Obesity is a medical condition in which there is excess storage of body fat resulting in a body mass index (BMI) that is significantly above the norm. It is associated with increased risk of illness, disability, and death. Medical professionals generally consider obesity, or being excessively overweight, to be a chronic illness requiring lifelong treatment and management. It is often grouped with other chronic conditions, such as high blood pressure and diabetes, which can be controlled but not cured.
The human body is composed of bone, muscle, special organ tissues, and fat. Together these comprise the total body mass, measured in pounds (lb) or kilograms (kg). Fat, or adipose tissue, is a combination of essential and storage fats. Essential fat is an energy source for the normal physiological function of cells and organs, protecting and insulating internal organs, and also is an important building block for all cells of the body. Storage fat is a reserve supply of energy. It accumulates in the chest and abdomen and, in much greater volume, under the skin. When the amount of energy consumed as food exceeds the amount of energy expended in everyday life and physical activity, storage fat accumulates in excessive amounts.
The human body needs fat for good health; however, sometimes the body accumulates too much fat. Thus, obesity is excessive body weight that develops over time as people consume more calories than they expend in energy over their daily lives. As excess energy accumulates in the body, people first become overweight, then excessively overweight, or obese.
In times of famine, the ability of the human body to store energy can mean the difference between life and death. This protective mechanism becomes a potential problem when food is readily available in unlimited quantities. This is evident in the increasing prevalence of obesity in modern society, particularly in the developed world. For instance, in the 2010s, the United States and Australia were considered to have two of the largest percentages of obese people in the world. Too much food, especially food high in sugar and fat content, and not enough exercise are reasons why people are becoming obese in these two countries and in other regions of the world. In fact, the US Centers for Disease Control and Prevention (CDC) state that obesity levels in the United States have doubled for adults and tripled for children over the past few decades. As obesity rates have increased, bariatrics, the branch of medicine that studies and treats obesity, has become a separate medical and surgical specialty.
Obesity was originally defined as body weight that was at least 20% above one's ideal weight, defined as the weight at which individuals of the same height, gender, and age had the lowest rate of death. Mild obesity was defined as 20%–40% over ideal weight, moderate obesity as 40%–100% over ideal, and gross or morbid obesity 100% over ideal weight.
Current guidelines use the body mass index (BMI) to define obesity. The BMI utilizes height and weight to compare the ratio of body fat to total body mass. To calculate BMI using metric units, weight in kilograms is divided by height in meters squared. To calculate BMI in English units, weight in pounds is divided by height in inches squared and then multiplied by 703. This calculated BMI is compared to the statistical distribution of BMIs for adults aged 20–29 to determine whether an individual is underweight, average, overweight, or obese. The 20- to 29-year age group was chosen as the standard because it represents fully developed adults at the point in their lives when they have the least amount of body fat. Ideally, body fat is about 15% of total body mass for adult males and about 20%–25% for adult females. A simple BMI calculator is available at https://nccd.cdc.gov/dnpabmi/Calculator.aspx . BMI does not distinguish between fat and muscle.
Adult BMIs are age- and gender-independent. All adults 20 years of age and older are evaluated on the same BMI scale:
Research has shown that adults with BMIs within the normal weight range live the longest and enjoy the best health. For a healthy life and a fit body, it is important that a normal weight is maintained consistently throughout one's life.
The BMI for children and teens is calculated in the same way as for adults, but the results are interpreted differently. A child's BMI is compared to those of other children of the same age and gender and assigned to a percentile. For example, a girl in the 75th percentile for her age group weighs more than 74 of every 100 girls her age and less than 25 of every 100 girls her age. The following is a guide to use for children and teens:
The CDC does not use the term obese for children and teens because the proportion of body fat fluctuates during growth and development and is slightly higher than in mature adults.
Obesity places stress on the body's organs and puts people at higher risk for many serious and potentially life-threatening health problems. Problems caused by obesity include:
Obese individuals have a shorter life expectancy than people of normal weight. Many diseases, especially degenerative diseases of the joints, heart, and blood vessels, tend to be more severe in obese individuals, increasing the need for some surgical procedures. Obesity is directly related to the increasing prevalence of type 2 diabetes in the United States and to the appearance of type 2 diabetes in children, previously a rarity.
