Obesity
Obesity is excess body weight, defined as a body mass index (BMI) of ≥ 30 kg/m2. Complications include cardiovascular disorders (particularly in people with excess abdominal fat), diabetes mellitus, certain cancers, cholelithiasis, fatty liver, cirrhosis, osteoarthritis, reproductive disorders in men and women, psychologic disorders, and, for people with BMI ≥ 35, premature death. Diagnosis is based on BMI. Treatment includes lifestyle modification (eg, in diet, physical activity, and behavior) and, for certain patients, drugs or bariatric (weight-loss) surgery.
Etiology of Obesity
Causes of obesity are probably multifactorial and include genetic predisposition. Ultimately, obesity results from a long-standing imbalance between energy intake and energy expenditure, including energy utilization for basic metabolic processes and energy expenditure from physical activity. However, many other factors appear to increase a person's predisposition to obesity, including endocrine disruptors (eg, bisphenol A [BPA]), gut microbiome, sleep/wake cycles, and environmental factors.
Genetic factors
Heritability of BMI is about 66%. Genetic factors may affect the many signaling molecules and receptors used by parts of the hypothalamus and gastrointestinal tract to regulate food intake. Genetic factors can be inherited or result from conditions in utero (called genetic imprinting). Rarely, obesity results from abnormal levels of peptides that regulate food intake (eg, leptin) or abnormalities in their receptors (eg, melanocortin-4 receptor).
Pathways Regulating Food Intake
Preabsorptive and postabsorptive signals from the gastrointestinal tract and changes in plasma nutrient levels provide short- and long-term feedback to regulate food intake:
Gastrointestinal hormones (eg, glucagon-like peptide 1 [GLP-1], cholecystokinin [CCK], peptide YY [PYY]) reduce food intake.
Genetic factors also regulate energy expenditure, including basal metabolic rate, diet-induced thermogenesis, and nonvoluntary activity–associated thermogenesis. Genetic factors may have a greater effect on the distribution of body fat, particularly abdominal fat (which increases the risk of metabolic syndrome), than on the amount of body fat.
Environmental factors
Weight is gained when caloric intake exceeds energy needs. Important determinants of energy intake include
Portion sizes
The energy density of the food
High-calorie foods (eg, processed foods), diets high in refined carbohydrates, and consumption of soft drinks, fruit juices, and alcohol promote weight gain. Diets high in fresh fruit and vegetables, fiber, complex carbohydrates, and lean proteins, with water as the main fluid consumed, minimize weight gain.
A sedentary lifestyle promotes weight gain.
Regulatory factors
Prenatal maternal obesity, prenatal maternal smoking, and intrauterine growth restriction can disturb weight regulation and contribute to weight gain during childhood and later. Obesity that persists beyond early childhood makes weight loss in later life more difficult.
The composition of the gut microbiome also appears to be an important factor; early use of antibiotics and other factors that alter the composition of the gut microbiome may promote weight gain and obesity later in life
Early exposure to obesogens, a type of endocrine-disrupting chemical (eg, cigarette smoke, bisphenol A, air pollution, flame retardants, phthalates, polychlorinated biphenyls) can alter metabolic set points through epigenetics or nuclear activation, increasing the propensity of developing obesity ( 2).
Adverse childhood events or abuse in early childhood increase risk of several disorders, including obesity. The Centers for Disease Control and Prevention's adverse childhood events study demonstrated that childhood history of verbal, physical, or sexual abuse predicted an 8% increase risk of BMI ≥ 30 and 17.3% of BMI ≥ to 40. Certain types of abuse carried the strongest risk. For example, frequent verbal abuse had the largest increase in risk (88%) for BMI > 40. Being often hit and injured increased the risk of BMI > 30 by 71% ( 3). Cited mechanisms for the association between abuse and obesity include neurobiologic and epigenetic phenomena ( 4).
About 15% of women permanently gain ≥ 20 lb with each pregnancy.
