Obesity is a condition in which excess body fat has accumulated to such an extent that health may be negatively affected. It is commonly defined as a body mass index (BMI = weight divided by height squared) of 30 kg/m2 or higher. This distinguishes it from being overweight as defined by a BMI of between 25–29.9 kg/m2.

Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis. As a result, obesity has been found to reduce life expectancy. The primary treatment for obesity is dieting and physical exercise. If this fails, anti-obesity drugs and (in severe cases) bariatric surgery can be tried.

Obesity arises from too much energy intake compared with a person's basal metabolic rate and level of physical exercise. Excessive caloric intake and a lack of physical activity in genetically susceptible individuals is thought to explain most cases of obesity, with purely genetic, medical, or psychiatric illness contributing to only a limited number of cases. With rates of adult and childhood obesity increasing, authorities view it as a serious public health problem.

Although obesity is often stigmatized in the modern Western world, it has been perceived as a symbol of wealth and fertility at other times in history.

Classification

Obesity, in absolute terms, is an increase of body adipose tissue (fat tissue) mass. In a practical setting it is difficult to determine this directly. Therefore obesity is typically assessed by BMI (body mass index) and in terms of its distribution via the waist circumference. In addition, the presence of obesity needs to be evaluated in the context of other risk factors such as medical conditions that could influence the risk of complications.

BMI

Body mass index or BMI is a simple and widely used method for estimating body fat mass. BMI was developed in the 19th century by the Belgian statistician and anthropometrist Adolphe Quetelet. BMI is an accurate reflection of body fat percentage in the majority of the adult population. It is less accurate in people such as body builders and pregnant women in whom body composition is affected.

BMI is calculated by dividing the subject's weight by the square of his or her height, typically expressed either in metric or US "Customary" units:

Metric: BMI = kg / m2

where kg is the subject's weight in kilograms and m is the subject's height in metres.

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.
BMI Classification
Less than 18.5 underweight
18.5–24.9 normal weight
25.0–29.9 is overweight
30.0–34.9 is class I obesity
35.0–39.9 class II obesity
Over 40.0 class III obesity

The most commonly used definitions, established by the WHO in 1997 and published in 2000, provide the values listed in the table at right. Some modifications to the WHO definitions have been made by particular bodies:

* A BMI of 35.0 or higher in the presence of at least one other significant comorbidity is also classified by some bodies as class III obesity.
* For Asians, overweight is a BMI between 23 and 29.9 kg/m2 and obesity a BMI >30 kg/m2.

The surgical literature breaks down "class III" obesity into further catergories.

* Any BMI > 40 is severe obesity
* A BMI of 40.0–49.9 is morbid obesity
* A BMI of >50 is super obese

Effects on health

Mortality

Obesity is one of the leading preventable causes of death. Mortality risk varies with BMI; the lowest risk is found at a BMI of 22–24 kg/m2 and increases with changes in either direction. A BMI of over 32 is associated with a doubling of risk of death[21] and obesity is estimated to cause an excess 111,909 to 365,000 death per year in the United States. Obesity on average reduces life expectancy by 6–7 years. Severe obesity (BMIs >40) reduces life expectancy by 20 years for men and 5 years for women.

Morbidity

A large number of physical and mental conditions have been associated with obesity. 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). 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.

Central obesity, characterized by its high waist to hip ratio, is an important risk for metabolic syndrome. Metabolic syndrome is a combination of medical disorders which often includes diabetes mellitus type 2, high blood pressure, high blood cholesterol, and triglyceride levels.

Obesity is related to a variety of other complications. Some of these are directly caused by obesity and others are indirectly related through mechanisms sharing a common cause such as 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 weight is behind 64% of cases of diabetes in men and 77% in women.

Causes

Most researchers agree that a combination of excessive calorie consumption and a sedentary lifestyle are the primary causes of obesity. In a minority of cases, increased food consumption can be attributed to genetic, medical, or psychiatric illness. Generally however the rising prevalence of obesity is attributed to the availability of an easily accessible and palatable diet, car culture and mechanized manufacturing. A 2006 review identifies ten other possible contributors to the recent increase of obesity: (1) insufficient sleep, (2) endocrine disruptors—food substances that interfere with lipid metabolism, (3) decreased variability in ambient temperature, (4) decreased rates of smoking as smoking suppresses appetite, (5) increased use of medication that leads to weight gain, (6) increased distribution of ethnic and age groups that tend to be heavier, (7) pregnancy at a later age, (8) intrauterine and intergenerational effects, (9) positive natural selection of people with a higher BMI, (10) assortative mating, heavier people tending to form relationships with each other.