Although acute complications of obesity are rare in children, childhood obesity is a risk factor for insulin resistance and type 2 diabetes, hypertension, hyperlipidemia, liver and renal disease, and reproductive dysfunction. Childhood obesity increases the risk of deformed bones in the legs and feet, and it can result in emotional disorders such as depression caused by social isolation and negative comments by peers. Childhood obesity also increases the risks of adult obesity and cardiovascular disease.
According to the CDC, the direct and indirect cost of obesity to the US economy in 2008 was estimated as high as US$147 billion. The study found that obese people spend an average of US$1,429 more each year for their medical care than do people with a normal weight range. This comparison finds that obese people have a 42% higher cost for medical care than do normal-weight people. The increasing prevalence of obesity and diabetes in children and young adults heralds spiraling healthcare costs in the future. The social costs of obesity, including decreased productivity, discrimination, depression, and low self-esteem, are less easily measured.
The US National Academies describe obesity as a “complex, multifactorial disease of appetite regulation and energy metabolism.” The following are positive outcomes from even relatively modest weight loss:
The Institute of Medicine has continued to emphasize the adverse consequences that obesity has on adults and children.
Obesity tends to run in families. Children of obese parents are about 13 times more likely than are other children to be obese. Additional obese family members, including siblings and grandparents, greatly increase the likelihood of childhood obesity. The tendency toward a body type with an unusually high number of fat cells—termed endomorphic—appears to be inherited. Other genetic factors influence appetite and the metabolic rate at which food is transformed into energy. As of late 2016, scientists had discovered more than 25 genes that strongly influence obesity risk and an additional 300 genes that make it easier to gain weight or harder to lose it.
Family eating habits are major contributors to the development of obesity. Although most adopted children have patterns of weight gain that more closely resemble those of their birth parents than those of their adoptive parents, normal-weight children adopted into obese families are more likely than other children to become obese. Longitudinal studies of juvenile-onset obesity have demonstrated parental and peer encouragement of overeating and even deliberate overfeeding of obese children. Low socioeconomic status is also a risk factor for adult-onset obesity.
Obesity is a serious public-health problem that affects both sexes and all ethnic, racial, age, and socioeconomic groups in the United States and around the world. According to the CDC, 36.5% of adults (well over 100 million in a total population of 304 million) in the United States are obese. More than 440,000 deaths a year are attributed to obesity, prompting public-health officials, such as former Surgeon General C. Everett Koop, to label obesity “the second leading cause of preventable deaths in the United States.” Smoking tobacco is the number one cause.
As of 2016, according to the National Institute of Diabetes and Digestive and Kidney Diseases, obesity affects all adult age groups. Based on 2010 national health data, men and women were nearly equal in obesity prevalence, but nearly 8% of women were considered extremely obese. The highest percentage of obesity is in the over-65 age group; the 45–64-year age group is a close second. Approximately 50% of black women and 43% of Hispanic women are obese, compared with about 33% of non-Hispanic white women of the same age. Black Americans have obesity rates nearing 50%, and Hispanics are at nearly 40%.
Obesity is the most common nutritional disorder among US children and teens. African American and Hispanic children are considerably more likely to be overweight than Caucasian American children.
According to the CDC, childhood obesity more than tripled from 1980 to 2008. As of 2016, estimates showed that 18% of children between age 6 and 19 were considered obese, and more than one-third of children in those age groups were overweight.
Similar trends were reported by the World Health Organization (WHO), who refers to the escalating global epidemic of obesity as “globesity.” WHO estimated that in 2014, more than 600 million adults were obese. The organization further found that, in 2014, nearly 41 million children under five years of age were overweight or obese.
Obesity is caused by consuming more calories than the body uses for energy. The excess calories are stored as adipose tissue. Although inheritance plays a role, a genetic predisposition toward weight gain does not in itself cause obesity. Eating habits, physical activity, and environmental, behavioral, social, and cultural factors all contribute to the development of obesity.
