Obesity

ByShauna M. Levy, MD, MS, Tulane University School of Medicine;
Michelle Nessen, MD, Tulane University School of Medicine
Reviewed/Revised Nov 2023 | Modified Dec 2023
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Obesity is a chronic, recurring complex disorder characterized by excess body weight.

  • Obesity is influenced by a combination of factors that includes genetics, hormones, behavior, and the environment.

  • Having the disease of obesity increases the risk of many disorders, such as diabetes, high blood pressure, heart disease, and certain cancers, and can result in early death.

  • Increasing activity and reducing caloric intake are important components of treating obesity.

  • Medications and weight-loss (bariatric) surgery are also important for long-term successful treatment for many people with obesity.

  • Losing as little as 5 to 10% of body weight can help lessen weight-related problems, such as diabetes, high blood pressure, and high cholesterol levels.

(See also Obesity in Adolescents.)

The body mass index (BMI) is used to define overweight and obesity. BMI is weight (in kilograms) divided by height (in meters squared):

  • Overweight is defined as a BMI of 25 to 29.9.

  • Obesity is defined as a BMI of 30 to 39.9.

  • Severe obesity is defined as a BMI of 40 or higher.

For people who are of Asian descent and some other ethnic groups, the BMIs that are considered normal and overweight are slightly lower. Definitions for children and adolescents are also different.

BMI does not distinguish between muscle (lean) and fat tissue. Thus, based on BMI alone, some people may be diagnosed with obesity when their percentage of body fat is very low. For example, some people, such as body builders, have a high BMI because they have a large amount of muscle (which weighs more than fat), even though they have very little fat. Such people are not considered to have obesity.

Obesity has become increasingly common throughout the world. In the United States, obesity is very common and has nearly doubled since the late 1970s. The national obesity rate for adults between 2017 and 2020 was 41.9%. The national youth obesity rate was 19.7% over this time frame. Also, severe obesity has become more common.

Obesity is much easier to prevent than treat. Once people gain excess weight, the body resists losing weight. For example, when people diet or reduce the number of calories they consume, the body compensates by increasing appetite and reducing the number of calories burned during rest.

Causes of Obesity

Obesity results from a combination of factors, including the reduced opportunity for physical activity, the increased availability of high-calorie foods, and the presence of genes that make obesity more likely. But ultimately, obesity results from consuming more calories than the body needs over a long period of time. Excess calories are stored in the body as fat (adipose tissue).

The number of calories needed varies from person to person, depending on age, sex, activity level, and metabolic rate. A person’s resting (basal) metabolic rate—the amount of calories the body burns while at rest—is determined by the amount of muscle (lean) tissue a person has and the person's total body weight. The more muscle people have, the higher their metabolic rate.

Changes in the bacteria that are normally present in the digestive system (called gut flora) may increase the risk of obesity. Normally, these bacteria help the body digest food among other things. Changes in the number and types of bacteria in the digestive system may change how the body processes food.

Where a person lives can affect lifestyle choices and behavior. Some communities do not have access to fresh fruits and vegetables. These communities tend to have a higher rate of obesity. Access to safe recreational spaces (parks and biking lanes) help encourage physical activity. Taking public transportation, instead of driving, can also help because it involves more walking and less sitting.

Obesogens are chemical compounds that disrupt normal development and metabolism (for example, cigarette smoke, bisphenol A, air pollution, flame retardants, phthalates, polychlorinated biphenyls). Being exposed to obesogens early in life can increase the risk of developing obesity.

Physical inactivity

In technologically advanced countries, lack of physical activity is common and contributes to the increase in obesity. Opportunities for physical activity have been engineered away by technological advances, such as elevators, cars, and remote controls. More time is spent doing sedentary activities, such as using the computer, watching television, and playing video games. Also, people’s jobs have become more sedentary as office or desk jobs have replaced manual labor. Sedentary people use fewer calories than more active people and thus require fewer calories in the diet. If caloric intake is not reduced accordingly, people gain weight.

