Obesity is a condition in which a person has an excess of body fat. It is one of the leading causes of preventable death, contributing to serious health problems such as diabetes, heart disease and cancer. Almost one-third of Americans are obese, according to the National Institutes of Health (NIH).

Weight gain occurs when energy intake exceeds the energy a person expends. When a person takes in an excessive amount of energy, it is stored in the fat cells of adipose tissue. People who are obese have more and larger fat cells than those who maintain healthier body weights.

A combination of high-fat, high-calorie diet (including large quantities of processed foods and refined sugar) and sedentary lifestyle (including lack of regular exercise) is among the main causes of obesity. Genetics also appears to play a role in obesity. If one parent is obese, a child’s chances of being overweight are 40 percent. That percentage doubles if both parents are obese.

Obesity is usually measured by the body mass index (BMI), a calculation based on an individual’s weight and height. BMI is the most common method of tracking weight problems and obesity among adults and is strongly connected to body fat. The criteria for determining obesity are the same for men and women.

People who are obese can usually gain significant health benefits by losing weight. The key is to change the person’s energy balance so that energy output exceeds energy intake. When this occurs the size of the person’s fat cells shrinks, leading to modest but significant weight loss. Even losing as little as 5 percent to 7 percent of total body weight can result in a dramatic decrease in risk for health conditions such as diabetes, cardiovascular disease and stroke.

Obesity is a condition in which people have an excess of body fat. According to the U.S. Centers for Disease Control and Prevention (CDC), the prevalence of obesity in the United States more than doubled between the years 1960 and 2000, with the greatest increase from 1980 forward. According to the National Institutes of Health, almost one-third of Americans are obese.

Obesity is also a growing problem across the globe. Worldwide, more than 300 million adults are obese, according to the World Health Organization (WHO). Formerly, being underweight was much more common throughout the world. Now, the world’s population is roughly divided in half between those who are underweight and those who are overweight (defined as having excess body weight that includes fat, bone and muscle).

The terms “obesity” and “overweight” are often used interchangeably. However, there is a distinct difference between the two. Overweight refers to an excess body weight that includes fat, bone, muscle, and/or body water. Therefore, a body builder whose muscle mass causes them to weigh significantly more than others of a similar height might be considered overweight despite a lack of body fat. In contrast, obesity is defined as an excess of body weight according to body mass index.

As a major risk factor, obesity is the second-leading cause of preventable death in the United States, surpassed only by smoking. At least 300,000 Americans die each year as a result of factors attributed to obesity, and annual healthcare costs attributed to obesity are around $100 billion, according to the American Obesity Association. People who are obese have a 10 percent to 50 percent increased risk of death from all causes when compared to people of normal weight, according to the National Institutes of Health (NIH). Most of this increased risk is associated with cardiovascular causes.

Obesity is a major risk factor for a number of serious health conditions, including:

• Coronary heart disease. Obesity causes a buildup of “bad” low-density lipoprotein (LDL) cholesterol and a decline in “good” high-density lipoprotein (HDL) cholesterol. Excess body fat can also cause levels of triglycerides to rise. As abnormal blood fats rise, so does the risk of fatty deposits in the arteries. This condition is known as atherosclerosis, and it increases the risk of coronary artery disease and stroke.
  
  
• Cancer. Obesity raises the risk of breast, uterine, cervical, ovarian and gallbladder cancer in women; and colon, rectum and prostate cancer in men.
  
  
• Diabetes. Excess fat makes the body resistant to insulin, the hormone that helps it maintain proper blood sugar levels.
  
  
• Fatty liver disease. Obesity causes fats to accumulate in the liver, leading to inflammation and scarring of the liver that can result in cirrhosis.
  
  
• Gallbladder disease. Obesity increases levels of cholesterol, which can be deposited in the gallbladder, raising the risk of gallstones.
  
  
• High blood pressure. Fatty tissue relies on oxygen and nutrients in the blood to survive. Blood flow through the body must increase to meet this demand, which raises blood pressure.

• Osteoarthritis. This condition involves a degeneration of cartilage and bone in joints. Increased weight associated with obesity puts added pressure on the joints of the knees, hips and lower back and wears away the cartilage that protects them.
  
  
• Stroke.
  
  
• Sleep apnea and other breathing problems.

Other health conditions linked to obesity include:

• Pregnancy complications (e.g., high birth weight)
• Asthma
• Excess body and facial hair
• Increased surgical risk
• Menstrual irregularities (e.g., missed periods)
• Psychological disorders (e.g., depression)
• Stress incontinence (leakage of urine)
• Digestive disorders (e.g., gastroesophageal reflux disease)
• Sleep apnea and other breathing problems
• Increased mortality

Recent research continues to uncover new links between obesity and health problems. For example, one recent study found a link between morbid obesity in toddlers and lower IQ scores, cognitive delays and brain lesions. Another study found that men with prostate cancer are more likely to experience poor treatment results when undergoing primary radiation therapy.