Sometimes obesity has a purely physiological cause. Some of the physiological causes are:
Some researchers have suggested that low levels of the neurotransmitter serotonin increase cravings for carbohydrates. In addition, a combination of genetics and early nutritional habits can result in a higher set point for body weight that causes obese individuals to feel hunger more often than others. Two peptide hormones, leptin and ghrelin, have been identified as involved in obesity. Leptin, produced by fat cells, affects hunger and eating behavior, and an insensitivity to leptin may contribute to obesity. Ghrelin is secreted by cells in the lining of the stomach and is important in appetite regulation and maintaining the body's energy balance. Other hormones are related to triggering hunger or slowing stomach emptying.
During the past several decades American eating habits have changed significantly, with many people consuming larger meals and more high-calorie processed foods. School and workplace cafeterias often have a poor selection of nutritional food offerings. Furthermore, it is estimated that in a given six-month period, 2%–5% of Americans binge eat. It has been estimated that approximately 15% of the mildly obese participating in weight-loss programs have binge-eating disorder and that the percentage is much higher among the morbidly obese.
Some studies suggest that the amount of fat in a person's diet may have a greater impact on weight than the total number of calories. Carbohydrates from cereals, breads, fruits, and vegetables, and protein from fish, lean meat, turkey breast, and skim milk are converted into fuel almost as soon as they are consumed. In contrast, most fats are immediately stored in fat cells that multiply and expand, adding to the body's weight and girth. Evidence indicates that weight gain depends primarily on total calories consumed, rather than the amount from carbohydrates versus fats, and that low-fat diets are no more effective for weight reduction than low-calorie diets.
Sedentary lifestyles that are a particular problem in richer societies, such as the United States, and also contribute to obesity. Rather than physical labor on farms and in factories, people are now employed at jobs in that involve sitting at computers and machines. People also spend more time in cars and move less because of technology, such as remote control devices, household electric appliances, and power tools, which have become standard equipment. One study found that the average Western European adult walks about 8,000–9,000 steps daily. In contrast, among the Amish of Pennsylvania who do not use cars or electricity, men accumulate 18,425 steps daily and have no obesity. Amish women walk 14,196 steps daily and have an obesity rate of only 9%.
Psychological factors, such as depression and low self-esteem, can contribute to overeating and obesity. People may eat compulsively to overcome fear or social maladjustment, express defiance, or avoid intimate relationships.
Some babies are born obese. This can be caused by excessive insulin production in the fetuses of diabetic mothers or excess transplacental nutrients in the case of obese mothers or those who gain excessive weight during pregnancy.
Babies can become obese because they are overfed. Because obese one-year-olds can be physically delayed in crawling and walking, they become less active toddlers, burning fewer calories. By the age of 10 years, obese boys and girls are taller than their peers by as much as 3.9 inches (10 cm). Their skeletal maturation, called “bone-age,” is also accelerated, so they stop growing earlier. Sexual maturation is advanced. It is not uncommon for obese girls to experience precocious menarche (early onset of menstruation), sometimes even before the age of 10 years. Parental separation and divorce or other psychological stresses can stimulate compensatory overeating in children. Obese teenagers and, increasingly, obese preteens might combine periods of binge eating with caloric deprivation, leading to bulimia or anorexia nervosa.
In developed countries, people generally experience increased BMI with age. The proportion of intraabdominal fat that correlates with disease and death increases progressively with age. There is also a progressive decline in daily total energy expenditure, associated with decreased physical activity and lower metabolic activity, especially in those with chronic disabilities and diseases.
The major symptoms of obesity are excessive weight and large amounts of fatty tissue. Common secondary symptoms include shortness of breath and lower back pain from carrying excessive body weight. Obesity can also give rise to secondary conditions including:
Obesity is usually diagnosed by observation of excessive stored fat and by calculating BMI from weight and height. Physicians observe how the excess weight is carried by comparing waist and hip measurements: “apple-shaped” patients, who store most of their weight around the waist and abdomen, are at greater risk for cancer, heart disease, stroke, and diabetes than “pear-shaped” patients, whose extra pounds settle primarily in their hips and thighs.
