Obesity has become a worldwide epidemic. Global prevalence of obesity nearly tripled between 1975 and 2016.[1] In the US, excess weight affects more than two-thirds of the adult population and one-third of the population between ages 6 and 19.[2]
The Framingham study has shown that, in the US, the current lifetime risk of obesity is 50% and risk of being overweight is 80%.[3] Weight conditions are typically classified based on body mass index (BMI), calculated with the following formula:

Overweight is defined as a BMI between 25 and 29.9 kg/m², and obesity is defined as a BMI of 30 kg/m² or greater. Class 1 obesity is defined by a BMI of 30 to < 35, Class 2 as a BMI of 35 to < 40, and Class 3 (or “severe”) obesity as a BMI ≥ 40 kg/m².[4]

In Asian populations, the risk for weight-related diseases begins to increase at lower BMI ranges, compared with other populations, so overweight among Asians is commonly defined as a BMI of 23 to 24.9, and obesity is defined as BMI ≥ 25.[5]

While genetics may predispose an individual to elevated body weight, diet and lifestyle are the major modifiable contributors to obesity. The number of people with overweight and obesity in the US increased by one-third between 1990 and 2000, and the rise in incidence of obesity since the 1970s has been attributed to larger portion sizes and increased availability of high-calorie foods such as meat, cheese, and soft drinks.[6]

Changes in eating habits have been sufficient to explain the recent US obesity epidemic. Perhaps surprisingly, changes in physical activity have played very little role according to large population studies, suggesting that the solution to obesity needs to focus on eating habits rather than primarily on exercise.[7][8][9]

Obesity is a strong risk factor for several chronic diseases, including dyslipidemia, cardiovascular and cerebrovascular disease, venous thromboembolism, hypertension, type 2 diabetes, cholelithiasis, gout, many types of cancer (particularly those arising in the breast, prostate, and colon), dementia, sleep apnea, polycystic ovary syndrome, osteoarthritis of the hip and knee, and infertility. Abdominal (visceral) fat, compared with other fat distributions, is generally a stronger indicator of health-problem risk.[10]

Risk Factors

Dietary factors are central to both the cause and the treatment of obesity and will be discussed in Nutrition Considerations below. In addition, genetic factors play an important role in predisposition to obesity. Specifically, genes involving the hypothalamic regulation of energy may play a significant role for some individuals.[11] Moreover, growing evidence from epigenetic research suggests that DNA expression is modified by environmental exposures (overnutrition and physical activity for example) that may influence inter-individual variability in obesity.[12]

Depression, anxiety, major life events, and eating disorders may also contribute to behaviors associated with unhealthy weight gain. The use of certain medications has been commonly associated with weight gain. Among the most common are glucocorticosteroids, antipsychotics, and some antidepressants, as well as some medications used in the treatment of diabetes mellitus, notably insulin and medications that cause insulin release, such as sulfonylureas.[8] Additionally, lifestyle factors that disrupt circadian rhythm such as night and shift work are associated with significant risk for developing obesity.[13][14]

Certain demographic groups are disproportionately affected by weight problems. In the US, the highest incidence is seen in non-Hispanic Black adults (50%), while non-Hispanic Asian adults have the lowest prevalence (16%).[4]

A history of childhood obesity and having parents with obesity are both risk factors for obesity in adulthood.[15]

Diagnosis

An evaluation of obesity requires a complete history and physical examination, with special attention to dietary habits, medications, herbal remedies, and exercise history, as well as risk factors for coronary artery disease, and family history of thyroid dysfunction, diabetes mellitus, and metabolic syndrome. Underlying medical conditions, psychosocial stressors, depression, and emotional eating should be assessed. Rare genetic disorders may contribute to obesity, and patients should be referred to a geneticist if such conditions are suspected.[16] Abnormal sleep patterns should be assessed, typically by screening for obstructive sleep apnea or by addressing sleep adequacy in the context of shift work.

BMI calculation is commonly used to estimate the severity of being overweight, though it can be misleading in very muscular people.

Anthropometric measures include waist circumference, waist-to-hip circumference ratio, and body-fat determination, which is usually based on skinfold thickness or bioimpedance. More accurate determinations such as DEXA scans, CT, and MRI are generally limited to research settings.[17]

Laboratory testing may include evaluation for diabetes and metabolic syndrome, Cushing syndrome, and thyroid abnormalities. It may also be appropriate to assess renal function, plasma lipid concentrations, a complete blood count, and liver enzymes to screen for nonalcoholic fatty liver disease. Female patients may warrant evaluation for polycystic ovary syndrome if clinically suspected.

Treatment

Dietary behavior is the principal cause of obesity and the most important consideration in treatment. This will be discussed in more detail in Nutritional Considerations, below. Other medical issues and approaches will be discussed in this section.

Any underlying medical or psychiatric condition contributing to obesity should be addressed, and therapeutic interventions should be included in the plan of action for treating obesity. Dietary changes are essential to weight loss.

