge eating and an overall increase in weight. Similarly those who workout but then stop can end up being fatter than those who never exercised.
Some obesity co-factors are resistant to the theory that the ‘epidemic’ is a new phenomenon. In particular, a class co-factor consistently appears across many studies. Comparing net worth with BMI scores, a 2002 study  (http://roa.sagepub.com/cgi/content/abstract/26/1/130) found obese subjects approximately half as wealthy as thin ones. When income differentials were factored out, the inequity persisted – thin subjects were inheriting more wealth than fat ones. Another study finds women who marry into higher status predictably thinner than women who married into lower status.