What is obesity?

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A condition in which the body carries abnormal or unhealthy amounts of fat tissue, leading the individual to weigh in excess of 20 percent more than his or her ideal weight.
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Causes and Symptoms

Obesity is a condition in which the body accumulates an abnormally large amount of adipose tissue, or fat. It is a multifactorial, chronic disease that is rapidly increasing and having devastating effects on health, especially in the United States. The disease has social, cultural, genetic, metabolic, behavioral, and psychological components. People who are obese also face stigma and discrimination in work and social settings. Obesity is the second leading cause of preventable deaths in the United States, and according to the American Journal of Public Health in 2013, obesity accounts for approximately 18 percent of all deaths in the United States.

Because it is not practical to measure body fat content directly but it is easy to measure weight and height, the body mass index (BMI), which correlates closely with body fat, is often used to identify and quantify obesity.

Being overweight and being obese are not the same condition. A BMI for an adult who measures 5' 9"of 25 to 29.9 is considered to be overweight. However, a BMI of 30 or more is considered obese, and a BMI of 40 or more is severely obese. In 2014 the Centers for Disease Control and Prevention (CDC) reported that more than one-third of US adults were obese, with non-Hispanic blacks having the highest obesity rate of 47.8 percent of the population. Hispanics and non-Hispanic whites followed with 42.5 percent and 32.6 percent, respectively. Non-Hispanic Asians had the lowest incidence of obesity with 10.8 percent of that population reportedly obese. The CDC further reported that obesity was highest among adults between the ages of forty and fifty-nine.

The CDC also cited a 2009 report in Health Affairs, which estimated that the annual cost of obesity in the United States was $147 billion in 2008 dollars. The National Institutes of Health funded approximately $836 million in obesity research for fiscal year 2012, and set funding estimates for fiscal year 2014 at about $843 million. The Patient Protection and Affordable Care Act of 2010 listed obesity screening and counseling among the preventative services that all new group health plans and individual market plans under the act are required to provide without patient cost sharing.

An important function of adipose tissue is to store energy. If the intake of energy in the form of food calories is greater than the expenditure of energy, then the excess calories are stored, mainly in the adipose tissue, with a resulting gain in weight. Expenditure of energy depends largely on the resting metabolic rate or resting energy expenditure, defined as the calories used each day to maintain normal body metabolism. Additional calories are expended by exercise or other activity, by the digestion and metabolism of food, and by other metabolic processes. Because of this simple relationship between energy intake, energy utilization, and energy storage, weight gain can occur only when there is increased caloric intake, decreased caloric expenditure, or both.

Genetic factors appear to be very important in determining the presence or absence of obesity. Body weight tends to be similar in close relatives, especially in identical twins, who share the same genetic makeup. The extent to which genetic factors affect food intake, activity level, or metabolic processes is not known.

One theory holds that each individual has a “set point” that determines body weight. When food intake is decreased, experiments have shown less weight loss than predicted by the caloric deficit, suggesting that the body has slowed its metabolic rate, thus minimizing the deviation from the original weight. Many believe that physiologic regulation of body weight, which tends to maintain a preferred weight for each individual, explains some of the difficulty in treating obesity. The discovery and role of leptin in regulating weight helps to explain this apparent set point of weight for each individual.

There are other causes of obesity as well. Producing lesions in the hypothalamus, a part of the brain, can make animals eat excessively and become obese, and rare cases of obesity in humans are attributable to disease of the hypothalamus. In hypothyroidism, a condition in which the thyroid gland produces too little thyroid hormone, the metabolic rate is slowed, which may cause a mild gain in weight. In Cushing’s syndrome, which is caused by excessive amounts of the adrenal hormone cortisol or by drugs that act like cortisol, there is an accumulation of excessive fat in the face and trunk, which disappears when the disease is cured or the drug is stopped. Weight gain has also occurred with the use of other drugs, including some antidepressants and tranquilizers.

While most physicians and the public assume that the main factor causing obesity is excessive food intake in relation to physical activity, it has not been possible to prove that overweight people eat more than slender people do. This may be the case because it is very difficult to measure food intake under normal conditions, or perhaps because obese individuals tend to underestimate their food intake when dietary histories are taken. Some experts believe, however, that differences in metabolic efficiency and the physiologic set point for body weight are the principal causes of obesity in some people, rather than excessive food intake. The basal metabolic rate (BMR) varies fairly widely among persons of the same age, sex, and body size, and studies have shown large differences in the daily caloric intake needed to maintain a constant body weight in normal people; these observations support the possibility that metabolic differences could contribute to obesity.