Diet

Energy-dense foods, which are foods that have a large number of calories in a relatively small amount (volume), promote weight gain. Most of these foods contain more processed carbohydrates, more fat, and less fiber. Fats, by nature, are energy dense. Fat has 9 calories per gram, but carbohydrates and proteins have 4 calories per gram. Energy-dense foods are common in technologically advanced countries.

Convenience foods, such as energy-dense snacks offered at vending machines and fast food restaurants, contribute to the increase in obesity. High-calorie beverages, including soda, juices, many coffee drinks, and alcohol, also contribute significantly. For example, a 12-ounce soda or bottle of beer has 150 calories, and a 12-ounce coffee beverage (containing dairy and sugar) or fruit smoothie can have 500 or more calories. High-fructose corn syrup (used to sweeten many bottled beverages) is often singled out as being particularly likely to cause obesity. However, recent studies show that it is no more likely to cause obesity than other foods with a similar number of calories in sugar.

Larger portion sizes at restaurants and in packaged foods and beverages encourage people to overeat. Also, restaurant and packaged foods are often prepared in ways that add calories. As a result, people may consume more calories than they realize.

Genes

Obesity tends to run in families. Genes help determine body mass index (BMI) in over 60% of people. However, families share not only genes but also environment, and separating the two influences is difficult. Genes can affect how quickly the body burns calories at rest and during exercise. They can also affect appetite and thus how much food is consumed. Genes may have a greater effect on where body fat accumulates, particularly fat around the waist and in the abdomen, than on how much body fat accumulates.

Many genes influence weight, but each gene has only a very small effect. Obesity rarely results when only one gene is abnormal.

Rarely, mutations in the following genes result in obesity:

  • The gene for the melanocortin 4 receptor: Receptors are structures on the surface of cells that inhibit or produce an action in the cell when certain substances (such as chemical messengers) bind with them. Melanocortin 4 receptors are located mainly in the brain. They help the body regulate its use of energy. A mutation in this gene may account for obesity in 1 to 4% of children.

  • The ob gene: This gene controls the production of leptin, a hormone made by fat cells. Leptin travels to the brain and interacts with receptors in the hypothalamus (the part of the brain that helps regulate appetite). The message carried by leptin is to decrease food intake and increase the amount of calories (energy) burned. A mutation in the ob gene prevents leptin production and results in severe obesity in a very small number of children. In these cases, administration of leptin reduces weight to a normal amount.

Background

Certain characteristics can increase the risk of becoming overweight or obese. They include the following:

  • Certain racial and ethnic backgrounds, such as Black, Hispanic, and Pacific Islander

  • A lower education level

  • Obesity during childhood, which tends to persist into adulthood

Adverse childhood events or a childhood history of verbal, physical, or sexual abuse are associated with a higher risk of obesity. According to the Centers for Disease Control and Prevention's adverse childhood events study, a history of verbal, physical, or sexual abuse during childhood increases the risk of obesity by 8% and the risk of severe obesity by 17.3%. 

Pregnancy and menopause

Gaining weight during pregnancy is normal and necessary. However, pregnancy can be the beginning of weight problems if women do not return to their prepregnancy weight. About 15% of women permanently gain 20 pounds or more with each pregnancy. Having several children close together may compound the problem. Breastfeeding can help women return to their prepregnancy weight.

If a pregnant woman is obese or smokes, weight regulation may be disturbed, contributing to weight gain during childhood and later.

After menopause, many women gain weight. This weight gain may result from reduced activity. Hormonal changes may cause fat to be redistributed and accumulate around the waist. Fat in this location increases the risk of health problems (such as metabolic syndrome).

Aging

Obesity becomes more common as people age (see ). As people age, body composition may change as muscle tissue decreases. The result is a higher percentage of body fat and a lower basal metabolic rate (because muscle burns more calories).

Lifestyle

Sleep deprivation or lack of sleep (usually considered less than 6 to 8 hours per night) can result in weight gain. Sleeplessness results in hormonal changes that increase appetite and cravings for energy-dense foods.