BMI and other measurements do not necessarily accurately reflect body composition and muscle mass. A heavily muscled football player might weigh far more than a sedentary man of similar height, but have significantly less body fat. Chronic dieters, who have lost significant muscle mass during periods of caloric deprivation, may look slim and weigh little, but have elevated body fat. Direct measurements of body fat are obtained using calipers to measure skin-fold thickness at the back of the upper arm and other sites that distinguish between muscle and adipose tissue.
The most accurate way to estimate body fat is hydrostatic weighing, or calculating the volume of water displaced by the body. The patient exhales as completely as possible and is immersed in water, and the relative displacement is measured. Women whose body fat exceeds 30%–32% of total body mass by this method and men whose body fat exceeds 25%–27% are generally considered obese. Since this method is unpleasant and impractical, it is usually used only in scientific studies.
Treatment of obesity aims to reduce weight to a BMI within the normal range (below 25). The best way to achieve weight loss is to reduce dietary caloric intake and increase physical activity. Obesity will return, however, unless the weight loss includes lifelong behavioral changes. Yo-yo dieting, in which weight is repeatedly lost and regained, has been shown to increase the likelihood of fatal health problems even more than no weight loss at all.
Behavioral treatment for obesity is goal-directed and process-oriented and relies heavily on self-monitoring, with emphasis on:
In 2016, researchers reported that they are aware of up to 59 different types of obesity and that the diet approach that works for one obese person might not work for another. Most mildly obese patients can make lifestyle changes independently with medical supervision. Others might use a commercial weight-loss program, such as Weight Watchers or Jenny Craig. The effectiveness of these programs is difficult to assess because they vary widely, dropout rates are high, and few employ medical professionals. Programs that emphasize realistic goals, gradual progress, sensible and healthy eating, and exercise can be very helpful and are recommended by many physicians. Programs that promise instant weight loss or utilize severely restricted diets are not effective and, in some cases, can be dangerous.
For morbidly obese patients, dietary changes and behavior modification may be accompanied by bariatric surgery. Gastroplasty involves inserting staples to decrease the size of the stomach. Gastric banding is an inflatable band inserted around the upper stomach to create a small pouch and narrow passage into the remainder of the stomach. Although bariatric surgery has become less risky with innovations in equipment and surgical techniques, it is performed only on patients for whom supervised diet and exercise strategies have failed, who are at least 100 lb (45 kg) overweight or twice their ideal body weight, and whose obesity seriously threatens their health. Risks and possible complications include infections, hernias, and blood clots. Overall, 10%–20% of patients who undergo weight-loss surgery require additional operations to correct complications, more than 33% develop gallstones, and 30% develop nutritional deficiencies such as anemia, osteoporosis, or metabolic bone disease.
Other bariatric surgical procedures, including liposuction (a purely cosmetic procedure in which a suction device removes fat from beneath the skin) and jaw wiring that can damage gums and teeth and cause painful muscle spasms, have no place in obesity treatment.
Weight loss is recommended for obese children over age seven and for obese children over age two who have medical complications. Weight maintenance is an appropriate goal for children over the age of two who have no medical complications. Most treatment approaches to childhood obesity involve a combination of caloric restriction, physical exercise, and behavioral therapy. Bariatric surgery is considered as a last resort only for adolescents who are fully grown. Reports in 2016 showed that the surgery can improve the long-term health of severely obese teens.
The short-term use of prescription medications can assist some individuals in managing their condition, but it is never the sole treatment for obesity, nor are drugs ever considered as a cure for obesity. Diet drugs are designed to help medically at-risk obese patients jumpstart their weight-loss efforts and lose 10% or more of their starting body weight, in combination with a diet and exercise regimen. Prescription weight-loss drugs are approved by the US Food and Drug Administration (FDA) only for patients with a BMI of 30 or above, or a BMI of 27 or above and an obesity-related condition such as high blood pressure, type 2 diabetes, or dyslipidemia (abnormal amounts of fats in the blood). The weight is usually regained as soon as the drugs are discontinued, unless eating and exercise habits have changed.