Generally, obesity develops when the amount of energy units (calories) consumed through food is greater than the amount of calories expended through metabolism and physical activity. Excess calories are stored in the body as fat; if too much body fat accumulates, weight gain is inevitable.

An enzyme called lipoprotein lipase (LPL) facilitates fat storage in both fat and muscle cells. Obese people typically have more of this activity in their fat cells than lean people do. The hormones estrogen and testosterone influence the activity of LPL. In women, fat cells in the breasts, hips and thighs produce large amounts of LPL. In men, fat cells in the abdomen produce the most LPL.

People also tend to gain more weight if they are older and/or have a low metabolic rate. Women often have a more difficult time losing fat than men, and may have particular trouble losing weight from the hips and thighs. Other factors (e.g., genetic, socioeconomic, psychological, medical factors) are also involved in obesity, but the relationship among these factors and the extent to which each contributes to obesity is less understood.

For most people, weight gain can be prevented or controlled by adequate diet and exercise. Ideally, good habits begin in childhood, because studies have shown that overweight children are likely to carry this condition into adulthood.

According to the National Institutes of Health (NIH), obesity rates in the United States are as follows:

African-Americans and Hispanic Americans have the highest rates of obesity.

Weight gain occurs when energy intake exceeds the energy a person expends. When a person takes in an excessive amount of energy, it is stored in the fat cells of adipose tissue. A person’s total fat stores reflect both the number and size of fat cells in the body. The number of fat cells increases fastest during late childhood and early puberty, when the child is growing. Additional fat cells may continue to develop throughout a person’s lifetime whenever the person consumes more energy than is expended.

People who are obese have more and larger fat cells than people who maintain healthier weights. A combination of high-fat, high-calorie diet (including large quantities of refined sugar) and sedentary lifestyle (including lack of regular exercise) is one of the main causes of excessive energy intake compared to energy output, an imbalance that tends to lead to obesity.

Although dietary fat receives a lot of attention from consumers and health professionals, a certain amount of fat is required for a balanced diet.

Another possible reason for the recent increase in obesity rates is the jump in daily consumption of calories. People tend to consume more calories in general and more calories from fat today than they did as recently as two decades ago. Portion sizes have increased substantially over that time, particularly in fast-food restaurants. This trend began in the 1970s and accelerated throughout the 1980s and 1990s.

Additionally, less than one-third of Americans meet the basic activity level, which is defined as 30 minutes of exercise a day on most days of the week. Technological advances such as escalators and remote controls, increased dependence on automobiles and more sedentary entertainment choices such as television and video games have all contributed to this inactivity. In some cases, obesity develops not because people eat too much, but because they move around too little.

Genetics also appears to play a role in obesity. If one parent is obese, a child’s chances of being overweight are 40 percent, according to the American Dietetic Association. That percentage doubles if both parents are obese. Studies of adopted children and twins have independently concluded that weight among biological siblings and parents is often very similar.

Two hormones have been identified as playing a part in the genetics that help determine obesity: leptin and ghrelin. Leptin is a hormone that regulates appetite. If an individual has consumed too much food and is gaining weight, leptin levels increase to decrease appetite and increase energy use. On the other hand, if an individual is losing weight, then leptin levels decrease to stimulate the appetite and reduce energy use. The point where a signal is sent to the brain to increase or decrease leptin levels is called the set point, resulting in a weight that is relatively stable in a given individual, but which may vary considerably among the general population. Not producing enough leptin, therefore, would increase the risk of being overweight or obese.

Studies have demonstrated that obese patients have a higher set point at which leptin levels are released, suggesting that many obese patients are biologically “programmed” to have a bigger appetite and a higher caloric intake. Additionally, many obese patients have a lower basal metabolic rate (BMR, amount of calories consumed at rest) and consume less calories at rest than non-obese patients. When patients attempt to lose weight by decreasing their caloric intake or by exercising, their BMR decreases to counteract their efforts to lose weight.

Ghrelin is a protein that behaves like a hormone. It is involved in food regulation through stimulating the appetite. Ghrelin is mainly secreted by stomach cells and helps individuals to maintain a stable body weight. Blood levels of ghrelin generally rise before and fall after a meal. Ghrelin levels are high when the body has a negative energy balance, such as occurs when an overweight person goes on a diet. The high levels of ghrelin in people trying to lose weight may explain why maintaining weight loss is difficult, because the appetite is being frequently stimulated by the activity of ghrelin.

As researchers come to better understand obesity, these two hormones may play a role in more effective prevention and treatment of the condition. In the meantime, overweight people are urged to speak to their physician. There are healthy treatment options that can significantly reduce the risk of developing cardiovascular disease and/or other serious health problems related to obesity.

