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Obesity is a condition in which the natural energy reserve of a mammal (such as a human), which is stored in fat tissue, is expanded far beyond usual levels to the point where it impairs health. Obesity in wild animals is relatively rare, but it is common in domestic animals like pigs and household pets who may be overfed and underexercised.

what is obesity?

Obesity is a concept that is being continually redefined. In humans, the most common statistical estimate of obesity is the body mass index (BMI).

symptoms of obesity and diagnosis

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 National Institutes of Health 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 30,000,000 Americans moved from "ideal" weight to being 1–10 pounds (0.45–4.55 kg) "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.

causes of obesity

Obesity is generally a result of a combination of factors:

• Genetic predisposition
• Energy-rich diet
• Limited exercise and sedentary lifestyle
• Weight cycling, caused by repeated attempts to lose weight by dieting
• Underlying illness (e.g. hypothyroidism)
• An eating disorder (such as binge eating disorder)
• Stressful mentality (debated)
• Insufficient sleeping (debated)

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.

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.

societal causes 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.

There are a number of theories as to the cause of this change since 1980. Most believe it is a combination of various factors.

• One of the most important is the much lower relative cost of foodstuffs: massive agricultural subsidies in the United States and Europe have led to food prices for consumers being lower than at any point in history. Sugar and corn syrup, two huge sources of food energy, are some of the most subsidized products by the United States government.
• Increased marketing has also played a role. In the early 1980s the Reagan administration lifted most regulations pertaining to advertising to children. As a result, the number of commercials seen by the average child increased greatly, and a large proportion of these were for fast food and candy.
• Changes in the price of mineral oil and petrol are also believed to have had an effect, as unlike during the 1970s it is now affordable in the United States to drive everywhere - at a time when public transit goes underused. At the same time more areas have been built without sidewalks and parks.
• The changing workforce as each year a greater percent of the population spends their entire workday behind a desk or computer, seeing virtually no exercise. In the kitchen the microwave has seen sales of unhealthy frozen convenience foods skyrocket and has encouraged more elaborate snacking.
• A social cause that is believed by many to play a role is the increasing number of two income households where one parent no longer remains home to look after the house. This increases the number of restaurant and take-out meals.
• Urban sprawl may be a factor: obesity rates increase as urban sprawl increases, possibly due to less walking and less time for cooking (Lopez 2004).
• Since 1980 both sit-in and fast food restaurants have seen dramatic growth in terms of the number of outlets and customers served. Low food costs, and intense competition for market share, led to increased portion sizes - for example, McDonalds french fries portions rose from 200 calories (840 kilojoules) in 1960 to over 600 calories (2,500 kJ) today.
• Increased food production is a likely factor. The U.S. produces three times more food than U.S. citizens eat.
• Increasing affluence itself (including many of the above factors as accompaniments of affluence) may be a cause, or contributing factor since obesity tends to flourish as a disease of affluence in countries which are developing and becoming westernised. This is supported by a dip in American GDP after 1990, the year of the first Iraq war, followed by an exponential increase. USA obesity statistics followed the same pattern, offset by two years.

Interestingly an 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. Nearly all diets fail, with participants resuming their previous eating habits or even engaging in binge eating. Many then see an overall increase in their weight. If the diet is then repeated and abandoned again, a pattern of rising and falling weight is establish, known as weight cycling. Similarly those who workout but then stop can end up being heavier than those who never exercised.