Stopping smoking usually results in weight gain and may deter people from stopping smoking. Nicotine decreases appetite and increases the metabolic rate. When nicotine is stopped, people may eat more food, and their metabolic rate decreases, so that fewer calories are burned. As a result, body weight may increase by 5 to 10%.

Hormones

Hormonal disorders rarely cause obesity. The following are among the most common:

  • Cushing syndrome is caused by excessive levels of cortisol in the body. The syndrome can result from a benign tumor in the pituitary gland (pituitary adenoma) or from a tumor in the adrenal gland or elsewhere, such as in the lungs. Cushing syndrome typically causes fat to accumulate in the face, making it look full (called moon face), and behind the neck (called a buffalo hump).

  • Polycystic ovary syndrome affects about 5 to 10% of women. Affected women tend to be overweight or obese. Levels of testosterone and other male hormones are increased, causing fat to accumulate in the waist and abdomen, which is more harmful than the fat that is distributed throughout the body.

Did You Know...

  • Hormonal disorders rarely cause obesity.

Eating disorders

Two eating disorders are associated with obesity:

  • Binge eating disorder is characterized by bingeing—eating large amounts of food during a short amount of time—and usually by feeling guilty, remorseful, or out of control. Most affected people do not purge (for example, by vomiting or using laxatives or diuretics). Binge eating disorder is diagnosed when bingeing episodes occur at least twice a week for 6 or more months.

  • Night-eating syndrome

Medications

Many medications used to treat common disorders promote weight gain. These medications include those used to treat the following:

Symptoms of Obesity

The most obvious symptom of obesity is a change in the person's appearance.

Complications

Having obesity increases the risk of many health problems. Virtually every organ system can be affected. These weight-related health problems can cause symptoms, such as shortness of breath, difficulty breathing during activity, snoring, skin abnormalities including stretch marks, and joint and back pain.

Obesity increases the risk of the following:

Obstructive sleep apnea can develop if excess fat in the neck compresses the airway during sleep. Breathing stops for a few moments, as often as hundreds of times a night. This disorder is often undiagnosed. It can cause loud snoring and excessive daytime sleepiness and increases the risk of high blood pressure, abnormal heart rhythms, metabolic syndrome, heart attacks, heart failure, and strokes.

Obesity can increase the risk of early death. The more severe the obesity, the higher the risk. It is the second most common cause of preventable death (cigarette smoking is the most common). Studies indicate that over a period of 15 years, the death rate is 30% lower for people who had weight loss surgery than for people who had not had it.

Obesity can lead to social, economic, and psychologic problems. For example, people with obesity may be underemployed or unemployed, or they may have a poor body image and low self-esteem.

If untreated, obesity tends to worsen, increasing the risk and severity of complications.

After weight loss, most people return to their pretreatment weight within 5 years. Also, when weight-loss medications are stopped, weight tends to be gained back.

Diagnosis of Obesity

  • Body mass index (BMI)

  • Waist circumference

  • Determination of body composition

Obesity is diagnosed by determining the body mass index (BMI). However, BMI has some limitations. The BMI does not take sex and age into consideration and makes only a few adjustments based on ethnic group. For people who are of Asian descent and some other ethnic groups, the BMI that is considered overweight is slightly lower.

Also, the BMI does not distinguish between lean and fat tissue. Therefore, doctors may be unsure whether a high BMI is due to muscle (for example, in body builders) or excessive fat. In such cases, they determine body composition (the percentage of body fat and muscle).

Waist circumference is measured. This measurement helps identify and quantify abdominal (visceral) obesity, which is fat that accumulates around the waist and in the abdomen. Abdominal obesity is much more harmful than fat that is distributed throughout the body under the skin (subcutaneous fat). Knowing how large the waist is and whether metabolic syndrome is present helps doctors estimate the risk of certain complications (such as heart disorders) better than knowing what the person's BMI is.

