Obesity

DEFINITION: A condition in which the body carries excessive and unhealthy amounts of fat tissue, leading the individual to weigh in excess of 20 percent more than their ideal weight

ANATOMY OR SYSTEM AFFECTED: Abdomen, blood vessels, circulatory system, endocrine system, gastrointestinal system, heart, intestines, joints, psychic-emotional system, respiratory system, stomach

Causes and Symptoms

Obesity is a condition in which the body accumulates an abnormally large amount of adipose tissue or fat. It is a multifactorial, chronic disease that can have devastating effects on individual wellbeing as well as public health worldwide. Being overweight and obesity are linked to more deaths around the globe than being underweight, and obesity is a major risk factor for cardiovascular disease, diabetes, musculoskeletal disorders such as osteoarthritis, and some cancers. The disease has social, cultural, genetic, metabolic, behavioral, and psychological components.

Obesity is preventable for some people, yet it is highly prevalent, affecting over 42 percent of American adults and about 13 percent of adults worldwide, according to the US Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO), respectively, in 2021. It is especially problematic in high- and middle-income countries. Obesity rates in the US and many other countries increased steadily in the late twentieth and early twenty-first centuries, leading many health experts to consider the issue a major social concern. Obesity and being overweight are considered to be among the leading causes of preventable deaths in the United States.

Because it is not practical to measure body fat content directly, but it is easy to measure weight and height, the body mass index (BMI), which correlates closely with body fat, is often used by medical professionals to identify and quantify obesity. Being overweight and being obese are not the same. According to the US Centers for Disease Control and Prevention (CDC), a BMI for an adult of 25 to 29.9 is considered to be overweight. A BMI of 30 to 34.9 is considered obese class 1, a BMI of 35 to 39.9 is considered obese class 2, and a BMI of 40 or more is obese class 3. However, many experts note that BMI is significantly flawed and is better as a screening tool than a direct indicator of whether or not a person's weight is healthy.

An important function of adipose tissue is to store energy. If the intake of energy in the form of food calories is greater than the expenditure of energy, then the excess calories are stored, mainly in the adipose tissue, with a resulting weight gain. Expenditure of energy depends largely on the resting metabolic rate or resting energy expenditure, defined as the calories used each day to maintain normal body metabolism. Additional calories are expended by exercise or other activity. Because of this simple relationship between energy intake, energy utilization, and energy storage, weight gain can occur only when there is increased caloric intake, decreased caloric expenditure, or both.

Genetic factors appear to be important in determining the presence or absence of obesity. Body weight tends to be similar in close relatives, especially in identical twins, who share the same genetic makeup. The extent to which genetic factors affect food intake, hunger levels, activity level, or metabolic processes is not fully understood. Eating a healthy diet and exercising can lower the risk of obesity in some people who have a genetic predisposition. What is not known, however, is why eating a healthy diet and exercising work well for some who are trying to treat obesity and less so for others.

Variations in certain genes involved with energy balance have been associated with obesity. It is rare for variation in a single gene to be linked to a higher occurrence of obesity in a family or monogenic obesity. While genomic studies have found obesity linked to more than fifty genes, obesity is most often due to multiple genetic and environmental factors.

One theory holds that each individual has a “set point” that determines body weight. When food intake is decreased sharply, experiments have shown less weight loss than predicted by the caloric deficit, suggesting that the body has slowed its metabolic rate, thus minimizing the deviation from the original weight. Many experts believe that physiologic regulation of body weight, which tends to maintain a preferred weight for each individual, explains some of the difficulty in treating obesity. However, it is possible to lower one's set point, especially when weight loss occurs slowly over the long term. The discovery and role of leptin in regulating weight helped to explain this apparent set point of weight for each individual. Nevertheless, as long as a caloric deficit is maintained over the long term, weight loss will occur. Some studies support losing no more than 10 percent of one's body weight and then maintaining that 10 percent weight loss for six months or more before losing more weight. Small, sustainable changes in one's daily habits and diet can help maintain weight loss over a lifetime.

