Obesity is a medical condition in which excess body fat has accumulated to an extent that it may have a negative effect on health.^[1] People are generally considered obese when their body mass index (BMI), a measurement obtained by dividing a person’s weight by the square of the person’s height—despite known allometric inaccuracies^[a]—is over 30 kg/m²; the range 25–30 kg/m² is defined as overweight.^[1] Some East Asian countries use lower values.^[10] Obesity is correlated with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis.^[2] High BMI is a marker of risk, but not proven to be a direct cause, for diseases caused by diet, physical activity, and environmental factors.^[11] A reciprocal link has been found between obesity and depression, with obesity increasing the risk of clinical depression and also depression leading to a higher chance of developing obesity.^[3]

Obesity has individual, socioeconomic, and environmental causes, including diet, physical activity, automation, urbanization, genetic susceptibility, medications, mental disorders, economic policies, endocrine disorders, and exposure to endocrine-disrupting chemicals.^{[1][4][12][13]} While a majority of obese individuals at any given time are attempting to lose weight and often successful, research shows that maintaining that weight loss over the long term proves to be rare.^[14] The reasons for weight cycling are not fully understood but may include decreased energy expenditure combined with increased biological urge to eat during and after caloric restriction.^[14] More studies are needed to determine if weight cycling and yo-yo dieting contribute to inflammation and disease risk in obese individuals.^[14]

Effects of Obesity on health

Excessive body weight is associated with various diseases and conditions, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, osteoarthritis,^[2] and asthma.^[2][36] As a result, obesity has been found to reduce life expectancy.^[2]

Mortality

Obesity is one of the leading preventable causes of death worldwide.^[38][39][40] A number of reviews have found that mortality risk is lowest at a BMI of 20–25 kg/m^{2[41][42][43]} in non-smokers and at 24–27 kg/m² in current smokers, with risk increasing along with changes in either direction.^[44][45] This appears to apply in at least four continents.^[43] In contrast, a 2013 review found that grade 1 obesity (BMI 30–35) was not associated with higher mortality than normal weight, and that overweight (BMI 25–30) was associated with “lower” mortality than was normal weight (BMI 18.5–25).^[46] Other evidence suggests that the association of BMI and waist circumference with mortality is U- or J-shaped, while the association between waist-to-hip ratio and waist-to-height ratio with mortality is more positive.^[47] In Asians the risk of negative health effects begins to increase between 22–25 kg/m².^[48] A BMI above 32 kg/m² has been associated with a doubled mortality rate among women over a 16-year period.^[49] In the United States, obesity is estimated to cause 111,909 to 365,000 deaths per year,^[2][40] while 1 million (7.7%) of deaths in Europe are attributed to excess weight.^[50][51] On average, obesity reduces life expectancy by six to seven years,^[2][52] a BMI of 30–35 kg/m² reduces life expectancy by two to four years,^[42] while severe obesity (BMI > 40 kg/m²) reduces life expectancy by ten years.^[42]

Morbidity

Obesity increases the risk of many physical and mental conditions. These comorbidities are most commonly shown in metabolic syndrome,^[2] a combination of medical disorders which includes: diabetes mellitus type 2, high blood pressure, high blood cholesterol, and high triglyceride levels.^[53]

Complications are either directly caused by obesity or indirectly related through mechanisms sharing a common cause such as a 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 body fat underlies 64% of cases of diabetes in men and 77% of cases in women.^[54]

Health consequences fall into two broad categories: those attributable to the effects of increased fat mass (such as osteoarthritis, obstructive sleep apnea, social stigmatization) and those due to the increased number of fat cells (diabetes, cancer, cardiovascular disease, non-alcoholic fatty liver disease).^[2][55] Increases in body fat alter the body’s response to insulin, potentially leading to insulin resistance. Increased fat also creates a proinflammatory state,^[56][57] and a prothrombotic state.^[55][58]