Body composition (the percentage of body fat and muscle) can be determined using the following:

  • Measurement of skinfold thickness and the circumference of the upper arm

  • Bioelectric impedance, which can be done in a doctor’s office

  • Underwater (hydrostatic) weighing

Skinfold thickness is usually measured over the triceps, at the back of the upper arm. The skinfold is the skin and layer of fat under it that are measured by pinching the skin.

Bioelectrical impedance analysis estimates the percentage of total body water directly and determines the percentage of body fat indirectly. It is most reliable in healthy people and in people with only a few chronic disorders, such as moderate obesity or diabetes mellitus.

Underwater weighing is the most accurate method for measuring percentage of body fat. However, it is costly and time-consuming. Thus, it is used more often in research than in clinical care.

Men are considered to have obesity when the body fat level is > 25%. In women, the level is > 32%.

Typically, blood tests are done. Blood sugar (glucose) is measured to check for prediabetes or diabetes, and cholesterol and other fat levels are measured to check for high cholesterol and other abnormal fat levels. Doctors also measure blood pressure to check for high blood pressure. These tests help doctors determine whether people have metabolic syndrome (which includes all three disorders).

Doctors also check for other disorders that are common among people who have obesity, such as obstructive sleep apnea, fatty liver, and depression.

Treatment of Obesity

  • Diet

  • Physical activity

  • Changes in behavior

  • Weight-loss medications

  • Metabolic and bariatric surgery

Initially, the treatment for obesity involves changes in lifestyle, which includes changes in diet, increased physical activity, and changes in behavior. Medications and weight-loss (bariatric) surgery are also important for long-term weight loss and are often underused because people may have trouble getting access or being reimbursed by insurance companies or because of the doctor's or person's preferences.

Losing as little as 5 to 10% of body weight can help reduce the risk or severity of weight-related health problems, such as diabetes, high blood pressure, and high cholesterol levels.

Successful weight loss requires motivation and a sense of readiness. People who are most successful have realistic goals and recognize that healthy weight loss can be achieved only with lifelong lifestyle changes rather than a magic bullet or fad diet that cannot be sustained.

Seeking the support of health care practitioners such as dieticians or doctors can be beneficial. Support from family members is also crucial.

Programs that require regular contact, such as WW (formerly known as Weight Watchers), increase accountability and can increase the likelihood of success. Typically, weekly meetings are conducted by counselors and supplemented with instructional and guidance materials.

Because people tend to gain weight back when treatment ends, obesity requires a lifelong management program similar to that for any other chronic disorder.

Did You Know...

  • Losing as little as 5 to 10% of body weight can reduce weight-related health risks.

Changes in diet

Healthy, balanced eating for weight loss requires reducing the number of calories consumed and choosing a wide range of foods that provide good nutrition.

Reducing the number of calories consumed by 500 to 1,000 calories a day may result in a healthy rate of weight loss. This approach usually means consuming 1,200 to 1,500 calories a day. However, the body may adjust to the decrease in calories (for example, by decreasing the metabolic rate). Thus, weight loss may be less than expected. Still, consuming a high-fiber diet plus reducing the number of calories by about 600 calories a day and substituting some carbohydrate for protein appears to be the best way to lose weight and keep it off. Weight can be lost more rapidly with a very low calorie diet, but such diets should be supervised by a doctor.

The following changes in diet are recommended:

  • Eating small meals and avoiding or carefully choosing snacks

  • Eating breakfast (skipping breakfast can lead to consuming too many calories later in the day)

  • Eating 5 or more servings of fruits and vegetables a day

  • Substituting fresh fruits and vegetables and salads for refined carbohydrates and processed food

  • Eating lean protein—for example, fish or chicken breast or vegetable protein, such as soy

  • Switching from full-fat to no-fat or low-fat dairy products

  • Eliminating high-calorie beverages, such as soda, juice, or alcohol, and drinking water instead

  • Limiting consumption of restaurant and fast food

  • Limiting alcohol consumption

  • Switching from harmful fats (such as saturated and trans fats) to good fats, such as monounsaturated fats (in olive and canola oils) and polyunsaturated fats (in deep-sea fish and vegetable oils), and limiting the amount of fat consumed.