A major cause of obesity is excessive food intake in relation to physical activity. However, there are other causes of obesity as well. For example, lesions in the hypothalamus can make animals eat excessively and become obese, and rare cases of obesity in humans are attributable to disease of the hypothalamus. In hypothyroidism, a condition in which the thyroid gland produces too little thyroid hormone, the metabolic rate is slowed, which may cause a mild gain in weight that can be offset by reduced caloric intake. In Cushing’s syndrome, which is caused by excessive amounts of the adrenal hormone cortisol or by drugs that act like cortisol, there is an accumulation of excessive fat in the face and trunk, which disappears when the disease is cured, or the drug is stopped. Weight gain has also occurred with the use of other drugs, including some antidepressants and tranquilizers.

There are also ways in which social determinants of health (SDOH), or the conditions in which people live and work, may contribute to obesity in some individuals. Inequalities and differences in SDOH categories in different populations may increase or decrease the risk of obesity in certain communities along class, racial, and other lines. For example, people who live in food deserts, which are areas underserved or not served at all by grocery stores, markets, farms, and other places to buy food, often have limited access to healthy food options and may be forced to rely more on processed food, which some researchers have linked to certain health risks, including obesity.

Many health problems are associated with obesity. Overweight and obesity are major risk factors for the development of non-insulin-dependent diabetes mellitus, and manifestations of the disease commonly improve or disappear if the individual succeeds in losing weight. Hypertension (high blood pressure) is more common with obesity, and weight loss may lower the blood pressure enough to lessen or avoid the need for medication. Arteriosclerosis, or “hardening of the arteries,” is more prevalent in obese people and causes an increased risk for heart attacks and strokes. Certain forms of cancer are more prevalent with obesity: cancer of the colon, rectum, and prostate in men and cancer of the uterus, gallbladder, ovary, and breast in women. Severe obesity can cause difficulties in breathing, with sleepiness resulting from inadequate oxygen delivery to the tissues and sometimes from interruption of sleep due to apnea at night. In addition, conditions such as arthritis may be worsened by the additional strain that excess weight places on weight-bearing parts of the body.

Furthermore, the effects of infectious diseases can be more severe in obese people. For instance, during the coronavirus disease 2019 (COVID-19) pandemic that began in 2020, the CDC and other health organizations emphasized that due to a connection between weight and immune system functioning as well as lung capacity, obese people who tested positive for COVID-19 faced increased risk of outcomes such as hospitalization, the need for ventilation, and even death. Other respiratory diseases such as influenza can also be signficantly complicated by obesity.

The distribution of excess adipose tissue differs among individuals. Two main patterns have been described: android obesity (more commonly affecting men), in which fat accumulates mainly in the abdomen and upper body, and gynoid obesity (more common in women), in which fat accumulates mainly in the hips, thighs, and lower body. This distinction has received much attention because persons with android obesity are more likely to suffer from diabetes, hypertension, and cardiovascular disease. The closest association with these diseases is seen when sensitive measurements of abdominal visceral fat mass are made with computed tomography (CT) scanning. A simple measurement of the waist circumference compared with the hip circumference—the waist-to-hip ratio—can also be used to identify those obese individuals at greater risk for diabetes and cardiovascular disease.

Treatment and Therapy

Some of the most useful methods for the treatment of obesity are those that decrease the intake or absorption of calories or those that increase the expenditure of calories. The basis for most long-term weight management programs is a balanced, lower-calorie diet that incorporates healthy choices. The average daily calorie requirement depends on a person's age, height, weight, and activity level and varies whether one is trying to lose, maintain, or gain weight. It is helpful to calculate one's basal metabolic rate (BMR), which is an estimate of a person's daily energy expenditure at absolute rest based on one's gender, weight, height, and age. A person's BMR is then multiplied by a factor depending on average daily activity level to estimate their total daily energy expenditure (TDEE). Decreasing an individual’s caloric intake below their TDEE, usually by 250 to 1,000 calories per day, will result in weight loss, provided that energy expenditure does not also decrease. A caloric deficit of 500 calories per day will result in one pound (0.45 kilograms) of weight loss per week. The Mayo Clinic recommends a balanced diet with 20 percent to 35 percent of one's daily calories derived from fat, which is considerably less fat than is found in the typical American diet.