Exercise alone without improved dietary habits is typically not sufficient for significant weight loss, but adding exercise to a healthful diet has been shown to help in attaining and maintaining a healthy weight.[18] Physical activity helps to increase and retain lean body mass and improve metabolic markers, and it may better prepare patients to keep weight off after the initial loss.[19][20]

A reasonable goal for most patients with obesity is to lose 1 pound or 0.45 kg. per week until they have lost 5-10% of their body weight. Patients will typically have health improvements even before they have achieved a healthy BMI.[21] A loss of 2.5% of original body weight, for example, can yield a 60% decreased risk of the development of diabetes over the course of 6 years.[22]

Pharmacotherapy

Several medical therapies are available, and more are under study. Recent evidence suggests that a combination of pharmacotherapy and behavior therapy achieves better results than either modality used alone.[23] Newer pharmacologic agents appear to be effective for weight loss, but side effects can be significant and weight is often regained once medications are discontinued.[24] A growing body of evidence suggests some medications provide benefit for co-existing conditions such as diabetes, heart disease, and renal disease, but inability to tolerate side effects may prohibit use.[25][26]

Medications that are commonly used (but not necessarily recommended) include:

Appetite suppressants (sympathomimetics). These medications are approved for short-term use. Examples are phentermine, benzphetamine, phendimetrazine, and diethylpropion.

The following combination therapies are approved by the Food and Drug Administration for long-term use:

A combination of topiramate extended-release and phentermine, marketed under the brand name Qsymia, allows for lower dosing of phentermine and is intended for longer-term use than high-dose phentermine.

A bupropion-naltrexone extended-release combination, marketed as Contrave, also has sympathomimetic properties.

Glucagon-like peptide 1 (GLP-1 receptor agonists) and dual-receptor agonists. High-dose semaglutide and liraglutide reduce appetite, slow gastric clearance, and improve insulin sensitivity. They are currently approved only as a subcutaneous injection (though an oral version of semaglutide is used for type 2 diabetes and has been shown to induce weight loss). Common side effects are related to delayed gastric clearance (nausea, gastroesophageal reflux disease). The medication tirzepatide, approved for weight loss in 2023, affects both the GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. Use of the weekly injectable medication lead to an average loss of > 25% of baseline body weight over 84 weeks in a large clinical trial.[27] Serious but rare potential side effects include pancreatitis, pancreatic and thyroid cancer, acute kidney injury (sometimes necessitating dialysis), and gallbladder disease. When the medications are discontinued, most lost weight typically returns.

Orlistat. This pancreatic lipase inhibitor decreases the absorption of dietary fat. When using this medicine, it is necessary to supplement the diet with fat-soluble vitamins and phytonutrients. The most common side effects include fecal incontinence, bloating, and anal leakage. Due to these side effects this medication is rarely tolerated or used.

Superabsorbent hydrogel particles, marketed as Plenity, are a hydrogel that acts as space-occupying cellulosic matrix in the stomach, increasing satiety.

Other medications may be useful in certain patients with diabetes, depression, or seizure disorders.

Surgery

Bariatric surgery has been used successfully in severely obese patients (BMI > 40) and in patients with BMI > 35 with comorbid conditions, such as diabetes mellitus, obstructive sleep apnea, or hypertension. Vertical sleeve gastrectomy was the most commonly performed procedure between 2011 and 2017. Gastric bypass, duodenal switch, and banding are also offered, though less frequently; however, complications such as nutrient deficiencies and infection are common. Mortality associated with these procedures has been reduced over the past several decades and is now estimated at 0.08%. Surgery is not recommended until failure of diet and lifestyle interventions has been established, in part because it necessitates lifelong medical monitoring.[28]

Lifestyle Modification

Certain personality and behavioral factors have been shown to be characteristic of those who succeed at maintaining weight loss. These include developing coping skills that prevent using food for emotional comfort, increasing self-efficacy with respect to weight control, engaging in high levels of physical activity (approximately 1 hour per day), limiting television viewing, choosing a low-calorie, low-fat diet, eating breakfast regularly, weight self-monitoring, and maintaining a consistent eating pattern through the 7-day week.[29][30] The characteristic behaviors of those who keep weight off are described and updated through the National Weight Control Registry, available at http://www.nwcr.ws/.[22]

Dietary Supplements

Patients should be advised to avoid dietary supplements promoting weight loss. Studies have shown the ineffectiveness of chitosan, chromium picolinate, Garcinia cambogia, glucomannan, guar gum, hydroxy-methylbutyrate, Plantago psyllium, pyruvate, yerba mate, and yohimbe.[31] Ephedra-containing formulas (prohibited in the United States) are effective for weight control, particularly when combined with aspirin, but the risk-to-benefit ratio of this combination is prohibitively high due to potential adverse cardiovascular effects of ephedra, as well as potential gastrointestinal damage caused by aspirin.