Poverty and obesity

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 2004 study by Zagorsky found obese American 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 married into higher status predictably thinner than women who married into lower status. This may be to the availability and low price of fast food. Healthier food tends to be more expensive, whilst anyone can walk into a McDonalds or other fast food restaurant and purchase a full meal for under a dollar.

long term effects of the symptoms of obesity:

Obesity is correlated (in population studies) with a variety of complications. For many of these complaints, it has not been clearly established to what extent they are caused directly by obesity itself, or have some other cause (such as limited exercise) that causes obesity as well. Most confidence in a direct cause is given to the mechanical complications in the following list, compiled by the American Medical Association for general physicians:

• Cardiovascular: High blood pressure, congestive heart failure, enlarged heart and its associated arrhythmia and dizziness, cor pulmonale, varicose veins, pulmonary embolism and coronary artery disease
• Endocrine: Syndrome X, diabetes mellitus type 2, dyslipidemia (high blood cholesterol and triglyceride levels, generally in the form of combined hyperlipidemia), polycystic ovarian syndrome (PCOS), menstrual disorders and infertility
• Gastrointestinal: gastroesophageal reflux disease (GERD), fatty liver disease, cholelithiasis (gallstones), hernia and colorectal cancer
• Renal and genitourinary: urinary incontinence, glomerulopathy, hypogonadism (male), breast cancer (female), uterine cancer (female), stillbirth
• Integument (skin and appendages): stretch marks, acanthosis nigricans, lymphedema, cellulitis, carbuncles, intertrigo
• Musculoskeletal: hyperuricemia (which predisposes to gout), immobility, osteoarthritis, low back pain
• Neurologic: stroke, meralgia paresthetica, headache, carpal tunnel syndrome, dementia (Whitmer et al 2005)
• Respiratory: dyspnea, obstructive sleep apnea, hypoventilation syndrome, Pickwickian syndrome, asthma
• Psychological: Depression, low self esteem, body image disorder, social stigmatization

While being severely obese has many health ramifications, those who are somewhat overweight face little increased mortality or morbidity. Some studies suggest that the somewhat "overweight" tend to live longer than those at their "ideal" weight. Osteoporosis is known to occur less in slightly overweight people.

The mainstay of treatment for obesity is an energy-limited diet and increased exercise. Although adherence to this regimen can cure obesity, many patients are unable to make the required sacrifices. There might be an additional behavioral factor at the brain level "forbidding" obesity patients from losing too much weight.

In a clinical practice guideline by the American College of Physicians (Snow et al 2005), the following five recommendations are made:

1. People with a BMI of over 30 should be counseled on diet, exercise and other relevant behavioral interventions, and set a realistic goal for weight loss.

2. If these goals are not achieved, pharmacotherapy can be offered. The patient needs to be informed of the possibility of side-effects and the unavailability of long-term safety and efficacy data.

3. Drug therapy may consist of sibutramine, orlistat, phentermine, diethylpropion, fluoxetine and bupropion. Evidence is not sufficient to recommend sertraline, topiramate or zonisamide.

4. In patients with BMI > 40 who fail to achieve their weight loss goals (with or without medication) and who develop obesity-related complications, referral for bariatric surgery may be indicated. The patient needs to be aware of the potential complications.

5. Those requiring bariatric surgery should be referred to high-volume referral centers, as the evidence suggests that surgeons who perform these procedures frequently have fewer complications.

Much research focuses on new drugs to combat obesity, which is seen as the biggest health problem facing developed countries. Some nutritionists feel that these research funds would be better devoted to advice on good nutrition, healthy eating and promoting a more active lifestyle.

Medication most commonly prescribed for diet/exercise-resistant obesity is orlistat (Xenical®, reduced intestinal fat absorption by inhibiting pancreatic lipase) and sibutramine (Reductil, Medaria, an anorectic). In the presence of diabetes mellitus, there is evidence that the anti-diabetic drug metformin (Glucophage®) can assist in weight loss - rather than sulfonylurea derivatives and insulin, which often lead to further weight gain. The thiazolidinediones (rosiglitazone or pioglitazone) can cause slight weight gain, but decrease the "pathologic" form of abdominal fat, and are therefore often used in obese diabetics.

Increasingly, bariatric surgery is being used to limit stomach capacity (and thus food intake); this can happen laparoscopically. Ileal bypass reduces the length of the intestine and hence absorbing surface, but has more complications.