Obesity increases the risk of developing serious illness from coronavirus disease 2019.^[59]

Causes of Obesity

At an individual level, a combination of excessive food energy intake and a lack of physical activity is thought to explain most cases of obesity.^[88] A limited number of cases are due primarily to genetics, medical reasons, or psychiatric illness.^[13] In contrast, increasing rates of obesity at a societal level are felt to be due to an easily accessible and palatable diet,^[89] increased reliance on cars, and mechanized manufacturing.^[90][91]

A 2006 review identified ten other possible contributors to the recent increase of obesity: (1) insufficient sleep, (2) endocrine disruptors (environmental pollutants that interfere with lipid metabolism), (3) decreased variability in ambient temperature, (4) decreased rates of smoking, because smoking suppresses appetite, (5) increased use of medications that can cause weight gain (e.g., atypical antipsychotics), (6) proportional increases in ethnic and age groups that tend to be heavier, (7) pregnancy at a later age (which may cause susceptibility to obesity in children), (8) epigenetic risk factors passed on generationally, (9) natural selection for higher BMI, and (10) assortative mating leading to increased concentration of obesity risk factors (this would increase the number of obese people by increasing population variance in weight).^[92] According to the Endocrine Society, there is “growing evidence suggesting that obesity is a disorder of the energy homeostasis system, rather than simply arising from the passive accumulation of excess weight”.^[93]

Sedentary lifestyle

A sedentary lifestyle plays a significant role in obesity.^[114] Worldwide there has been a large shift towards less physically demanding work,^{[115][116][117]} and currently at least 30% of the world’s population gets insufficient exercise.^[116] This is primarily due to increasing use of mechanized transportation and a greater prevalence of labor-saving technology in the home.^{[115][116][117]} In children, there appear to be declines in levels of physical activity due to less walking and physical education.^[118] World trends in active leisure time physical activity are less clear. The World Health Organization indicates people worldwide are taking up less active recreational pursuits, while a study from Finland^[119] found an increase and a study from the United States found leisure-time physical activity has not changed significantly.^[120] A 2011 review of physical activity in children found that it may not be a significant contributor.^[121]

In both children and adults, there is an association between television viewing time and the risk of obesity.^{[122][123][124]} A review found 63 of 73 studies (86%) showed an increased rate of childhood obesity with increased media exposure, with rates increasing proportionally to time spent watching television.^[125]

Genetics

A 1680 painting by Juan Carreno de Miranda of a girl presumed to have Prader–Willi syndrome[126]

Like many other medical conditions, obesity is the result of an interplay between genetic and environmental factors.^[127] Polymorphisms in various genes controlling appetite and metabolism predispose to obesity when sufficient food energy is present. As of 2006, more than 41 of these sites on the human genome have been linked to the development of obesity when a favorable environment is present.^[128] People with two copies of the FTO gene (fat mass and obesity associated gene) have been found on average to weigh 3–4 kg more and have a 1.67-fold greater risk of obesity compared with those without the risk allele.^[129] The differences in BMI between people that are due to genetics varies depending on the population examined from 6% to 85%.^[130]

Obesity is a major feature in several syndromes, such as Prader–Willi syndrome, Bardet–Biedl syndrome, Cohen syndrome, and MOMO syndrome. (The term “non-syndromic obesity” is sometimes used to exclude these conditions.)^[131] In people with early-onset severe obesity (defined by an onset before 10 years of age and body mass index over three standard deviations above normal), 7% harbor a single point DNA mutation.^[132]

Studies that have focused on inheritance patterns rather than on specific genes have found that 80% of the offspring of two obese parents were also obese, in contrast to less than 10% of the offspring of two parents who were of normal weight.^[133] Different people exposed to the same environment have different risks of obesity due to their underlying genetics.^[134]