Insufficient sleep (usually considered < 6 to 8 hours/night) can result in weight gain by changing the levels of satiety hormones that promote hunger.
Drugs, including corticosteroids, lithium, traditional antidepressants (tricyclics, tetracyclics, monoamine oxidase inhibitors [MAOIs]), benzodiazepines, antiseizure drugs, thiazolidinediones (eg, rosiglitazone, pioglitazone), beta-blockers, and antipsychotic drugs, can cause weight gain.
Uncommonly, weight gain is caused by one of the following disorders:
- Brain damage caused by a tumor (especially a craniopharyngioma) or an infection (particularly those affecting the hypothalamus), which can stimulate consumption of excess calories
- Hyperinsulinism due to pancreatic tumors
- Hypercortisolism due to Cushing syndrome, which causes predominantly abdominal obesity
- Hypothyroidism (rarely a cause of substantial weight gain)
- Eating disorders
- At least 2 pathologic eating patterns may be associated with obesity:
- Binge eating disorder is consumption of large amounts of food quickly with a subjective sense of loss of control during the binge and distress after it. This disorder does not include compensatory behaviors, such as vomiting. Binge eating disorder occurs in about 3.5% of women and 2% of men during their lifetime and in about 10 to 20% of people entering weight reduction programs. Obesity is usually severe, large amounts of weight are frequently gained or lost, and pronounced psychologic disturbances are present.
- Night-eating syndrome consists of morning anorexia, evening hyperphagia, and insomnia, with eating in the middle of the night. At least 25 to 50% of daily intake occurs after the evening meal. About 10% of people seeking treatment for severe obesity may have this disorder. Rarely, a similar disorder is induced by use of a hypnotic such as zolpidem.
- Similar but less extreme patterns probably contribute to excess weight gain in more people. For example, eating after the evening meal contributes to excess weight gain in many people who do not have night-eating syndrome.
Complications of Obesity
Complications of obesity include the following:
- Metabolic syndrome
- Diabetes mellitus
- Cardiovascular disorders
- Liver disorders ( nonalcoholic steatohepatitis [fatty liver], which may lead to cirrhosis)
- Gallbladder disease ( cholelithiasis)
- Gastroesophageal reflux
- Obstructive sleep apnea
- Reproductive system disorders, including infertility in both sexes and a low serum testosterone level in men; obesity is a risk factor for polycystic ovary syndrome in women
- Many cancers (especially colon cancer and breast cancer)
- Osteoarthritis
- Tendon and fascial disorders
- Skin disorders (eg, intertriginous infections)
- Social, economic, and psychologic problems
- Depression, anxiety, low self-esteem, poor body image, stigma, and discrimination
- Insulin resistance, dyslipidemias, and hypertension (metabolic syndrome) can develop, often leading to diabetes mellitus and coronary artery disease. These complications are more likely in patients with fat that is concentrated abdominally (visceral fat), a high serum triglyceride level, a family history of type 2 diabetes mellitus or premature cardiovascular disease, or a combination of these risk factors.
- Obstructive sleep apnea can result if excess fat in the neck compresses the airway during sleep. Breathing stops for moments, as often as hundreds of times a night. This disorder, often undiagnosed, can cause loud snoring and excessive daytime sleepiness and increases the risk of hypertension, cardiac arrhythmias, and metabolic syndrome.
- Obesity may cause the obesity-hypoventilation syndrome (Pickwickian syndrome). Impaired breathing leads to hypercapnia, reduced sensitivity to carbon dioxide in stimulating respiration, hypoxia, cor pulmonale, and risk of premature death. This syndrome may occur alone or secondary to obstructive sleep apnea.
- Skin disorders are common; increased sweat and skin secretions, trapped in thick folds of skin, are conducive to fungal and bacterial growth, making intertriginous infections especially common.
- Being overweight probably predisposes to gout, deep venous thrombosis, and pulmonary embolism.