Many unbalanced diets, or “fad diets,” have enjoyed periods of popularity, although many of these diets either lack scientific backing or, in some cases, have even drawn criticism from medical professionals for their possible risks. Rice diets, low-carbohydrate diets, vegetable diets, and other special diets may produce rapid weight loss, but long-term persistence with an unbalanced diet is rare, and any lost weight is often regained; other health risks or downsides can also occur with some of these diets. Maintaining a balanced diet and not consuming excessive calories tends to be a more sustainable approach to weight loss than avoiding particular foods.

Many patients fail to lose weight with low-calorie diets, and high levels of caloric restriction are unsafe in the long term. Nutrients should be supplemented with vitamins and minerals. These requirements can be met with special formula diets under careful medical supervision. Such a program may be recommended for severely obese patients who are otherwise healthy enough to tolerate this degree of caloric restriction.

Although diets can provide some people with a useful tool for fighting obesity and improving their overall health, some diets have also been criticized for a number of reasons. The concept of dieting in general has been criticized by some people for perpetuating unrealistic or unhealthy body standards, as well as contributing to body shaming and other forms of weight-based discrimination. Activists involved in the Anti-Diet Movement and similar causes instead sought to promote equal access to health care for heavier people, who often faced barriers and discrimination in the healthcare system. Other aspects of this movement also focused on the promotion of mental wellbeing and sustainable eating habits, which they argued is more broadly helpful than focusing solely on weight loss. Many activists associated with this movement emphasized the importance of body positivity. However, some observers have expressed concerns that the language of the anti-diet movement can be co-opted by food industry marketers to deflect criticism of major food companies' heavy reliance on additives, sugar, and artificial sweeteners in their products.

Because it can be difficult to sustain restricted calorie diets in the long-term, behavioral management programs may be combined with dietary restrictions; some of these management strategies can help with disordered eating, such as compulsive or binge-eating. Dieters can be taught techniques for self-monitoring of food intake, such as keeping a daily log of meals and exercise, which will increase the awareness of eating behavior as well as point out ways in which that behavior can be modified. There are techniques for reducing exposure to food and the stimuli associated with eating, such as keeping food out of sight, keeping food handling and preparation to a minimum, and eliminating the occasions when food is eaten out of habit or as part of a social routine. Increasing the social support of friends and family for weight-losing behavior and for reinforcement of compliance with dietary restrictions can be helpful.

Physical activity can also help people manage obesity and maintain a healthier weight. More vigorous exercise, such as running, swimming, or calisthenics, could contribute to more rapid weight loss, but it might not be advisable for every person because of the increased prevalence of certain health problems in obese individuals, such as heart disease, hypertension, and musculoskeletal disorders. For this reason, any exercise program that involves vigorous physical activity should be undertaken following medical consultation.

Exercise as part of a weight-loss regimen has additional benefits. The function of the cardiovascular system may be improved, and muscles may be strengthened. Exercise could contribute to loss of adipose tissue and gain in lean body mass as weight is lost, a change in body composition that is beneficial to overall health. Although some fear that physical activity can contribute to an increase in appetite, exercise is a beneficial supplement to calorie restriction.

Medications that decrease appetite are occasionally used to help people comply with a low-calorie diet. Some appetite suppressants act like adrenaline and may cause side effects, such as nervousness, irritability, and increased heart rate and blood pressure. Other drugs may stimulate serotonin, a chemical transmitter in the central nervous system that decreases appetite and may cause drowsiness as a side effect. The use of these medications was long controversial because of their side effects and their limited effectiveness in promoting weight loss. In the 2020s, several drugs originally marketed for diabetes were discovered to also reduce patients’ body weight by a large percentage. These drugs, notably including the brand name Ozempic (semaglutide), became massively popular, even leading to shortages of the medication. Some observers touted them as a major breakthrough for treating obesity, but many experts insisted that long-term studies were necessary to better determine the drugs’ safety and efficacy as a weight-loss drug and raised concerns about equitable access.

