Body mass index.

The body mass index (BMI), which is derived from the patient’s weight and height, is a simple way to estimate body fat content. Studies indicate a close correlation between BMI and total body fat. The BMI is calculated by dividing a patient’s weight (in kilograms) by the square of the patient’s height (in meters). Hence, BMI is expressed in units of kg/m². BMI can also be calculated using the patient’s weight in pounds and height in inches (Fig. 82–1). According to the federal guidelines, a BMI of 30 or higher indicates obesity. Individuals with a BMI of 25 to 29.9 are considered overweight, but not obese. There is good evidence that the risk of cardiovascular disease and other disorders rises significantly when the BMI exceeds 25. When the BMI exceeds 30, there is an increased risk of death. These specific associations between BMI and health risk do not apply to elderly adults, growing children, or women who are pregnant or lactating. Nor do they apply to competitive athletes or bodybuilders, who are heavy because of muscle mass rather than excess fat. Table 82–1 summarizes weight classifications based on BMI.

Waist circumference.

Waist circumference (WC) is an indicator of abdominal fat content, an independent risk factor for obesity-related diseases. Accumulation of fat in the upper body, and especially within the abdominal cavity, poses a greater risk to health than does accumulation of fat in the lower body (hips and thighs). People with too much abdominal fat are at increased risk of insulin resistance, diabetes, hypertension, coronary atherosclerosis, ischemic stroke, and dementia. Fat distribution can be estimated simply by looking in the mirror: an apple shape indicates too much abdominal fat, whereas a pear shape indicates fat on the hips and thighs. Measurement of WC provides a quantitative estimate of abdominal fat. A WC exceeding 40 inches (102 cm) in men or 35 inches (88 cm) in women signifies an increased health risk—but only for people with a BMI between 25 and 34.9 (see Table 82–1).

Risk status.

Overall weight-related health risk is determined by BMI, WC, and the presence of weight-related diseases and cardiovascular risk factors. Certain weight-related diseases—established coronary heart disease, other atherosclerotic diseases, type 2 diabetes, and sleep apnea—confer a very high risk for complications and mortality. Other weight-related diseases—gynecologic abnormalities, osteoarthritis, gallstones, and stress incontinence—confer less risk. Cardiovascular risk factors—smoking, hypertension, high levels of low-density lipoprotein (LDL) cholesterol, low levels of high-density lipoprotein (HDL) cholesterol, high fasting glucose, family history of premature coronary heart disease, physical inactivity, and advancing age—confer a high risk when three or more of these factors are present.

Not surprisingly, health risk rises as BMI gets larger (see Table 82–1). In addition, the risk is increased by the presence of an excessive WC. The risk is further increased by weight-related diseases and cardiovascular risk factors. In the absence of an excessive WC and other risk factors, health risk is minimal with a BMI below 25, and relatively low with a BMI below 30. Conversely, a BMI of 30 or more indicates significant risk. In the presence of an excessive WC, health risk is high for all individuals with a BMI above 25.

Caloric restriction.

A reduced-calorie diet is central to any weight-loss program. As noted, the only way to lose weight is to take in fewer calories than we burn. Depending on the individual, the caloric deficit should range from 300 to 1000 kcal/day. Because fats contain more calories than either carbohydrates or proteins (on an ounce-for-ounce basis), reducing dietary fat is the easiest way to reduce calorie intake.

What’s the best diet for losing weight? Answer: The one that you actually stick to. In one study, 160 overweight subjects were randomized to follow one of four popular diet plans: Atkins (low-carbohydrate), Ornish (fat-restricted), Weight Watchers (portion- and calorie-restricted), and Zone (low-glycemic-index). Mean weight loss after 1 year was modest—ranging from 4.6 to 7.3 pounds—and did not differ significantly between the plans. However, what did matter was adherence: There was a direct correlation between weight lost and adherence to the plan, regardless of which plan was followed.

To succeed at losing weight, it helps to know just how many calories you take in each day and how many you burn. The following web sites, which are free, have databases on foods and physical activities, along with tools to calculate and log calories taken in and calories burned:

Exercise.

Physical activity should be a component of all weight-loss and weight-maintenance programs. Exercise makes a modest contribution to weight loss by increasing energy expenditure. In addition, exercise can help reduce abdominal fat, increase cardiorespiratory fitness, and maintain weight once loss has occurred. According to 2009 guidelines from the American College of Sports Medicine, people trying to lose weight should exercise at least 150 minutes per week (and preferably more), and those trying to maintain weight loss should exercise 200 to 300 minutes per week.

Behavior modification.

Behavior therapy is directed at modifying eating and exercise habits. As such, behavior therapy can strengthen a program of diet and exercise. In the absence of continuing behavior therapy, most patients regain lost weight when treatment stops. Techniques of behavior therapy include self-monitoring of eating and exercise habits, stress management (because stress can trigger eating), and stimulus control (limiting exposure to stimuli that promote eating). There is no evidence that any one of these techniques is superior to others.

Drug therapy.

Drugs can be used as an adjunct to diet and exercise—but only for people at increased health risk, and only after a 6-month program of diet and exercise has failed. Drugs should never be used alone. Rather, they should be part of a comprehensive weight-reduction program—one that includes exercise, behavior modification, and a reduced-calorie diet.

Candidates for drug therapy should be at increased health risk owing to excessive body fat. Specifically, drugs should be reserved for patients whose BMI is 30 or greater (in the absence of additional risk factors), or 27 or greater (in the presence of additional risk factors). Drugs are not appropriate for patients whose BMI is relatively low.

Benefits of drugs are usually modest. Weight loss attributable to drugs generally ranges between 4.4 and 22 pounds, although some people lose significantly more. As a rule, the majority of weight loss occurs during the first 6 months of treatment.

Expert opinion regarding duration of therapy has changed. In the past, drug therapy was limited to a few months. Today, long-term treatment is recommended. Why? Because we now know that, when drugs are discontinued, most patients regain lost weight. This is similar to the return of high blood pressure when antihypertensive drugs are withdrawn. Accordingly, when treatment has been effective and well tolerated, it should continue indefinitely. At this time, only one drug—orlistat—is approved for long-term use, and hence is preferred to other agents.

Not everyone responds to drugs, and hence regular assessment is required. Patients should lose at least 4 pounds during the first 4 weeks of drug treatment. If this initial response is absent, further drug use should be questioned. For patients who do respond, ongoing assessment must show that (1) the drug is effective at maintaining weight loss, and that (2) serious adverse effects are absent. Otherwise, drug therapy should cease.

In theory, drugs can promote weight loss in three ways: They can suppress appetite, reduce absorption of nutrients, or increase metabolic rate. With one exception—orlistat—all of the drugs used for obesity work by suppressing appetite. Orlistat works by reducing absorption of fat. None of the available drugs increases metabolic rate.

Table 82–2 lists Food and Drug Administration (FDA)–approved drugs for obesity, and summarizes their approved indications, mechanism of action, major side effects, and status under the Controlled Substances Act (CSA).

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