Many health problems are associated with obesity. The majority of people who develop non-insulin-dependent diabetes mellitus are overweight or obese, and manifestations of the disease commonly improve or disappear if the individual succeeds in losing weight. Hypertension (high blood pressure) is more common with obesity, and weight loss may lower the blood pressure enough to lessen or avoid the need for medication. Arteriosclerosis, or “hardening of the arteries,” is more prevalent in obese persons and causes an increased risk for heart attacks and strokes. Certain forms of cancer are more prevalent with obesity: cancer of the colon, rectum, and prostate in men and cancer of the uterus, gallbladder, ovary, and breast in women. Severe obesity can cause difficulties in breathing, with sleepiness resulting from inadequate oxygen delivery to the tissues and sometimes from interruption of sleep at night. In addition, conditions such as arthritis may be worsened by the additional strain that obesity places on weight-bearing parts of the body.

The distribution of excess adipose tissue differs among individuals. Two main patterns have been described: android obesity (more commonly affecting men), in which fat accumulates mainly in the abdomen and upper body; and gynoid obesity (more common in women), in which fat accumulates mainly in the hips, thighs, and lower body. This distinction has received much attention because persons with android obesity are more likely to suffer from diabetes, hypertension, and cardiovascular disease. The closest association with these diseases is seen when sensitive measurements of abdominal visceral fat mass are made with computed tomography (CT) scanning. A simple measurement of the waist circumference compared with the hip circumference—the waist-to-hip ratio—can also be used to identify those obese individuals at greater risk for diabetes and cardiovascular disease.

Treatment and Therapy

Many obese people are highly motivated to lose weight because of the common perception that a slim body build is more attractive than an obese one. Many other overweight individuals desire to lose weight because of health problems related to obesity. As a result, the human and financial resources devoted to weight loss efforts are extensive. Unfortunately, the long-term results of the treatment of obesity are successful in only a minority of cases.

The only measures useful in the treatment of obesity are those that decrease the intake or absorption of calories or those that increase the expenditure of calories. The basis for any long-term weight management program is a low-calorie diet. The average daily calorie requirement in the United States is between 1,800 and 2,400 calories for adult women and between 2,400 and 3,000 calories for adult men; decreasing an individual’s intake, usually by 800 to 1,500 calories, will result in weight loss, provided that energy expenditure does not decrease. The Mayo Clinic recommends a balanced diet with 20 percent to 35 percent of one's daily calories derived from fat, which is considerably less fat than is found in the typical American diet. Many unbalanced diets, or “fad diets,” have enjoyed periods of popularity. Rice diets, low carbohydrate diets, vegetable diets, and other special diets may produce rapid weight loss, but long-term persistence with an unbalanced diet is rare and any lost weight is often regained.

Many patients fail to lose weight with low-calorie diets. More severe calorie restriction can be achieved with very-low-calorie diets that provide only 400 to 800 calories daily. This level of caloric restriction is unsafe unless a very high proportion of the diet consists of high-quality protein, with correct amounts of other nutrients such as vitamins and minerals. These requirements can be met with special formula diets under careful medical supervision. Such a program is recommended for severely obese patients who are otherwise healthy enough to tolerate this degree of caloric restriction.

Because most people find it difficult to lower their calorie intake, behavioral management programs may be combined with dietary restrictions. Dieters can be taught techniques for self-monitoring of food intake, such as keeping a daily log of meals and exercise, which will increase the awareness of eating behavior as well as point out ways in which that behavior can be modified. There are techniques for reducing exposure to food and the stimuli associated with eating, such as keeping food out of sight, keeping food handling and preparation to a minimum, and eliminating the occasions when food is eaten out of habit or as part of a social routine. Ways can be sought to increase the social support of friends and family for weight-losing behavior and for reinforcement of compliance with dietary restrictions. Interestingly, the “diet merry-go-round” which many mildly obese individuals experience—restricting their caloric intake until a weight goal is achieved, ending the diet only to resume overeating and regain the weight lost—often results in higher weight. Over time, such a pattern can “cycle” the individual to a dangerously high weight. Such individuals tend to experience more success if they can adjust their long-range eating behavior to moderate, rather than restrictive, intake of food. Many physicians would prefer to see their obese patients remain relatively stable in weight, reducing slowly over time, to avoid physical stress and ensure success.

Because obesity is caused by an excess of calorie intake over calorie expenditure, another approach to weight loss is to increase energy utilization by increasing physical activity. Some studies have shown that overweight individuals are less active than their nonobese counterparts. This fact could contribute to their obesity, since less energy utilization results in more energy available for storage as fat. Decreased activity could also be a result of obesity, since a heavier person must do more work, by carrying more pounds, than a nonobese person who walks or climbs the same distance.

Each pound of fat contains energy equal to about four thousand calories. If an obese person expends four hundred extra calories each day by walking briskly for one hour, it will take ten days for this activity to result in the loss of one pound. In a year, this increased calorie expenditure would result in a thirty-six-pound weight loss. More vigorous exercise, such as running, swimming, or calisthenics, would lead to more rapid weight loss, but might not be advisable for every person because of the increased prevalence of certain health problems in obese individuals, such as heart disease, hypertension, and musculoskeletal disorders. For this reason, any exercise program that involves vigorous physical activity should be undertaken with medical supervision.

