Obesity and Overweight

Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it negatively affects health. People are classified as obese when their body mass index (BMI)—a person’s weight divided by the square of the person’s height—is over 30 kg/m²; the range 25–30 kg/m² is defined as overweight.^[1] Some East Asian countries use lower values to calculate obesity.^[11] Obesity is a major cause of disability and is correlated with various diseases and conditions, particularly cardiovascular diseases, type 2 diabetes, obstructive sleep apnea, certain types of cancer, and osteoarthritis.

Obesity has individual, socioeconomic, and environmental causes. Some known causes are diet, physical activity, automation, urbanization, genetic susceptibility, medications, mental disorders, economic policies, endocrine disorders, and exposure to endocrine-disrupting chemicals.

While a majority of obese individuals at any given time attempt to lose weight and are often successful, maintaining weight loss long-term is rare. There is no effective, well-defined, evidence-based intervention for preventing obesity. Obesity prevention requires a complex approach, including interventions at societal, community, family, and individual levels. Changes to diet as well as exercising are the main treatments recommended by health professionals. 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, if these dietary choices are available, affordable, and accessible. Medications can be used, along with a suitable diet, to reduce appetite or decrease fat absorption. 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.

Obesity is a leading preventable cause of death worldwide, with increasing rates in adults and children.  In 2015, 600 million adults (12%) and 100 million children were obese in 195 countries. Obesity is more common in women than in men.  Today, obesity is stigmatized in most of the world. Conversely, some cultures, past and present, have a favorable view of obesity, seeing it as a symbol of wealth and fertility.  Nevertheless, in 2013, several medical societies, including the American Medical Association and the American Heart Association, classified obesity as a disease.

Classification

 

Obesity is typically defined as a substantial accumulation of body fat that could impact health. Medical organizations tend to classify people as obese based on body mass index (BMI) – a ratio of a person’s weight in kilograms to the square of their height in meters. For adults, the World Health Organization (WHO) defines “overweight” as a BMI 25 or higher, and “obese” as a BMI 30 or higher. The U.S. Centers for Disease Control and Prevention (CDC) further subdivides obesity based on BMI, with a BMI 30 to 35 called class 1 obesity; 35 to 40, class 2 obesity; and 40+, class 3 obesity.

For children, obesity measures take age into consideration along with height and weight. For children aged 5–19, the WHO defines obesity as a BMI two standard deviations above the median for their age (a BMI around 18 for a five-year old; around 30 for a 19-year old).  For children under five, the WHO defines obesity as a weight three standard deviations above the median for their height.

Some modifications to the WHO definitions have been made by particular organizations. The surgical literature breaks down class II and III or only class III obesity into further categories whose exact values are still disputed.

• Any BMI ≥ 35 or 40 kg/m² is severe obesity.
• A BMI of ≥ 35 kg/m² and experiencing obesity-related health conditions or ≥ 40 or 45 kg/m² is morbid obesity.
• A BMI of ≥ 45 or 50 kg/m² is super obesity.

As Asian populations develop negative health consequences at a lower BMI than Caucasians, some nations have redefined obesity; Japan has defined obesity as any BMI greater than 25 kg/m^2[11] while China uses a BMI of greater than 28 kg/m².^[27]

The preferred obesity metric in scholarly circles is the body fat percentage (BF%) – the ratio of the total weight of person’s fat to his or her body weight, and BMI is viewed merely as a way to approximate BF%.^[29] Levels in excess of 32% for women and 25% for men are generally considered to indicate obesity.

BMI ignores variations between individuals in amounts of lean body mass, particularly muscle mass. Individuals involved in heavy physical labor or sports may have high BMI values despite having little fat. For example, more than half of all NFL players are classified as “obese” (BMI ≥ 30), and 1 in 4 are classified as “extremely obese” (BMI ≥ 35), according to the BMI metric.^[30] However, their mean body fat percentage, 14%, is well within what is considered a healthy range.^[31] Similarly, Sumo wrestlers may be categorized by BMI as “severely obese” or “very severely obese” but many Sumo wrestlers are not categorized as obese when body fat percentage is used instead (having <25% body fat).^[32] Some Sumo wrestlers were found to have no more body fat than a non-Sumo comparison group, with high BMI values resulting from their high amounts of lean body mass.