Eating foods with a low glycemic index and foods that contain fish oils (including deep-sea fish such as salmon and tuna) or monounsaturated fats derived from plants (such as olive oil) may reduce the risk of heart disorders and diabetes.

No-fat or low-fat dairy products, which provide vitamin D, should be included to help prevent a deficiency of this vitamin.

Using meal replacements, regularly or once in a while, can help some people lose weight and keep it off.

Physical activity

Increasing physical activity can help people lose weight in a healthy way and keep it off. Physical activity includes not only exercise (that is, structured physical activity) but also lifestyle activities, such as taking the stairs instead of the elevator, gardening, and walking instead of driving when possible. Lifestyle activities can burn a considerable number of calories. People who do not exercise while dieting are more likely to regain the weight they lose.

Aerobic exercise, such as jogging, walking briskly (3 to 4 miles [5 to 6 kilometers] an hour), biking, singles tennis, skating, and cross-country skiing, burn more calories than less active exercises (see Choosing the Right Exercise). For example, vigorous walking can burn about 4 calories per minute, so that 1 hour of brisk walking per day burns about 240 calories. Running burns about 6 to 8 calories per minute (about 360 to 480 calories per hour). As a general guide, people need to walk at least 150 minutes each week to promote health. To lose weight and keep it off, people need to spend 300 to 360 minutes each week doing moderate physical activity or 150 minutes each week doing vigorous aerobic exercise (such as running or using an elliptical machine). Other health benefits of vigorous aerobic exercise include reducing the risk of coronary artery disease and increasing endurance.

To get the most benefit from exercise, people should do strength training (with weights or another form of resistance) about 3 days of the week. Strength training increases the amount of muscle tissue, which increases the metabolic rate, so that the body burns more calories when at rest.

Changes in behavior

Ultimately, for weight loss to be effective and long-lasting, people must change their behavior. Weight-loss programs that help people change their behavior are the most effective. To change behavior, people need certain skills, such as

  • Problem solving

  • Stress management

  • Self-monitoring

  • Contingency management

  • Stimulus control

Problem solving involves identifying and planning ahead for situations that make unhealthy eating more likely (such as going out to dinner or traveling) or that reduce the opportunity for physical activity (such as driving cross country).

To manage stress, people can learn to identify stressful situations and develop ways to manage the stress that do not involve eating—for example, by going for a walk, meditating, or taking deep breaths.

To monitor themselves, people may keep a food log, including the number of calories in the foods, and weigh themselves regularly. They may record where and when they eat, what their mood is when they eat, and who is with them. With this information, they can observe and record patterns of behavior and eating and may be able to avoid situations that lead to weight gain or unhealthy eating.

Contingency management involves providing rewards (other than food) for behavior that contributes to weight loss or maintenance. For example, if people walk more or eat less of certain foods, they may reward themselves by getting new clothes or going to a movie. Rewards may also come from other people—for example, praise from family members or members of a support group. People can get support by using social media to connect with each other and health care professionals.

To control stimuli that can trigger unhealthy eating, people can learn to identify obstacles to healthy eating and an active lifestyle. Then they can develop strategies to overcome them. For example, people may avoid going by a fast food restaurant on their way to work or not keep sweets in the house. To develop an active lifestyle, they may take up an active hobby (such as gardening), walk more, make a habit of taking the stairs instead of elevators, or park at the far end of parking lots (resulting in a longer walk).

Internet resources, applications for mobile devices, and other technological devices may also help people develop an active lifestyle and maintain weight loss. Applications can help people set a weight-loss goal, monitor their progress, track food consumption, and record physical activity.

Medications

For people who have obesity or overweight and weight-related disorders, weight-loss medications (also called anti-obesity medications) can be useful. Medications are most effective when used with changes in diet, increased physical activity, and structured programs that include changes in behavior.

Some weight-loss medications are intended to be used for a short time. Others are intended to be used for a long time. Weight-loss medications should be stopped or changed if people do not lose weight after 12 weeks of treatment.