Diet

Despite the widespread availability of nutritional information in schools, doctors' offices, on the internet and on product packaging, it is evident that overeating remains a substantial problem. In the period of 1971–2000, obesity rates in the United States increased from 14.5% to 30.9%. During the same time period, an increase occurred in the average amount of calories consumed. For women, the average increase was 335 calories per day (1542 calories in 1971 and 1877 calories in 2004), while for men the average increase was 168 calories per day (2450 calories in 1971 and 2618 calories in 2004). Most of these extra calories came from an increase in carbohydrate consumption rather than an increase in fat consumption.

The primary sources of these extra carbohydrates are sweetened beverages, which now accounts for almost 25 percent of daily calories in young adults. Dietary trends have changed with reliance on energy-dense fast-food meals tripling between 1977 and 1995, and calorie intake from fast food quadrupling over the same period. In the early 1980s, the administration of Ronald Reagan lifted regulations limiting the advertising of sweets and fast food to children, and advertisement of these products directed towards children has increased. Agricultural policy and techniques in the United States and Europe have led to lower food prices. In the United States, subsidization of corn, soy, wheat, and rice through the U.S. farm bill has made the main sources of processed food relatively cheap compared to fruits and vegetables.

There is little evidence to support the commonly expressed view that some obese people eat little yet gain weight due to a slow metabolism. What has been found, however, is that some obese people underreport how much food they consume compared to those of normal weight.

Sedentary lifestyle

A sedentary lifestyle plays a significant role in obesity. Obese people are less active than those of normal weight. For example in Canada, 27.0% of sedentary men are obese as opposed to 19.6% of active men.[53] Normal weight people are more fidgety then their obese conterparts; this relationship is maintained even if normal weight people eat more or the obese person loses weight.

In 2000 the CDC estimated that more than 40% of the US population was sedentary, another 30% was active but not sufficiently and less than 30% had an adequate level of physical activity. There has been a trend toward decreased physical activity in part due to increasingly mechanized forms of work, changing modes of transportation, and increasing urbanization.

A study from China found urbanization reduces daily energy expenditure by about 300–400 kcal and going to work by car or bus reduced it by a further 200 kcal. Obesity rates have increased in relation to expanding suburbs. This has been attributed to increased time spent commuting, leading to less exercise and less meal preparation at home. Driving one's children to school has become increasingly popular. In the USA the proportion of children who walk or bike to school declined between 1969 (42%) and 2001 (16%) resulting in less exercise. Studies in children and adults have found an association between the number of hours of television watched and the prevalence of obesity.

Genetics

Like many other medical conditions, obesity is the result of an interplay between genetic and environmental factors. Polymorphisms in various genes controlling appetite and metabolism may predispose to obesity when sufficient calories are present. Obesity is a major feature in a number of rare genetic conditions: Prader-Willi syndrome, Bardet-Biedl syndrome, MOMO syndrome, leptin receptor mutations, congenital leptin deficiency, and melanocortin receptor mutations. In a people with early-onset severe obesity (defined by an onset before ten years of age and body mass index over three standard deviations above normal), 7% harbor a single locus mutation.

Apart from the above syndromes, an association has been found between an FTO gene polymorphism and weight. The adults in the study who were homozygous for this allele weighed about 3 kilograms more and had a 1.6-fold greater rate of obesity than those who had not inherited this trait. The association disappeared, though, when those with FTO polymorphisms participated in moderately intensive physical activity equivalent to 3 to 4 hours of brisk walking. One study found that 80% of the offspring of two obese parents were obese, in contrast to less then 10% of the offspring of two parents who were of normal weight.

The percentage of obesity that can be attributed to genetics varies from 6% to 85% depending on the population examined. The thrifty gene hypothesis postulates that certain ethnic groups may be more prone to obesity in an equivalent environment. Their ability to take advantage of rare periods of abundance by storing energy as fat would be advantageous during times of varying food availability, and individuals with greater adipose reserves would be more likely survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies. This is the presumed reason that Pima Indians, who evolved in a desert ecosystem, developed some of the highest rates of obesity when exposed to a Western lifestyle.

Medical and psychiatric illness

Certain physical and mental illnesses and the pharmaceutical substances used to treat them can increase risk of obesity. Medical illnesses that increase obesity risk include several rare genetic syndromes (listed above) as well as some congenital or acquired conditions: hypothyroidism, Cushing's syndrome, growth hormone deficiency, and the eating disorders: binge eating disorder and night eating syndrome. However, obesity is not regarded as a psychiatric disorder, and therefore is not listed in the DSM-IVR as a psychiatric illness.

Certain medications may cause weight gain or changes in body composition; these include insulin, sulfonylureas, thiazolidinediones, atypical antipsychotics, antidepressants, steroids, sulfonylureas, certain anticonvulsants (phenytoin and valproate), pizotifen, and some forms of hormonal contraception.