- Obesity leads to social, economic, and psychologic problems as a result of prejudice, discrimination, poor body image, and low self-esteem. For example, people may be underemployed or unemployed.
Diagnosis of Obesity
- Body mass index (BMI)
- Waist circumference
- Sometimes body composition analysis
- In adults, BMI, defined as weight (kg) divided by the square of the height (m2), is used to screen for overweight or obesity (see table Body Mass Index):
Overweight = 25 to 29.9 kg/m2
Obesity = ≥ 30 kg/m2
However, BMI is a crude screening tool and has limitations in many subpopulations. Some experts think that BMI cutoffs should vary based on ethnicity, sex, and age. For example, in certain nonwhite populations, complications of obesity develop at a much lower BMI than in whites.
In children and adolescents, overweight is defined as BMI at the ≥ 95th percentile, based on the Centers for Disease Control and Prevention's age- and sex-specific growth charts.
Body Mass Index (BMI)
Asians and many aboriginal populations have a lower cut-off (23 kg/m2) for overweight. In addition, BMI may be high in muscular athletes, who lack excess body fat, and may be normal or low in formerly overweight people who have lost muscle mass.
Waist circumference and the presence of metabolic syndrome appear to predict risk of metabolic and cardiovascular complications better than BMI does.
The waist circumference that increases risk of complications due to obesity varies by ethnic group and sex.
Body composition analysis
Body composition—the percentage of body fat and muscle—is also considered when obesity is diagnosed. Although probably unnecessary in routine clinical practice, body composition analysis can be helpful if clinicians question whether elevated BMI is due to muscle or excessive fat.
The percentage of body fat can be estimated by measuring skinfold thickness (usually over the triceps) or determining mid upper arm muscle area.
Bioelectrical impedance analysis (BIA) can estimate percentage of body fat simply and noninvasively. BIA estimates percentage of total body water directly; percentage of body fat is derived indirectly. BIA is most reliable in healthy people and in people with only a few chronic disorders that do not change the percentage of total body water (eg, moderate obesity, diabetes mellitus). Whether measuring BIA poses risks in people with implanted defibrillators is unclear.
Underwater (hydrostatic) weighing is the most accurate method for measuring percentage of body fat. Costly and time-consuming, it is used more often in research than in clinical care. To be weighed accurately while submerged, people must fully exhale beforehand.
Imaging procedures, including CT, MRI, and dual-energy x-ray absorptiometry (DXA), can also estimate the percentage and distribution of body fat but are usually used only for research.
Other testing
Obese patients should be screened for common comorbid disorders, such as obstructive sleep apnea, diabetes, dyslipidemia, hypertension, fatty liver, and depression. Screening tools can help; for example, for obstructive sleep apnea, clinicians can use an instrument such as the STOP-BANG questionnaire (see table STOP-BANG Risk Score for Obstructive Sleep Apnea) and often the apnea-hypopnea index (total number of apnea or hypopnea episodes occurring per hour of sleep). Obstructive sleep apnea is often underdiagnosed, and obesity increases the risk.
Prognosis of Obesity
Untreated, obesity tends to progress. The probability and severity of complications are proportional to
- The absolute amount of fat
- The distribution of the fat
- Absolute muscle mass
- After weight loss, most people return to their pretreatment weight within 5 years, and accordingly, obesity requires a lifelong management program similar to that for any other chronic disorder.
Treatment of Obesity
- Dietary management
- Physical activity
- Behavioral interventions
- Drugs (eg, phentermine, orlistat, lorcaserin [not available in US because of possible cancer risk], phentermine/topiramate, naltrexone/bupropion extended-release, liraglutide, semaglutide)
- Bariatric surgery
- Weight loss of even 5 to 10% improves overall health, helps reduce risk of developing cardiovascular complications (eg, hypertension, dyslipidemia, insulin resistance) and helps lessen their severity, and may lessen the severity of other complications and comorbid disorders such as obstructive sleep apnea, fatty liver, infertility, and depression.