Nutritional Considerations

The increased prevalence of this condition during the last century (particularly in the last 3 decades) indicates that obesity is clearly related, not to genetics, but to dietary factors; US government figures highlight the role of increased consumption of cheese, meat, oils, shortening, and frozen desserts.[32][33][34] The consumption of energy-dense foods activates reward systems in the brain, up-regulates the expression of hunger signals, and blunts the response to satiety signals, promoting overconsumption.[35]

Common short-term diets focusing on limiting calories or portion sizes rarely produce long-term weight loss and may have deleterious health effects.[36][37][38][39] A better approach is a permanent change in the type of foods individuals select. Individuals who consume foods lower in energy density and higher in water and fiber (particularly plant-based foods), instead of foods high in energy density, experience early satiety and spontaneously decrease food intake. In addition, high-fiber plant-based foods have been shown to increase the excretion of unabsorbed energy[40] and to increase postprandial metabolism, as described below. This strategy has produced weight loss in several clinical studies.[41][42][43] By allowing for the intake of larger portions that provide satiety, it fosters continued adherence.[44][45]

The following steps reduce the energy density of the diet and promote weight control:

Following low-fat, plant-based diets. Several studies have found that individuals following vegan diets tend to be slimmer than omnivores, which is not surprising given that grains, legumes, vegetables, and fruits are low in fat and high in complex carbohydrates and fiber.[46] Randomized trials show that low-fat vegan diets promote greater weight loss than typical weight-loss diets, and improve plasma lipids, insulin sensitivity, and other measures.[32][47] A study of a near-vegan diet in heart patients, used in combination with exercise and stress management, showed sustained weight loss over a 5-year period.[48]

The effect of a low-fat vegan diet on body weight is mainly attributable to its low energy density. Because the diet is very low in fat (which is energy-dense) and high in fiber (which has essentially no calories), energy intake falls without a person needing to intentionally cut calories or even to be aware of the change. A secondary contributor to weight loss is the tendency of a low-fat vegan diet to increase postprandial energy expenditure. In the after-meal period, as foods are digested and their nutrients are absorbed, energy expenditure rises—an effect called the thermic effect of food. This effect can be larger or smaller depending on the foods consumed during a specific meal and also depending on the habitual diet. Transitioning to a low-fat vegan diet causes the thermic effect of food to increase by roughly 15%.[49] Plant-based meals have been shown to increase GLP-1 levels, which in turn reduce appetite.[50]

Ad-libitum, low-fat vegan diets are much more effective for weight control, compared with Mediterranean diets. In a randomized cross-over trial including overweight participants, a low-fat vegan diet led to a weight loss of 6.0 kg over 16 weeks, compared with no net weight loss on a Mediterranean diet.[51]

Reducing dietary fat. Dietary fat has more than twice as many calories per gram as protein and carbohydrate (9 g compared with 4 g) and promotes passive overconsumption of energy. These factors may explain why the prevalence of overweight worldwide is directly related to the percentage of fat in the diet, and why low-fat diets have been consistently shown to promote moderate weight loss.[52] Common sources of fat are meats, dairy products, fried foods, and added oils.

Choosing foods high in complex carbohydrates and fiber. Populations in Asia, Africa, and elsewhere with diets high in complex carbohydrates tend to have a low incidence of obesity. The whole grains and legumes in these diets also provide fiber that is filling but contributes little to overall energy intake. Studies show that fiber intake is inversely associated with body weight and body fat.[53]

Minimizing sugars. Sucrose, high-fructose corn syrup, and other simple sugars add calories without producing satiety. A systematic review of epidemiological and clinical studies found positive associations between intake of sugar-sweetened beverages and both weight gain and obesity in children and adults.[54]

Low-carbohydrate diets have been popular, but they have not been found superior to either low-fat, high-carbohydrate, or calorie-controlled diets over a 12-month period.[55] A controlled feeding study in a metabolic ward using isocaloric, energy-restricted diets found that a low-fat diet resulted in more body fat loss than a low-carbohydrate diet.[56] Low-carbohydrate diets increase plasma low-density lipoprotein concentrations, sometimes severely, in approximately one-third of users.[57] They also cause a sustained increase in urinary calcium losses.[58] In the Women’s Health Initiative Dietary Modification Trial, weight loss was greatest in women who both decreased fat intake and consumed more fruit, vegetables, and fibrous carbohydrate.[59]

Steps for preventing obesity should begin in early childhood—including pregnancy. It is important to select foods that help maintain healthy weight gain during pregnancy and to feed newborns breast milk versus formula whenever possible. Parents should role-model healthful eating habits and physical activity and encourage these behaviors in their children (see Life Cycle chapter).[60]

Orders

See Basic Diet Orders chapter.

Low-fat vegan diet.

What to Tell the Family

Obesity contributes to many chronic illnesses, but it may be prevented and successfully treated in most individuals through a diet low in fat and simple sugars, yet high in fiber, along with regular physical activity. Well-planned, low-fat vegan diets are particularly healthful and effective.

The family plays an essential role in supporting the diet and lifestyle changes that can prevent and treat weight problems. Family members are likely to benefit from these same changes.

References