Exercise as part of a weight-loss program has additional benefits. The function of the cardiovascular system may be improved, and muscles may be strengthened. Exercise will lead to loss of adipose tissue and gain in lean body mass as weight is lost, a change in body composition that is beneficial to overall health. Although some fear that physical activity will lead to an increase in appetite, studies show that any increase in food intake that occurs after exercise is usually not great enough to match the calories expended by the exercise.

Medications that decrease appetite are occasionally used to help people comply with a low-calorie diet. Some appetite suppressants act like adrenaline and may cause such side effects as nervousness, irritability, and increased heart rate and blood pressure. Other drugs may stimulate serotonin, a chemical transmitter in the central nervous system that decreases appetite, and may cause drowsiness as a side effect. The use of these medications is controversial because of their side effects and their limited effectiveness in promoting weight loss.

Several surgical procedures, collectively referred to as bariatric surgery, have been used to treat severe obesity that has impaired the patient’s health and has resisted other treatment. The operation now most commonly performed is gastroplasty, which creates a small pouch in the stomach with a narrow outlet through which all food must pass. This procedure decreases the effective volume of the stomach, causing fullness and nausea if more than small amounts of solid food are eaten. Patients have lost about half of their excess weight after one and one-half to two years, but some weight may be regained after this period. Gastroplasty has produced fewer serious complications than an older form of treatment, no longer done, called intestinal bypass. Care must be taken to avoid certain foods that might cause blockage of the narrowed opening from the surgically created stomach pouch, and the benefit of the operation can be overcome by eating soft or liquid foods, which can be consumed in large quantities. The long-term benefit of this procedure is being evaluated, but a significant number of obese patients with diabetes essentially cured their diabetes mellitus after having the surgery and losing weight..

Perspective and Prospects

Fat has several important functions in the human body. It serves as a cushion for the body frame and internal organs, it provides insulation against heat loss, and it is a storage site for energy. Fat stores energy very efficiently since it contains approximately nine calories per gram, compared with approximately four calories per gram in protein and carbohydrate. The presence of reserve stores of energy in the form of fat is particularly important when regular food intake is interrupted and the body becomes dependent on its fat deposits to maintain a source of fuel for daily metabolism and physical activity.

In affluent, culturally advanced societies, however, where food is abundant and modern conveniences greatly reduce the need for physical exertion, many people tend to accumulate excessive amounts of fat, since energy that is taken in but not utilized is stored in the adipose tissue. In the early twenty-first century, health officials were concerned by findings that showed one in every fifty Americans were “extremely obese,” meaning their BMI measured at least 50 and they were at least one hundred pounds overweight. This number had quadrupled since the 1980s. Obesity is a critical public health problem because it increases the risk of diabetes, hypertension, cardiovascular disease, and other illnesses. Also, many overweight men and women are distressed by the effects of their weight on their social interactions and self-image, and, despite laws, face discrimination in workplace settings. Therefore, many obese individuals desire to lose weight.

Unfortunately, the results of weight-loss programs and countless individual efforts at dieting to achieve this goal have often been disappointing. Short-term weight loss can often be achieved; programs utilizing low-calorie diets, behavior modification, exercise, and sometimes appetite-suppressing drugs usually lead to a weight loss of ten to thirty pounds or more over a period of several weeks or months. The problem is that after a year or more, the great majority of these dieters have regained the lost weight. It appears that the maintenance of a low-calorie diet and an increase in physical activity require a degree of commitment and willingness to endure inconvenience, self-deprivation, and sometimes even physical discomfort that most people can accept for short periods of time but not indefinitely. There are exceptions—some people do succeed in maintaining long-term weight loss—but more commonly dieters return to or surpass their original weight. It is as if the body’s set point can be overcome temporarily by intense effort, but not permanently.

Because of the poor prognosis for long-term weight loss, some experts now question the extent to which efforts should be devoted to the treatment of obesity. Nevertheless, because one cannot predict which obese individuals will succeed in achieving long-term weight reduction and because of the important health benefits of maintaining a normal body weight, most physicians agree that serious efforts should be made to treat obesity. Overweight individuals should identify the modifications in their diet and lifestyle that would be most beneficial and should attempt, with medical supervision, to initiate and maintain the behavior needed to bring about permanent weight loss.

In 2006, in an effort to reduce the incidence rate of obesity in the United States, the Alliance for a Healthier Generation, the William J. Clinton Foundation, and the American Heart Association announced an agreement to fight childhood obesity. Five leading food manufacturers vowed to reformulate their products in order to provide more nutritious choices for children in schools. In 2010 President Barack Obama and First Lady Michelle Obama each announced further initiatives to help prevent childhood obesity.


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