- Support from health care practitioners, peers, and family members and various structured programs can help with weight loss and weight maintenance.
- Diet
- Balanced eating is important for weight loss and maintenance.
Strategies include
- Eating small meals and avoiding or carefully choosing snacks
- Substituting fresh fruits and vegetables and salads for refined carbohydrates and processed food
- Substituting water for soft drinks or juices
- Limiting alcohol consumption to moderate levels
- Including no- or low-fat dairy products, which are part of a healthy diet and help provide an adequate amount of vitamin D
- Low-calorie, high-fiber diets that modestly restrict calories (by 600 kcal/day) and that incorporate lean protein appear to have the best long-term outcome. Foods with a low glycemic index (see table Glycemic Index of Some Foods) and marine fish oils or monounsaturated fats derived from plants (eg, olive oil) reduce the risk of cardiovascular disorders and diabetes.
- Use of meal replacements can help with weight loss and maintenance; these products can be used regularly or intermittently.
- Diets that are overly restrictive are unlikely to be maintained or to result in long-term weight loss. Diets that limit caloric intake to < 50% of basal energy expenditure (BEE), described as very low calorie diets, can have as few as 800 kcal/day. A very low calorie diet may be indicated for obese patients; however, such diets must be supervised by a physician, and after weight is lost, intake must be increased gradually to prevent patients from regaining weight.
Physical activity
Exercise increases energy expenditure, basal metabolic rate, and diet-induced thermogenesis. Exercise also seems to regulate appetite to more closely match caloric needs. Other benefits associated with physical activity include
- Increased insulin sensitivity
- Improved lipid profile
- Lower blood pressure
- Better aerobic fitness
- Improved psychologic well-being
- Decreased risk of breast and colon cancer
- Increased life expectancy
- Exercise, including strengthening (resistance) exercises, increases muscle mass. Because muscle tissue burns more calories at rest than does fat tissue, increasing muscle mass produces lasting increases in basal metabolic rate. Exercise that is interesting and enjoyable is more likely to be sustained. A combination of aerobic and resistance exercise is better than either alone. Guidelines suggest physical activity of 150 minutes/week for health benefits and 300 to 360 minutes/week for weight loss and maintenance. Developing a more physically active lifestyle can help with weight loss and maintenance.
Behavioral interventions
Clinicians can recommend various behavioral interventions to help patients lose weight. They include
- Support
- Self-monitoring
- Stress management
- Contingency management
- Problem solving
- Stimulus control
- Support may come from a group, a buddy, or family members. Participation in a support group can improve adherence to lifestyle changes and thus increase weight loss. The more frequently people attend group meetings, the greater the support, motivation, and supervision they receive and the greater their accountability, resulting in greater weight loss.
Self-monitoring may include keeping a food log (including the number of calories in foods), weighing regularly, and observing and recording behavioral patterns. Other useful information to record includes time and location of food consumption, the presence or absence of other people, and mood. Clinicians can provide feedback about how patients may improve their eating habits.
Stress management involves teaching patients to identify stressful situations and to develop strategies to manage stress that do not involve eating (eg, going for a walk, meditating, deep breathing).
Contingency management involves providing tangible rewards for positive behaviors (eg, for increasing time spent walking or reducing consumption of certain foods). Rewards may be given by other people (eg, from members of a support group or a health care practitioner) or by the person (eg, purchase of new clothing or tickets to a concert). Verbal rewards (praise) may also be useful.
Problem solving involves identifying and planning ahead for situations that increase the risk of unhealthy eating (eg, travelling, going out to dinner) or that reduce the opportunity for physical activity (eg, driving across country).
