In its simplest expression, obesity is a physical condition where the amount of fat stored within the cells of the body significantly impairs the overall health of that individual. Obese persons are visibly overweight, with their physical ability to move without strain significantly reduced. Obesity is a designation that is a part of a body mass continuum that begins with the term underweight, or skinny, continuing through ideal weight, overweight, obese, and an ultimate condition, morbid obesity. Obesity is a significant health problem throughout the industrialized world, and in many countries it has replaced smoking and the illnesses related to that habit as the most important societal health concern to be addressed, especially among young people.

Obesity has become so prevalent that, in the last 30 years, the study of obesity and its causes has evolved into a distinct medical specialty, bariatrics, a term derived from the Greek word baros, meaning weight.

There is no unanimous agreement as to how obesity should be defined in terms of the physical size, the percentage of body fat, or physical capabilities of a person. A common tool used in the assessment of obesity is the formula known as the body mass index (BMI). The BMI was developed as a method to assist in the determination of a particular person's weight relative to a range of ideal weights, calculated in reference to height. BMI is determined by dividing the weight of the person by their height squared. Subject to a number of physical variables, BMI defines the positions on the index using the following reference points: ideal weight, 18.5-25 BMI; overweight, greater than 25 BMI; obese, greater than 30 BMI; and morbidly obese, greater than 35 BMI.

The BMI definitions are subject to significant individual factors, as a person's body type and other genetic features may influence weight without necessarily creating the adverse health concerns of obesity. An example is the large, muscular athlete who will often possess a ratio of lean body mass to fat that is far higher than the average sedentary person; the BMI may otherwise categorize this athlete as overweight or obese. Conversely, a sedentary female adolescent may be tall and appear very slim, but due to the combination of an unhealthy diet and little exercise, she may possess a significant amount of body fat.

An alternative definition of obesity is determined through the determination of body fat percentage, the relationship between stored fat and overall body mass. Body fat can be measured with some degree of precision, using tools such as skin calipers, to measure skin folds at the places on the body where fat-storing adipose tissue are most concentrated, such as the chest, abdomen, triceps, and upper thighs. Water displacement machines can also accurately determine body fat percentage. While there is no strict consensus in the medical community on what amount of body fat will render a person obese, most definitions set 20% or greater as a baseline.

Whatever measure is employed as yardstick to determine whether a person is obese or is significantly overweight, the body and its ability to function are severely compromised, both in the immediate term and with respect to the longevity of the person. The most common health problems that arise as a direct result of obesity are cardiovascular disease, diabetes, and various forms of osteoarthritis.

Cardiovascular function is the first casualty of a body whose weight is far in excess of the ideal. The heart is required to pump blood through an oversized structure, which places significant stress on overall cardiac function. The typical fat-rich diet of the obese person leads to the creation of low-density lipoproteins (LDLs), a form of cholesterol that forms plaque within the blood vessels, resulting in narrower arterial passages and a greater risk of stroke.

The incidence of diabetes in the Western world has risen by over 50% since 1970; the prevalence of juvenile diabetes has climbed even more dramatically. Diabetes is generally caused through the inability of the body to generate insulin, the hormone produced by the pancreas that regulates the glucose levels within the bloodstream. The onset of diabetes is often due to a combination of factors, with obesity and a sedentary lifestyle chief contributors.

Osteoarthritis and other diseases of the joints and the connective tissues are a common result of obesity. The mechanisms are very simple—if the body is genetically designed to support a particular mass, the body will eventually break down as it is not be able to adequately support the excess weight.

The health risks and the limitations of obesity are not restricted to the observable examples. Bariatric science has established that the overall health risks associated with being persistently overweight, if not clinically obese, are far higher than those for the balance of the population. Numerous medical studies, including those of the American Medical Association, confirm that persons who were overweight through their middle age (approximately 40 years to 60 years of age), were at a significantly greater risk of dying from heart disease or diabetes by age 65.

The obvious cure to obesity is to restrict the number of calories consumed and to adhere to an exercise program. As obesity so often carries with it a host of psychological and physical health problems, many obese people undergo bariatrics surgery, which involves either restricting the size of the stomach to limit intake through the insertion of a gastric band, or physically stapling the stomach to create a smaller-sized organ.

SEE ALSO Diet; Health; Juvenile obesity; Obesity and insulin resistance; Weight gain.