Effects on health

Obesity increases a person’s risk of developing various metabolic diseases, cardiovascular disease, osteoarthritis, Alzheimer disease, depression, and certain types of cancer. Depending on the degree of obesity and the presence of comorbid disorders, obesity is associated with an estimated 2–20 year shorter life expectancy. High BMI is a marker of risk for, but not a direct cause of, diseases caused by diet and physical activity.

Mortality

Graphs are temporarily unavailable due to technical issues.

Relative risk of death over 10 years in white people who have never smoked in the United States by BMI. The BMI range 22.5 to 24.9 is set as the reference.

Obesity is one of the leading preventable causes of death worldwide. The mortality risk is lowest at a BMI of 20–25 kg/m²  in non-smokers and at 24–27 kg/m² in current smokers, with risk increasing along with changes in either direction. This appears to apply in at least four continents.  Other research 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.  In Asians the risk of negative health effects begins to increase between 22 and 25 kg/m².  In 2021, the World Health Organization estimated that obesity caused at least 2.8 million deaths annually. On average, obesity reduces life expectancy by six to seven years,  a BMI of 30–35 kg/m² reduces life expectancy by two to four years,^[34] while severe obesity (BMI ≥ 40 kg/m²) reduces life expectancy by ten years.^[34]

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.^[47] A study from the RAK Hospital found that obese people are at a greater risk of developing long COVID.^[48] The CDC has found that obesity is the single strongest risk factor for severe COVID-19 illness.

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.

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). Increases in body fat alter the body’s response to insulin, potentially leading to insulin resistance. Increased fat also creates a proinflammatory state, and a prothrombotic state.

Parts of this article (those related to table below) need to be updated. Please help update this article to reflect recent events or newly available information. (March 2022)

Metrics of health

Newer research has focused on methods of identifying healthier obese people by clinicians, and not treating obese people as a monolithic group.^[79] Obese people who do not experience medical complications from their obesity are sometimes called (metabolically) healthy obese, but the extent to which this group exists (especially among older people) is in dispute.^[80] The number of people considered metabolically healthy depends on the definition used, and there is no universally accepted definition.^[81] There are numerous obese people who have relatively few metabolic abnormalities, and a minority of obese people have no medical complications.^[81] The guidelines of the American Association of Clinical Endocrinologists call for physicians to use risk stratification with obese patients when considering how to assess their risk of developing type 2 diabetes.^{[82]: 59–60}

In 2014, the BioSHaRE–EU Healthy Obese Project (sponsored by Maelstrom Research, a team under the Research Institute of the McGill University Health Centre) came up with two definitions for healthy obesity, one more strict and one less so:^[80][83]

To come up with these criteria, BioSHaRE controlled for age and tobacco use, researching how both may effect the metabolic syndrome associated with obesity, but not found to exist in the metabolically healthy obese.^[84] Other definitions of metabolically healthy obesity exist, including ones based on waist circumference rather than BMI, which is unreliable in certain individuals.^[81]

Another identification metric for health in obese people is calf strength, which is positively correlated with physical fitness in obese people.^[85] Body composition in general is hypothesized to help explain the existence of metabolically healthy obesity—the metabolically healthy obese are often found to have low amounts of ectopic fat (fat stored in tissues other than adipose tissue) despite having overall fat mass equivalent in weight to obese people with metabolic syndrome.^[86]: 1282

Survival paradox

Although the negative health consequences of obesity in the general population are well supported by the available research evidence, health outcomes in certain subgroups seem to be improved at an increased BMI, a phenomenon known as the obesity survival paradox.^[87] The paradox was first described in 1999 in overweight and obese people undergoing hemodialysis^[87] and has subsequently been found in those with heart failure and peripheral artery disease (PAD).^[88]

In people with heart failure, those with a BMI between 30.0 and 34.9 had lower mortality than those with a normal weight. This has been attributed to the fact that people often lose weight as they become progressively more ill.^[89] Similar findings have been made in other types of heart disease. People with class I obesity and heart disease do not have greater rates of further heart problems than people of normal weight who also have heart disease. In people with greater degrees of obesity, however, the risk of further cardiovascular events is increased.^[90][91] Even after cardiac bypass surgery, no increase in mortality is seen in the overweight and obese.^[92] One study found that the improved survival could be explained by the more aggressive treatment obese people receive after a cardiac event.^[93] Another study found that if one takes into account chronic obstructive pulmonary disease (COPD) in those with PAD, the benefit of obesity no longer exists.^[88]