Several surgical procedures, collectively referred to as bariatric surgery, have been used to treat severe obesity that has impaired the patient’s health and has resisted other treatment. The operation now most commonly performed is vertical banded gastroplasty, which creates a small pouch in the stomach with a narrow outlet through which all food must pass. This procedure decreases the effective volume of the stomach, causing fullness and nausea if more than small amounts of solid food are eaten. Patients have lost about half of their excess weight after one and one-half to two years, but some weight may be regained after this period. Gastroplasty has produced fewer serious complications than an older form of treatment, called intestinal bypass. Care must be taken to avoid certain foods that might cause blockage of the narrowed opening from the surgically created stomach pouch, and the benefit of the operation can be overcome by eating soft or liquid foods, which can be consumed in large quantities. The long-term benefit of this procedure continues to be evaluated, but a significant number of obese patients with diabetes essentially cured their diabetes mellitus after having the surgery and losing weight.

Perspective and Prospects

Fat has several important functions in the human body. It serves as a cushion for the body frame and internal organs, provides insulation against heat loss, and is a storage site for energy. Fat stores energy very efficiently since it contains approximately nine calories per gram, compared with approximately four calories per gram in protein and carbohydrate. The presence of reserve stores of energy in the form of fat is particularly important when regular food intake is interrupted, and the body becomes dependent on its fat deposits to maintain a source of fuel for daily metabolism and physical activity. In many affluent societies, however, where food is abundant and modern conveniences greatly reduce the need for physical exertion, obesity rates rose sharply in the twentieth and twenty-first centuries. By the 2020s, rising obesity rates in many developing countries also raised concerns.

The issue has generated strong public attention in the United States. In the early twenty-first century, health officials noted findings that showed one in every fifty Americans were “extremely obese,” meaning their BMI measured at least 50 and they were at least one hundred pounds overweight. This number had quadrupled since the 1980s.

Many efforts have been launched to reduce the incidence rate of obesity in the United States. The Patient Protection and Affordable Care Act of 2010 listed obesity screening and counseling among the preventative services that all new group health plans and individual market plans under the act were required to provide without patient cost sharing. The CDC and other health organizations seek to publicize information on obesity and provide resources for both preventing and overcoming the disease. Groups such as the WHO have taken similar steps at the international level.

In particular, childhood obesity has been targeted as a major public health crisis. In 2006 the Alliance for a Healthier Generation, the William J. Clinton Foundation, and the American Heart Association announced an agreement to fight childhood obesity. Five leading food manufacturers vowed to reformulate their products in order to provide more nutritious choices for children in schools. In 2010 President Barack Obama and First Lady Michelle Obama each announced further initiatives to help prevent childhood obesity, including the "Let's Move!" initiative and the Task Force on Childhood Obesity. However, such efforts had limited immediate impact. According to the CDC, between 2017 and 2018, approximately 19.3 percent of children between the ages of two and nineteen were obese.

Concerns regarding the increased prevalence of obesity among both adults and children were further heightened during the COVID-19 pandemic. As people's lifestyles abruptly changed due to lockdowns and restrictions that included gym closures as well as regularly working from home, many were experiencing changing diets as well as amounts of physical activity (at times potentially linked to increased mental health issues such as depression and anxiety). Experts such as those affiliated with the Mayo Clinic saw the pandemic as exacerbating the challenges to weight management.

Worldwide, obesity nearly tripled between 1975 and 2020, according to data published by the WHO in 2021. In 2021 the CDC reported that 42.4 percent of American adults were obese between 2017 and 2018, with non-Hispanic Black Americans having the highest obesity rate of 49.6 percent of the population. Hispanic and non-Hispanic White Americans followed with 44.8 percent and 42.2 percent, respectively. Non-Hispanic Asian Americans had the lowest incidence of obesity with 17.4 percent of that population reportedly obese. The CDC further reported that obesity was highest among adults between the ages of forty and fifty-nine. The annual cost of obesity in the United States was estimated at $147 billion in 2008 US dollars.

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