Stimulus control involves identifying obstacles to healthy eating and an active lifestyle and developing strategies to overcome them. For example, people may avoid going by a fast food restaurant or not keep sweets in the house. For a more active lifestyle, they may take up an active hobby (eg, gardening), enroll in scheduled group activities (eg, exercise classes, sports teams), walk more, make a habit of taking the stairs instead of elevators, and 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 with adherence to lifestyle changes and weight loss. Applications can help patients set a weight-loss goal, monitor their progress, track food consumption, and record physical activity.
Risk of obesity persisting into adulthood depends partly on when obesity first develops:
During infancy: Low risk
Between 6 months and 5 years: 25%
After 6 years: > 50%
During adolescence if a parent is obese: > 80%
In children, preventing further weight gain, rather than losing weight, is a reasonable goal. Diet should be modified, and physical activity increased. Increasing general activities and play is more likely to be effective than a structured exercise program. Participating in physical activities during childhood may promote a lifelong physically active lifestyle. Limiting sedentary activities (eg, watching TV, using the computer or handheld devices) can also help. Drugs and surgery are avoided but, if complications of obesity are life threatening, may be warranted.
Measures that control weight and prevent obesity in children may have the largest public health benefits. Such measures should be implemented in the family, schools, and primary care programs.
Older adults
In the US, the percentage of obese older people has been increasing.
With age, body fat increases and is redistributed to the abdomen, and muscle mass is lost, largely because of physical inactivity, but decreased androgens and growth hormone (which are anabolic) and inflammatory cytokines produced in obesity may also play a role.
Risk of complications depends on
Body fat distribution (increasing with a predominantly abdominal distribution)
Duration and severity of obesity
Associated sarcopenia
Increased waist circumference, suggesting abdominal fat distribution, predicts morbidity (eg, hypertension, diabetes mellitus, coronary artery disease) and mortality risk better in older adults than does BMI. With aging, fat tends to accumulate more in the waist.
For older adults, physicians may recommend that caloric intake be reduced and physical activity be increased. However, if older patients wish to substantially reduce their caloric intake, their diet should be supervised by a physician. Physical activity also improves muscle strength, endurance, and overall well-being and reduces the risk of developing chronic disorders such as diabetes. Activity should include strengthening and endurance exercises.
Regardless of whether caloric restriction is considered necessary, nutrition should be optimized.
Weight-loss drugs are often not studied specifically in older adults, and possible benefits may not outweigh the adverse effects. However, orlistat may be useful for obese older patients, particularly those with diabetes mellitus or hypertension. Surgery can be considered in healthy older patients with good functional status.
Prevention of Obesity
Regular physical activity and healthy eating improve general fitness, can control weight, and help prevent diabetes mellitus and obesity. Even without weight loss, exercise decreases the risk of cardiovascular disorders. Dietary fiber decreases the risk of colon cancer and cardiovascular disorders.
Sufficient and good-quality sleep, management of stress, and moderation of alcohol intake are also important.
Key Points
- Obesity increases the risk of many common health problems and causes up to 300,000 premature deaths each year in the US, making it second only to cigarette smoking as a preventable cause of death.
- Excess caloric intake and too little physical activity contribute the most to obesity, but genetic susceptibility and various disorders (including eating disorders) may also contribute.
- Screen patients using BMI and waist circumference and, when body composition analysis is indicated, by measuring skinfold thickness or using bioelectrical impedance analysis.
- Screen obese patients for common comorbid disorders, such as obstructive sleep apnea, diabetes, dyslipidemia, hypertension, fatty liver, and depression.
- Encourage patients to lose even 5 to 10% of body weight by changing their diet, increasing physical activity, and using behavioral interventions if possible.
- Try treating patients with orlistat, phentermine, phentermine/topiramate, naltrexone/bupropion, liraglutide, or semaglutide if BMI is ≥ 30 or if BMI is ≥ 27 and they have complications (eg, hypertension, insulin resistance); however, for severe obesity, surgery is most effective.
- Encourage all patients to exercise, to eat healthily, to get enough sleep, and to manage stress
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