Socioeconomic

While genetic influences are important to understanding obesity, they cannot explain the current dramatic increase seen within specific countries or globally. Though it is accepted that calorie consumption in excess of calorie expenditure leads to obesity on an individual basis, the cause of the shifts in these two factors on the societal scale is much debated. There are a number of theories as to the cause but most believe it is a combination of various factors.

The correlation between social class and BMI varies globally. A review in 1989 found that in developed countries women of a high social class were less likely to be obese. No significant differences were seen among men of different social classes. In the developing world, women, men, and children from high social classes had greater rates of obesity. An update of this review carried out in 2007 found the same relationships, but they were weaker. The decrease in strength of correlation was felt to be due to the effects of globalization.

Many explanations have been put forth for associations between BMI and social class. It is thought that in developed countries, the wealthy are able to afford more nutritious food, they are under greater social pressure to remain slim, and have more opportunities along with greater expectations for physical fitness. In the developing world the ability to afford food, high energy expenditure with physical labor, and cultural values favoring a larger body size are believed to contribute to the observed patterns. Attitudes toward body mass held by people in one's life may also play a role in obesity. A correlation in BMI changes over time has been found between friends, siblings, and spouses.

Smoking has a significant effect on a individual's weight. Those who quit smoking gain an average of 4.4 kilograms for men and 5.0 kg for women over ten years. Changing rates of smoking however have had little effect on the overall rates of obesity.

Neurobiological mechanisms

Flier summarizes the many possible pathophysiological mechanisms involved in the development and maintenance of obesity. This field of research had been almost unapproached until leptin was discovered in 1994. Since this discovery, many other hormonal mechanisms have been elucidated that participate in the regulation of appetite and food intake, storage patterns of adipose tissue, and development of insulin resistance. Since leptin's discovery, ghrelin, orexin, PYY 3-36, cholecystokinin, adiponectin, as well as many other mediators have been studied. The adipokines are mediators produced by adipose tissue; their action is thought to modify many obesity-related diseases.

Leptin and ghrelin are considered to be complementary in their influence on appetite, with ghrelin produced by the stomach modulating short-term appetitive control (i.e. to eat when the stomach is empty and to stop when the stomach is stretched). Leptin is produced by adipose tissue to signal fat storage reserves in the body, and mediates long-term appetitive controls (i.e. to eat more when fat storages are low and less when fat storages are high). Although administration of leptin may be effective in a small subset of obese individuals who are leptin deficient, most obese individuals are thought to be leptin resistant and have been found to have high levels of leptin. This resistance is thought to explain in part why administration of leptin has not been shown to be effective in suppressing appetite in most obese subjects.

While leptin and ghrelin are produced peripherally, they control appetite through their actions on the central nervous system. In particular, they and other appetite-related hormones act on the hypothalamus, a region of the brain central to the regulation of food intake and energy expenditure. There are several circuits within the hypothalamus that contribute to its role in integrating appetite, the melanocortin pathway being the most well understood. The circuit begins with an area of the hypothalamus, the arcuate nucleus, that has outputs to the lateral hypothalamus (LH) and ventromedial hypothalamus (VMH), the brain's feeding and satiety centers, respectively.

The arcuate nucleus contains two distinct groups of neurons. The first group coexpresses neuropeptide Y (NPY) and agouti-related peptide (AgRP) and has stimulatory inputs to the LH and inhibitory inputs to the VMH. The second group coexpresses pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) and has stimulatory inputs to the VMH and inhibitory inputs to the LH. Consequently, NPY/AgRP neurons stimulate feeding and inhibit satiety, while POMC/CART neurons stimulate satiety and inhibit feeding. Both groups of arcuate nucleus neurons are regulated in part by leptin. Leptin inhibits the NPY/AgRP group while stimulating the POMC/CART group. Thus a deficiency in leptin signaling, either via leptin deficiency or leptin resistance, leads to overfeeding and may account for some genetic and acquired forms of obesity.

Management

The main treatment for obesity consists of dieting and physical exercise. Diet programs may produce weight loss over the short term, but keeping this weight off can be a problem. It often requires making exercise and a lower calorie diet a permanent part of a person's lifestyle. In the general population only 20% are successful at long-term weight loss maintenance. In a more structured setting, however, 67% of people who lost greater then 10% of their body mass maintained or continued to lose weight one year later. An average maintained weight loss of more then 3 kg or 3% of total body mass could be sustained for five years. There are significant benefits to weight loss. In a prospective study, intentional weight loss of any amount was associated with a 20% reduction in all-cause mortality.