Causes

The “a calorie is a calorie” model of obesity posits a combination of excessive food energy intake and a lack of physical activity as the cause of most cases of obesity.^[94] A limited number of cases are due primarily to genetics, medical reasons, or psychiatric illness.^[15] In contrast, increasing rates of obesity at a societal level are felt to be due to an easily accessible and palatable diet,^[95] increased reliance on cars, and mechanized manufacturing.^[96][97]

Other possible contributors to the recent increase of obesity are: (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).^[98] 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”.^[99] Societal pressure to diet as a means of maintaining or losing weight has also been suggested as a possible contributor.^[100]

Diet

1961

2001–03

Map of dietary energy availability per person per day in 1961 (left) and 2001–2003 (right)^[101] Calories per person per day (kilojoules per person per day)

No data   <1,600 (<6,700)   1,600–1,800 (6,700–7,500)   1,800–2,000 (7,500–8,400)   2,000–2,200 (8,400–9,200)   2,200–2,400 (9,200–10,000)   2,400–2,600 (10,000–10,900)

2,600–2,800 (10,900–11,700)   2,800–3,000 (11,700–12,600)   3,000–3,200 (12,600–13,400)   3,200–3,400 (13,400–14,200)   3,400–3,600 (14,200–15,100)   >3,600 (>15,100)

 

Excess appetite for palatable, high-calorie food (especially fat, sugar, and certain animal proteins) is seen as the primary factor driving obesity worldwide, likely because of imbalances in neurotransmitters affecting the drive to eat.^[102] Dietary energy supply per capita varies markedly between different regions and countries. It has also changed significantly over time.^[101] From the early 1970s to the late 1990s the average food energy available per person per day (the amount of food bought) increased in all parts of the world except Eastern Europe. The United States had the highest availability with 3,654 calories (15,290 kJ) per person in 1996.^[101] This increased further in 2003 to 3,754 calories (15,710 kJ).^[101] During the late 1990s, Europeans had 3,394 calories (14,200 kJ) per person, in the developing areas of Asia there were 2,648 calories (11,080 kJ) per person, and in sub-Saharan Africa people had 2,176 calories (9,100 kJ) per person.^[101][103] Total food energy consumption has been found to be related to obesity.^[104]

The widespread availability of dietary guidelines^[105] has done little to address the problems of overeating and poor dietary choice.^[106] From 1971 to 2000, obesity rates in the United States increased from 14.5% to 30.9%.^[107] During the same period, an increase occurred in the average amount of food energy consumed. For women, the average increase was 335 calories (1,400 kJ) per day (1,542 calories (6,450 kJ) in 1971 and 1,877 calories (7,850 kJ) in 2004), while for men the average increase was 168 calories (700 kJ) per day (2,450 calories (10,300 kJ) in 1971 and 2,618 calories (10,950 kJ) in 2004). Most of this extra food energy came from an increase in carbohydrate consumption rather than fat consumption.^[108] The primary sources of these extra carbohydrates are sweetened beverages, which now account for almost 25 percent of daily food energy in young adults in America,^[109] and potato chips.^[110] Consumption of sweetened beverages such as soft drinks, fruit drinks, and iced tea is believed to be contributing to the rising rates of obesity^[111][112] and to an increased risk of metabolic syndrome and type 2 diabetes.^[113] Vitamin D deficiency is related to diseases associated with obesity.^[114]

As societies become increasingly reliant on energy-dense, big-portions, and fast-food meals, the association between fast-food consumption and obesity becomes more concerning.^[115] In the United States, consumption of fast-food meals tripled and food energy intake from these meals quadrupled between 1977 and 1995.^[116]

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 cheap compared to fruits and vegetables.^[117] Calorie count laws and nutrition facts labels attempt to steer people toward making healthier food choices, including awareness of how much food energy is being consumed.

Obese people consistently under-report their food consumption as compared to people of normal weight.^[118] This is supported both by tests of people carried out in a calorimeter room^[119] and by direct observation.