Most appetite suppressants are based on amphetamine. They increase levels of serotonin or catecholamine, brain chemicals that control feelings of fullness. Serotonin also regulates mood and may be linked to mood-related eating behaviors. Prescription weight-loss medications include:
These drugs can cause unwanted and even serious side effects and should be taken only as directed. Although most of the immediate side effects of appetite suppressants are harmless, their long-term effects are less apparent. Dexfenfluramine hydrochloride (Redux), fenfluramine (Pondimin), and the fenfluramine-phentermine combination (Fen/Phen) were taken off the market after they were shown to cause potentially fatal cardiac effects. Phenylpropanolamine, a component of many nonprescription weight-loss and cold and cough medications (Acutrim, Dex-A-Diet, Dexatrim, Phenldrine, Phenoxine, PPA, Propagest, Rhindecon, Unitrol) was removed from shelves because of an increased risk of stroke. Appetite suppressants can be habit-forming and have the potential for abuse. They should not be used by patients taking monoamine oxidase inhibitors (MAOIs) and are not recommended for children.
Side effects of prescription and over-the-counter weight-loss products may include:
Unlike appetite suppressants, orlistat is a lipase inhibitor that reduces the breakdown and absorption of dietary fat in the intestines. It is available over the counter (without a prescription). Side effects of orlistat may include abdominal cramping, gas, fecal urgency, oily stools, frequent bowel movements, and diarrhea.
Other drugs are sometimes prescribed off-label for treating obesity. For example, fluoxetine (Prozac) is an antidepressant that sometimes aids in temporary weight loss. Side effects of this medication include diarrhea, fatigue, insomnia, nausea, and thirst.
Functional food diets are newer, as yet unproven, approaches to weight loss. These include:
Various herbs and supplements are promoted for weight loss. Some of these include:
Acupressure and acupuncture can suppress food cravings. Visualization and meditation can create and reinforce a positive self-image that can enhance a patient's determination to lose weight. By improving physical strength, mental concentration, and emotional serenity, yoga can provide the same benefits. Patients who play soft slow music during meals often find that they eat less food but enjoy it more.
Eating the correct ratios of protein, carbohydrates, and high-quality fats is important for weight loss. It also is important to replace high-calorie choices with natural foods, namely fruits and vegetables. Support and self-help groups, such as Overeaters Anonymous and TOPS (Taking Off Pounds Sensibly), that promote nutritious, balanced diets can help patients maintain proper eating regimens.
Fad dieting can have harmful health effects. Weight should be lost gradually and steadily by decreasing calories while maintaining an adequate nutrient intake and level of physical activity. A daily caloric intake of 1,000–1,200 calories for women and 1,200–1,600 for men enables most people to lose weight safely. A loss of about 2 lb (1 kg) per week is recommended. Diets of less than 800 calories a day should never be attempted unless prescribed and monitored by a physician.
At least 60–90 minutes of daily moderate-intensity physical activity is recommended to maintain weight loss. Obese people who have led sedentary lives may need monitoring to avoid injury as they begin to increase their physical activity. Exercise should be increased gradually, perhaps starting by climbing stairs instead of taking elevators, followed by walking, biking, or swimming at a slow pace. Eventually, 15-minute walks can be built up to brisk, 45–60-minute walks.
The American Academy of Family Physicians offers advice for families with children who need to maintain or lose weight. This advice includes:
The primary factor in achieving and maintaining weight loss is a lifelong commitment to sensible eating habits and regular exercise. As many as 85% of dieters who do not exercise on a regular basis regain their lost weight within two years and 90% regain it within five years. Short-term diet programs and repeatedly losing and regaining weight encourage the storage of fat and may increase the risk of heart disease.