Diet

Diets to promote weight loss are generally divided into four categories: low-fat, low-carbohydrate, low-calorie, and very low calorie. A meta-analysis of six randomized controlled trials found no difference between the main diet types (low calorie, low carbohydrate, and low fat), with a 2–4 kilogram weight loss in all studies.

Low-fat diets

Low-fat diets involve the reduction of the percentage of fat in one's diet. Calorie consumption is reduced but not purposely so. Diets of this type include NCEP Step I and II. A meta-analysis of 16 trials of 2–12 months' duration found that low-fat diets resulted in weight loss of 3.2 kg over eating as normal.

Low-carbohydrate diets

Low carbohydrate diets such as Atkins and Protein Power are relatively high in fat and protein. They are very popular in the press but are not recommended by the American Heart Association. A review of 94 trials found that weight loss was associated with decreased calorie consumption rather than any special properties of reduced carbohydrate consumption. No adverse affect from low carbohydrate diets were detected.

Low-calorie diets

Low-calorie diets usually produce an energy deficit of 500–1000 calories per day, which can result in a 0.5 kilogram weight loss per week. They include the DASH diet and Weight Watchers among others. The National Institutes of Health reviewed 34 randomized controlled trials to determine the effectiveness of low-calorie diets. They found that these diet lowered total body mass by 8% over 3–12 months.

Very low-calorie diets

Very low calorie diets provide 200–800 kcal/day while maintaining protein intake and limiting calories from both fat and carbohydrates. They subject the body to starvation and produce an average weekly weight loss of 1.5–2.5 kilograms. These diets are not recommended for general use as they are associated with adverse side effects such as loss of lean muscle mass, increased risks of gout, and electrolyte imbalances. People attempting these diets must be monitored closely by a physician to prevent complications.

Exercise

With use, muscles consume energy derived from both fat and glycogen. Due to the large size of leg muscles, walking, running, and cycling are the most effective means of exercise to reduce body fat.

A meta-analysis of 43 randomized controlled trials by the Cochrane Collaboration found that exercising alone led to limited weight loss. In combination with diet, however, it resulted in a 1 kilogram weight loss over dieting alone. A 1.5-kilogram loss was observed with a greater degree of exercise. Even though exercise as carried out in the general population has only modest effects, a dose response curve is found, and very intense exercise can lead to substantial weight loss. During 20 weeks of basic military training with no dietary restriction, obese military recruits lost 12.5 kg.

Medication

There are two commonly prescribed medications for obesity. One is orlistat, which reduces intestinal fat absorption by inhibiting pancreatic lipase; the other is sibutramine, which is a specific inhibitor of the neurotransmitters norepinephrine, serotonin, and dopamine in the brain (very similar to some anti-depressants), therefore decreasing appetite. Rimonabant, a third drug, works via a specific blockade of the endocannabinoid system. It has been approved in Europe for the treatment of obesity but has not yet received approval in the United States or Canada due to safety concerns. Weight loss with these drugs is modest; over the longer term, average weight loss on orlistat is 2.9 kg, sibutramine is 4.2 kg and rimonabant is 4.7 kg. Orlistat and rimonabant lead to a reduced incidence of diabetes, and all drugs have some effect on cholesterol. There is little data on how these drugs affect the longer-term complications of obesity. It is common for weight loss drugs to be tried and if there is little or no benefit from them to discontinue treatment. A meta-analysis of randomized controlled trials by the Cochrane Collaboration concluded that in diabetic patients fluoxetine (Prozac), orlistat and sibutramine could achieve modest but significant weight loss over 12–57 weeks. The long-term health benefits remained unclear.

Obesity may also influence the choice of drugs used to treat diabetes. Metformin may lead to mild weight loss in comparison to sulfonylureas and insulin. It has been show to reduce the risk of cardiovascular disease in obese type 2 diabetics.[89] The thiazolidinediones, on the other hand, may cause weight gain, but decrease central obesity and therefore can be used in obese diabetics. Ephedrine (Ma Huang) is a stimulant effective for weight loss; however it is not recommended due to potential side effects.

Clinical protocols

In a clinical practice guideline by the American College of Physicians, 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. For more severe cases of obesity, stronger drugs such as amphetamine and methamphetamine may be used on a selective basis. Evidence is not sufficient to recommend sertraline, topiramate, or zonisamide.
4. In patients with BMI over 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 frequently perform these procedures have fewer complications.

A clinical practice guideline by the US Preventive Services Task Force (USPSTF) concluded that the evidence is insufficient to recommend for or against routine behavioral counseling to promote a healthy diet in unselected patients in primary care settings, but that intensive behavioral dietary counseling is recommended in those with hyperlipidemia and other known risk factors for cardiovascular and diet-related chronic disease. Intensive counseling can be delivered by primary care clinicians or by referral to other specialists, such as nutritionists or dietitians. Source: Wikipedia