Sedentary lifestyle

A sedentary lifestyle may play a significant role in obesity.^[120] Worldwide there has been a large shift towards less physically demanding work,^{[121][122][123]} and currently at least 30% of the world’s population gets insufficient exercise.^[122] This is primarily due to increasing use of mechanized transportation and a greater prevalence of labor-saving technology in the home.^{[121][122][123]} In children, there appear to be declines in levels of physical activity (with particularly strong declines in the amount of walking and physical education), likely due to safety concerns, changes in social interaction (such as fewer relationships with neighborhood children), and inadequate urban design (such as too few public spaces for safe physical activity).^[124] 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 research from Finland^[125] found an increase and research from the United States found leisure-time physical activity has not changed significantly.^[126] Physical activity in children may not be a significant contributor.^[127]

In both children and adults, there is an association between television viewing time and the risk of obesity.^{[128][129][130]} Increased media exposure increases the rate of childhood obesity, with rates increasing proportionally to time spent watching television.^[131]

Genetics

This section needs to be updated. Please help update this article to reflect recent events or newly available information. (July 2021)

 

Like many other medical conditions, obesity is the result of an interplay between genetic and environmental factors.^[133] 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.^[134] 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.^[135] The differences in BMI between people that are due to genetics varies depending on the population examined from 6% to 85%.^[136]

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.)^[137] 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.^[138]

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.^[139] Different people exposed to the same environment have different risks of obesity due to their underlying genetics.^[140]

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.^{[medical citation needed]} 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.^{[medical citation needed]}

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,^[141] 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.^[142] The risk of overweight and obesity is higher in patients with psychiatric disorders than in persons without psychiatric disorders.^[143] Obesity and depression influence each other mutually, with obesity increasing the risk of clinical depression, and also depression leading to a higher chance of developing obesity.^[3]

Drug-induced obesity

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

 

 

While genetic influences are important to understanding obesity, they cannot completely explain the dramatic increase seen within specific countries or globally.^{[144][better source needed]} Though it is accepted that energy consumption in excess of energy expenditure leads to increases in body weight 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. Research 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.^{[better source needed][145]} In 2007 repeating the same research found the same relationships, but they were weaker. The decrease in strength of correlation was felt to be due to the effects of globalization.^[146] 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.^[147]

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.^[146] 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.^[148] Stress and perceived low social status appear to increase risk of obesity.^{[147][149][150]}

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.^[151] However, changing rates of smoking have had little effect on the overall rates of obesity.^[152]

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.^[153] This could be partly explained by the fact that having dependent children decreases physical activity in Western parents.^[154]

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%.^[155] In part, this may be because of urban design issues (such as inadequate public spaces for physical activity).^[124] Time spent in motor vehicles, as opposed to active transportation options such as cycling or walking, is correlated with increased risk of obesity.^[156][157]

Malnutrition in early life is believed to play a role in the rising rates of obesity in the developing world.^[158] Endocrine changes that occur during periods of malnutrition may promote the storage of fat once more food energy becomes available.^[158]

Gut bacteria

The study of the effect of infectious agents on metabolism is still in its early stages. Gut flora has been shown to differ between lean and obese people. There is an indication that gut flora can affect the metabolic potential. This apparent alteration is believed to confer a greater capacity to harvest energy contributing to obesity. Whether these differences are the direct cause or the result of obesity has yet to be determined unequivocally.^[159] The use of antibiotics among children has also been associated with obesity later in life.^[160][161]

An association between viruses and obesity has been found in humans and several different animal species. The amount that these associations may have contributed to the rising rate of obesity is yet to be determined.^[162]

Other factors

Not getting enough sleep is also associated with obesity.^[163][164] Whether one causes the other is unclear.^[163] Even if short sleep does increase weight gain, it is unclear if this is to a meaningful degree or if increasing sleep would be of benefit.^[165]

Some have proposed that chemical compounds called “obesogens” may play a role in obesity.

Certain aspects of personality are associated with being obese.^[166] Loneliness,^[167] neuroticism, impulsivity, and sensitivity to reward are more common in people who are obese while conscientiousness and self-control are less common in people who are obese.^[166][168] Because most of the studies on this topic are questionnaire-based, it is possible that these findings overestimate the relationships between personality and obesity: people who are obese might be aware of the social stigma of obesity and their questionnaire responses might be biased accordingly.^[166] Similarly, the personalities of people who are obese as children might be influenced by obesity stigma, rather than these personality factors acting as risk factor for obesity