Obesity is a condition in which a mammal’s natural energy reserve, which is stored in fat, is stretched far beyond normal levels to the point where it causes health stress. Obesity in wild animals is relatively rare, but it is common in domestic animals such as castrates and pets that can be castrated, overfed and underexercised.
Obesity is a concept that is continually being redefined. In men, the most common statistical measure of obesity is body mass index (BMI).
A person with a BMI greater than 25.0 kg/m2 is considered overweight; a BMI greater than 30.0 kg/m2 is considered obese. Another threshold at 40.0 kg/m2 is identified as an 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 person (although many 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 sometimes redefined and may differ from one country to another. In June 1998, the NIH officially defined the U.S. category in accordance with those used by the WHO, moving the U.S. “overweight” cut-off point from BMI 27 to BMI 25. Approximately 30 million Americans moved from “ideal” weight to be 1-10 pounds “overweight” accordingly.
BMI cannot provide a complete diagnosis because it does not take into account the distribution of fat in the body and the relative contribution of fat muscle to total body weight. A powerful athlete may be classified as obese by the BMI with two heavy muscles, while a false “normal” may be diagnosed in the case of an elderly person with very low lean body mass, which masks excess adiposity. The BMI score alone is therefore insufficient as a diagnostic tool. In practice, in most examples of overweight that can be detrimental to health, both doctor and patient can see “in the eye” that fat is a problem. In these cases, BMI thresholds provide simple targets that all patients can understand. Physicians can also use a simple waist circumference measurement (which is a better predictor of complications such as insulin resistance to both visceral fat and visceral fat – see Janssen et al, 2004); the skin fold 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 performed in specialized clinics.
Such clinical data are rarely available in the statistical raw materials required for large public health studies, however – while height and weight are 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 comparative analyses.
Obesity is usually the result of a combination of factors:
Limited exercise and sedentary lifestyle
Underlying illness (e.g. hypothyroidism)
An eating disorder (such as an eating binge)
A stressful mentality (debated)
Although there is no definitive explanation for the recent obesity epidemic, the evolutionary hypothesis comes closest to understanding this phenomenon. At a time when food was scarce, the ability to take advantage of rare periods of abundance and to use such abundance by storing efficient energy was undoubtedly an evolutionary advantage. This is the exact opposite of what is required in a sedentary society where high energy food is available in abundance in the context of reduced exercise. Although many people may have a genetic propensity for obesity, it is only with the reduction of physical activity and a movement towards the high calorie diets of modern society, it has become widespread. Significant proportions (up to 30%) of the population in rich countries are now obese, and be considered at risk of poor health.
Eating disorders can lead to obesity, particularly overeating disorders (OED). As the name implies, patients with this disorder are prone to overeating, often in the binges. One proposed mechanism is that eating is used to reduce anxiety, and some parallels with substance abuse can be drawn. An important additional factor is that LGB patients often lack the ability to recognize hunger and satisfaction, which is normally learned in childhood. Learning theory suggests that early childhood concepts can lead to an association between food and a calm mental state.
Some recent research has suggested that human obesity may be caused by a viral infection. The adenovirus virus vectors AD-36 and AD-37 stati identified as a cause of obesity in animals and as potential stimulants on human pre-adipocytes (Vangipuram et al 2004). Although these viruses occur in humans, there is no clear evidence that their presence leads to an increased risk of obesity.