Prudent dieting and exercise are not quick cures for obesity. With decreased caloric intake, the body breaks down muscle for carbohydrates. Much of the early weight loss on a very low-calorie diet represents loss of muscle tissue rather than fat. Similarly, fat is not easily accessed as fuel for exercise.
The chronically or habitually obese tend to come from families with a larger number of risk factors for obesity and have a much more difficult time losing weight than the newly obese. Likewise, previously obese people have a high probability of reverting to obesity.
When obesity develops in childhood, the total number of fat cells increases (hyperplastic obesity), whereas in adulthood the total amount of fat in each cell increases (hypertrophic obesity). Patients who were obese as children may have up to five times as many fat cells as a patient who became obese as an adult. Decreasing the amount of energy (food) consumed or increasing the amount of energy expended reduces the amount of fat in the cells, but does not reduce the number of fat cells already present, and this process is slow, just like the accumulation of excess fat.
Neonatal obesity does not necessarily translate into childhood or adult obesity, but the probability increases if the child is born or adopted into a family with multiple obese members. Likewise, excess weight in a child under age three does not necessarily predict adult obesity unless one of the parents is obese.
Summer camps for habitually obese children, especially girls, have little long-term success in reducing obesity and a high degree of recidivism for habitual overeating and underexercising. About 30% of overweight girls eventually develop eating disorders.
According to the Obesity Prevention Center at the University of Minnesota, obesity-control programs that rely on educational messages encouraging greater physical activity and a healthier diet have been only modestly successful. The best outcomes have been with children's programs that have high levels of physical activity.
Prevention is far superior to any available treatment for obesity. Obesity can be prevented by eating a healthy diet, being physically active, and making lifestyle changes that help maintain a normal weight. Examples include:
Obesity experts suggest that monitoring fat consumption, as well as counting calories, is a key to preventing excess weight gain. Fewer than 30% of calories should be from fat and only one-third of those should be saturated fats. High concentrations of saturated fats are found in meat, poultry, and dairy products. Fat replacers or substitutes are now added to many foods. They reduce the amount of fat and usually reduce the number of calories.
Total caloric intake cannot be ignored because it is usually the slow accumulation of excess calories, regardless of the source, that results in obesity. A single daily cookie providing 25 excess calories will result in a 5-lb. weight gain by the end of one year. Because most people eat more than they think they do, keeping a detailed and honest food diary on paper or in an app is a useful way to assess eating habits. Eating three balanced, moderate-portion meals a day, with the main meal at midday, is a more effective way to prevent obesity than fasting or crash diets that trick the body into believing it is starving. After 12 hours without food, the body has depleted its stores of readily available energy. It then begins to protect itself for the long term. Metabolic rate starts to slow and muscle tissue is broken down for the raw materials needed for energy maintenance.
The U.S. Department of Agriculture (USDA) introduced the MyPlate program to replace the long-time food pyramid called MyPyramid. Although the new symbol looks very different than the old one, it still contains recommendations for a diet based on the Dietary Guidelines for Americans. The first use of the new symbol followed 2010 guidelines. The USDA later released its 2015–2020 dietary guidelines, which are based largely on scientific evidence. The guidelines recommend at least an hour of physical activity a day for children age 6–17 years and at least 150 minutes a week of moderately intense activity for adults (or 75 minutes of vigorous exercise). Increasing activity adds to health.
The dietary guidelines focus on five key recommendations:
Adults age 65 and older should follow typical activity guidelines, choosing activities that improve balance and that match their fitness level. Studies have shown that weight loss in seniors can lower the incidence of arthritis, diabetes, and other conditions; reduce cardiovascular risk factors; and improve wellbeing. Increased physical activity in the elderly also improves muscle strength and endurance.
The poor prognosis for reversing adult obesity makes childhood prevention imperative. Unhealthy eating patterns and behaviors associated with obesity can be addressed by programs in nutrition, exercise, and stress management involving the entire family.
See also Calories ; Fat .
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Rosalyn Carson-DeWitt, MD
Revised by William Atkins, BB, BS, MBA
Revised by Teresa Odle, BA, ELS