Anti-obesity medications that are currently available include

These medications are used if people have a body mass index (BMI) of 30 or higher or if people have a BMI of 27 or higher and have complications such as high blood pressure or diabetes.

fat-soluble vitamins: A, D, E, and K. If not enough vitamin D

reduces appetite by affecting chemical messengers in the part of the brain that controls appetite. It is available by prescription only. It is taken only for a short time. It can increase blood pressure and heart rate and cause insomnia, anxiety, and constipation.

(used to treat seizures and migraines) is available by prescription only. This combination results in weight loss for up to 2 years. However, it can cause birth defects, so women of childbearing age should take it only if they are using birth control and are tested monthly for pregnancy. These medications can cause problems with sleep and concentration and can increase heart rate.

NaltrexoneBupropion can also decrease appetite. Side effects of the combination medication include increased blood pressure, nausea, vomiting, and headache. People who have uncontrolled high blood pressure, who have had seizures, or who have a seizure disorder should not take this medication.

is used to treat type 2 diabetesmedullary thyroid carcinoma

is an injectable medication used to treat type 2 diabetesinsulinsemaglutide include nausea and diarrhea. Semaglutide should not be used by people who have had medullary thyroid cancer or have relatives who have had it. Also, people who have an endocrine system disorder called multiple endocrine neoplasia syndrome type 2 (MEN 2) should not take semaglutide.

Some over-the-counter diet aids, including medicinal herbs, claim to enhance weight loss by increasing metabolism or by increasing a feeling of fullness. These supplements have not been shown to be effective and may contain harmful additives or stimulants (such as ephedra, caffeine, guarana, and phenylpropanolamine) and should be avoided.

Obesity in Older Adults

In the United States, the percentage of older adults who have obesity has been increasing. Obesity in older adults is a concern because excess weight increases the risk of certain health problems that tend to become more common as people age: diabetes, cancer, abnormal levels of fats (lipids) in the blood (dyslipidemia), high blood pressure, heart failure, coronary artery disease, and joint disorders.

Several age-related changes contribute to gaining weight:

  • Decreased physical activity: Some reasons for decreased activity are related to aging. They include retiring, becoming physically unable to exercise, developing disorders that make movement painful (such as arthritis), and having problems with balance. Other factors may also limit physical activity. For example, people may not want to walk because there are no sidewalks, there is too much traffic, or safety concerns.

  • Loss of muscle tissue: Muscle tissue is lost partly because levels of growth hormone and sex hormones (estrogen in women and testosterone in men) decrease. But the main reason older adults lose muscle tissue is physical inactivity. The less muscle tissue people have, the fewer calories their body burns when resting and the easier it is to gain weight.

  • Increased body fat: When the amount of muscle tissue decreases, the percentage of fat in the body increases. Fat tissue burns fewer calories. Also, the higher percentage of fat means that older adults with a normal body mass index (BMI), which is based only on weight and height, may have a higher risk of weight-related health problems than expected. Waist circumference predicts health risks better than BMI in older adults.

  • Shifting of body fat to the waist: With aging, body fat tends to shift to the waist. Fat that accumulates around the waist and abdomen (as opposed to the hips and thighs) increases the risk of health problems, such as high blood pressure, diabetes, and coronary artery disease.

For older adults who need to lose weight, doctors recommend increasing physical activity and changing the diet. Physical activity improves muscle strength, endurance, and overall well-being and reduces the risk of developing chronic disorders such as diabetes. Activity should include strength training and endurance exercises.

Older adults are at greater risk of undernutrition than younger people. Therefore, when they try to lose weight, they should be sure to consume a healthy and balanced diet.

diabetes or high blood pressure who have overweight or obesity. Weight-loss (bariatric) surgery has been shown to be safe and effective for older adults who are functioning well.

Whether weight loss in older adults has health risks is controversial. Doctors help older adults devise weight-loss strategies based on their individual circumstances. In older adults, weight loss is best supervised by a doctor.

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