The thrifty gene hypothesis postulates that, due to dietary scarcity during human evolution, people are prone to obesity. 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 to survive famine. This tendency to store fat, however, would be maladaptive in societies with stable food supplies.^[135] This theory has received various criticisms, and other evolutionarily-based theories such as the drifty gene hypothesis and the thrifty phenotype hypothesis have also been proposed.^[136][137]

Other illnesses

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,^[138] and some eating disorders such as binge eating disorder and night eating syndrome.^[2] However, obesity is not regarded as a psychiatric disorder, and therefore is not listed in the DSM-IVR as a psychiatric illness.^[139] The risk of overweight and obesity is higher in patients with psychiatric disorders than in persons without psychiatric disorders.^[140]

Certain medications may cause weight gain or changes in body composition; these include insulin, sulfonylureas, thiazolidinediones, atypical antipsychotics, antidepressants, steroids, certain anticonvulsants (phenytoin and valproate), pizotifen, and some forms of hormonal contraception.^[2]

Social determinants

The disease scroll (Yamai no soshi, late 12th century) depicts a woman moneylender with obesity, considered a disease of the rich.

Obesity in developed countries is correlated with economic inequality

While genetic influences are important to understanding obesity, they cannot explain the current dramatic increase seen within specific countries or globally.^[141] Though it is accepted that energy consumption in excess of energy 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.^[142] 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.^[143] Among developed countries, levels of adult obesity, and percentage of teenage children who are overweight, are correlated with income inequality. A similar relationship is seen among US states: more adults, even in higher social classes, are obese in more unequal states.^[144]

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 undeveloped countries 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.^[143] Attitudes toward body weight held by people in one’s life may also play a role in obesity. A correlation in BMI changes over time has been found among friends, siblings, and spouses.^[145] Stress and perceived low social status appear to increase risk of obesity.^{[144][146][147]}

Smoking has a significant effect on an individual’s weight. Those who quit smoking gain an average of 4.4 kilograms (9.7 lb) for men and 5.0 kilograms (11.0 lb) for women over ten years.^[148] However, changing rates of smoking have had little effect on the overall rates of obesity.^[149]

In the United States the number of children a person has is related to their risk of obesity. A woman’s risk increases by 7% per child, while a man’s risk increases by 4% per child.^[150] This could be partly explained by the fact that having dependent children decreases physical activity in Western parents.^[151]

In the developing world urbanization is playing a role in increasing rate of obesity. In China overall rates of obesity are below 5%; however, in some cities rates of obesity are greater than 20%.^[152]

Malnutrition in early life is believed to play a role in the rising rates of obesity in the developing world.^[153] Endocrine changes that occur during periods of malnutrition may promote the storage of fat once more food energy becomes available.^[153]

Consistent with cognitive epidemiological data, numerous studies confirm that obesity is associated with cognitive deficits.^[154][155]

Whether obesity causes cognitive deficits, or vice versa is unclear at present.

Public health

Obesity prevention requires a complex approach, including interventions at community, family, and individual levels.^[1][11] Changes to diet and exercising are the main treatments recommended by health professionals.^[2] Diet quality can be improved by reducing the consumption of energy-dense foods, such as those high in fat or sugars, and by increasing the intake of dietary fiber.^[1] However, large-scale analyses have found an inverse relationship between energy density and energy cost of foods in developed nations.^[15] Low-income populations are more likely to live in neighborhoods that are considered “food deserts” or “food swamps” where nutritional groceries are less available.^[16] Medications can be used, along with a suitable diet, to reduce appetite or decrease fat absorption.^[5] If diet, exercise, and medication are not effective, a gastric balloon or surgery may be performed to reduce stomach volume or length of the intestines, leading to feeling full earlier or a reduced ability to absorb nutrients from food.^[6][17]

