Obesity: Overview and Risk Factors

Obesity has become a worldwide epidemic. In the United States, more than 60 percent of the population is overweight or obese. Weight problems are typically classified based on body mass index (BMI), which is calculated by the following formula, in which weight is measured in kilograms and height is in meters:

BMI = weight
height²

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.

Although genetic factors influence body weight, diet and lifestyle have a major effect as well. North Americans have long been heavier, compared with people in Asia or Africa where plant-based diets prevail, and vegetarians are typically slimmer than omnivores. The number of overweight and obese people in the United States increased by one-third between 1990 and 2000, due in part to larger portion sizes and increased availability of high-calorie foods, such as cheese and soft drinks.

Obesity is a strong risk factor for several chronic diseases, including high cholesterol, hypertension, heart disease, stroke, type 2 diabetes, gallstones, several types of cancer (particularly those arising in the breast, prostate, and colon), dementia, sleep apnea, infertility, and arthritis of the hips and knees.

Risk Factors

In addition to the contributions of diet and lifestyle, genetic factors play an important role. Dozens of genes coding for hormones, neurotransmitters, and receptors have been associated with body weight. Several are being investigated as a basis for possible pharmacologic therapies. These include leptin, ghrelin, and melanocortin. Depression, anxiety, and eating disorders may also contribute to disordered eating habits.

Diagnosis

• A complete history and physical examination are necessary with special attention to medications, herbal remedies, nutritional and exercise history, risk factors for coronary artery disease, and family history of thyroid and cardiac diseases.
  
• Measures of the severity of obesity include BMI, waist circumference, waist-to-hip circumference ratio, and body fat determination, which is based on skin-fold thickness or bioimpedance.
  
• Blood testing includes fasting glucose and insulin concentration, thyroid hormone testing, renal function testing, liver function testing, cholesterol levels, and complete blood count.

Treatment

• Diet, exercise, and lifestyle modification that reduce energy intake and increase energy expenditure are the essentials of treatment. Nutritional interventions are discussed below in Nutritional Considerations.
  
  Physical activity helps individuals retain lean body mass, when compared with food restriction alone, and may better prepare them to keep weight off after the initial loss. Because it takes a great deal of exercise to burn off even a modest amount of weight, physical activity is not a substitute for diet changes.
  
• Several medical therapies are available. However, long-term results of drug therapy have been disappointing, and weight gain occurs once medications are discontinued. Recent evidence suggests that a combination of drugs and behavior therapy has better results than either modality used alone.
  
  Medications that are commonly used (but not necessarily recommended) include appetite suppressants (e.g., sibutramine, phentermine, benzphetamine, phendimetrazine, diethylpropion) and orlistat, which decreases the absorption of dietary fat.
  
• Bariatric surgery ("stomach stapling") has been used successfully in morbidly obese individuals (BMI >40). However, complications (such as nutrient deficiencies and infection) are common and can be fatal.
  
• Dietary supplements that promise extraordinary weight loss should be avoided. Research studies indicate a lack of weight-loss efficacy for chitosan, chromium picolinate, Garcinia cambogia, glucomannan, guar gum, hydroxy-methylbutyrate, plantago psyllium, pyruvate, yerba mate, yohimbe, and others.
  
  Although ephedra-containing formulas have been found effective for weight control, they carry a high risk of heart disease and are now illegal in the United States. 

Obesity: Nutritional Considerations

The Western diet, which emphasizes highly energy-dense foods rich in fat and sugar, promotes over-consumption and weight gain.

The best strategy for permanent weight control is to change the type of foods you eat and increase physical activity. When foods lower in energy density and higher in water and fiber (e.g., salads, soups, vegetables, and fruits) are consumed instead of foods high in energy density, early fullness occurs, along with decreased food intake. This strategy has produced weight loss in several clinical studies.

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

• Reducing dietary fat: Fat contains more than twice as many calories as protein and carbohydrate (9 calories per gram of fat, compared with 4 for protein or carbohydrate). Dietary fat often results in overconsumption of calories. The addition of fats to meals results in fat storage rather than fat burning. These mechanisms may explain why the prevalence of overweight individuals 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. 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 whose diets are high in complex carbohydrates tend to have a low incidence of obesity. The whole grains and legumes in these diets also provide fiber. Fiber is filling, but contributes little to overall calorie intake. Studies show that fiber intake is inversely associated with body weight and body fat (i.e., individuals who eat the most fiber tend to have better weight control, and vice versa).
• Following low-fat, vegetarian diets: Many studies have found that vegetarians tend to be slimmer than individuals who eat meat, which is not surprising given that grains, legumes, vegetables, and fruits are low in fat and high in complex carbohydrate and fiber. Randomized trials show that low-fat vegan diets promote greater weight loss than typical low-fat diets, and they also improve cholesterol levels and blood sugar. A study of a vegetarian diet in heart patients, used in combination with exercise and stress management, showed sustained weight loss over a five-year period.
• Minimizing sugars: Sucrose, high-fructose corn syrup, and other sugars add calories without producing fullness. In particular, an increasing intake of sweetened beverages is strongly associated with a rise in obesity.

Low-carbohydrate diets, such as Atkins and South Beach, are not recommended. They have not been found to be superior to either low-fat, high-carbohydrate, or calorie-controlled diets over a 12-month period. Further, these types of diets have been shown to increase cholesterol levels, particularly LDL (the "bad") cholesterol, sometimes severely, in approximately one-third of users. They can also cause calcium loss from the body.

Certain personality and behavioral factors are characteristic of those who succeed at maintaining weight loss. These factors include developing coping skills that prevent using food for comfort; engaging in high levels of physical activity (approximately one hour per day); eating a low-calorie, low-at diet; eating breakfast regularly; self-monitoring weight on a regular basis; and maintaining a consistent eating pattern through the seven-day week. The characteristic behaviors of those who keep weight off are documented and updated through the National Weight Control Registry, available at: http://www.nwcr.ws/.