Obesity is a leading preventable cause of death worldwide, with increasing rates in adults and children.^[1][18] In 2015, 600 million adults (12%) and 100 million children were obese in 195 countries.^[7] Obesity is more common in women than in men.^[1] Authorities view it as one of the most serious public health problems of the 21st century.^[19] Obesity is stigmatized in much of the modern world (particularly in the Western world), though it was seen as a symbol of wealth and fertility at other times in history and still is in some parts of the world.^[2][20] In 2013, several medical societies, including the American Medical Association and the American Heart Association, classified obesity as a disease.^[21][22][23]

The World Health Organization (WHO) predicts that overweight and obesity may soon replace more traditional public health concerns such as undernutrition and infectious diseases as the most significant cause of poor health.^[169] Obesity is a public health and policy problem because of its prevalence, costs, and health effects.^[170] The United States Preventive Services Task Force recommends screening for all adults followed by behavioral interventions in those who are obese.^[171] Public health efforts seek to understand and correct the environmental factors responsible for the increasing prevalence of obesity in the population. Solutions look at changing the factors that cause excess food energy consumption and inhibit physical activity. Efforts include federally reimbursed meal programs in schools, limiting direct junk food marketing to children,^[172] and decreasing access to sugar-sweetened beverages in schools.^[173] The World Health Organization recommends the taxing of sugary drinks.^[174] When constructing urban environments, efforts have been made to increase access to parks and to develop pedestrian routes.^[175] There is low quality evidence that nutritional labelling with energy information on menus can help to reduce energy intake while dining in restaurants.^[176]

Management of Obesity

The main treatment for obesity consists of weight loss via calorie restricted dieting and physical exercise.^{[21][88][183][184]} Dieting, as part of a lifestyle change, produces sustained weight loss, despite slow weight regain over time.^{[21][185][186][187]} Although 87% of participants in the National Weight Control Registry were able to maintain 10% body weight loss for 10 years,^[188] the most appropriate dietary approach for long term weight loss maintenance is still unknown.^[189] Intensive behavioral interventions combining both dietary changes and exercise are recommended.^{[21][183][190]} Intermittent fasting has no additional benefit of weight loss compared to continuous energy restriction.^[189] Adherence is a more important factor in weight loss success than whatever kind of diet an individual undertakes.^[189][191]

Several hypo-caloric diets are effective.^[21] In the short-term low carbohydrate diets appear better than low fat diets for weight loss.^[192] In the long term, however, all types of low-carbohydrate and low-fat diets appear equally beneficial.^[192][193] A 2014 review found that the heart disease and diabetes risks associated with different diets appear to be similar.^[194] Promotion of the Mediterranean diets among the obese may lower the risk of heart disease.^[192] Decreased intake of sweet drinks is also related to weight-loss.^[192] Success rates of long-term weight loss maintenance with lifestyle changes are low, ranging from 2–20%.^[195] Dietary and lifestyle changes are effective in limiting excessive weight gain in pregnancy and improve outcomes for both the mother and the child.^[196] Intensive behavioral counseling is recommended in those who are both obese and have other risk factors for heart disease.^[197]

Medical interventions

The most effective treatment for obesity is bariatric surgery.^[6][21] The types of procedures include laparoscopic adjustable gastric banding, Roux-en-Y gastric bypass, vertical-sleeve gastrectomy, and biliopancreatic diversion.^[198] Surgery for severe obesity is associated with long-term weight loss, improvement in obesity-related conditions,^[203] and decreased overall mortality, however, improved metabolic health results from the weight loss, not the surgery.^[204] One study found a weight loss of between 14% and 25% (depending on the type of procedure performed) at 10 years, and a 29% reduction in all cause mortality when compared to standard weight loss measures.^[205] Complications occur in about 17% of cases and reoperation is needed in 7% of cases.^[203] Due to its cost and risks, researchers are searching for other effective yet less invasive treatments including devices that occupy space in the stomach.^[206] For adults who have not responded to behavioral treatments with or without medication, the US guidelines on obesity recommend informing them about bariatric surgery.^[183]