The development of obesity is complex, and there are many reasons, including environmental and inherited factors, that can lead to a person becoming obese. Some of the most common are:

• Age. As people get older, the amount of muscle in the body decreases, which in turn leads to a decrease in metabolism. Older people are also less active. People who do not decrease their calorie intake as they get older are likely to gain weight.
  
  
• Sedentary lifestyle. An inactive lifestyle that includes a great deal of sitting and little or no exercise can lead to obesity.
  
  
• Alcohol. Consuming too much alcohol can lead to obesity because alcohol tends to be high in calories and offers no nutritional benefit.
  
  
• Medications. Certain drugs have been linked to obesity, including the use of corticosteroids, as well as some medications used for psychiatric conditions.
  
  
• Eating disorders or other psychological issues. An estimated 10 percent of mildly obese people who attempt to lose weight through weight loss programs or on their own have binge eating disorders, according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Among those who are severely obese, this disorder is even more common. Also, some people eat, even when they are not hungry, as a way of dealing with anger, sadness or other emotions.
  
  
• Medical conditions. An overproduction of hormones by the adrenal glands (Cushing’s syndrome), low thyroid function (hypothyroidism), polycystic ovarian syndrome, low metabolic rate or other medical problems can all lead to obesity. However these conditions account for a small percentage of all cases of obesity.
  
  
• Physiological influences. Metabolic rates vary among individuals. This means that two people with the same height, weight and body type can require a different number of calories to maintain a normal body weight. A person with a slower metabolism is more likely to have a weight problem.
  
  
• Pregnancy. Some women who gain weight during pregnancy may find it difficult to lose the weight afterward. In some cases, this weight provides the foundation for additional weight gain that results in obesity.
  
  
• Quitting smoking. People who quit smoking often gain weight that eventually can lead to obesity. This may be partially due to nicotine’s ability to raise a person’s metabolic rate and the fact that foods often taste better once a person stops smoking and their sense of taste and smell is no longer impaired. People who quit smoking may also turn to food to fill the smoking void, resulting in weight gain. However, despite this weight gain, most experts agree that the risks of smoking far outweigh any risks associated with weight gain following quitting.

Studies continue to look at other potential causes of obesity. In particular, researchers are devoting much time to uncovering the factors responsible for the surge in childhood obesity. For example, researchers have found growing evidence of a link between a mother’s prompting her infant to eat and eventual childhood obesity. One recent study found that when mothers ask their children to take another bite of a food, the child complies two-thirds of the time, even if the child may be full.

Other research into childhood obesity is looking at how gene mutations may cause a failure in regulation of leptin, the so-called “fat hormone” that regulates energy caloric intake in the body. Experts are also examining the role that lack of sleep may play in disturbing metabolism in children, which may trigger obesity.

Researchers also continue to look at risk factors that make obesity more likely. For example, a recent study concluded that obesity is more likely in people who live in rural areas and report feeling isolated from recreational facilities, stores, churches and schools.

Body mass index (BMI) is the standard most often used to determine if a person is overweight or obese. BMI is a number that shows body weight adjusted for height. It is a metric measure used to estimate the amount of body fat the person carries. It is arrived at by dividing weight by height. Using standard measurement, divide weight (in pounds) by height (in inches squared) and multiply by 705. Using the metric system, divide weight (in kilograms) by height (in meters squared) and multiply by 705.

For example, a man who weighs 250 pounds and is 6 feet tall would have a BMI of 33.9. This is 250 divided by 5,184 (which is 72 inches squared) and multiplied by 705.

In general, overweight is defined as a BMI greater than or equal to 25, and obesity is a BMI greater than or equal to 30. BMI ranges include the following classifications, according to the National Institutes of Health (NIH):

The BMI standard of measurement was developed by the NIH in 1998 in an effort to define classes of excess body weight and obesity. Most physicians believe that BMI offers a better way to calculate body fat and the associated health risks than the previous standard of using height and weight tables.

However, BMI is only an estimate of body fat and does not take a number of factors into account, including the amount of muscle mass. Extremely muscular individuals (e.g., body builders, weightlifters, athletes) may have a high BMI but are not obese and do not have increased risks for disease. Conversely, BMI may not provide an accurate measurement of people who have high concentrations of abdominal fat or those with low muscle mass (e.g., the elderly).

Waist measurement may be used in combination with BMI to calculate a person's risk for diabetes. The following chart provides healthy versus obese ranges for men and women:

A third tool for estimating body fat is the waist-to-hip ratio. This ratio is determined by measuring the circumference of the waist and dividing it by the circumference of the hips. If the stomach measurement is 1.5 times larger than the hip measurement, indicating abdominal or visceral fat (android obesity), the risk of various diseases (e.g., diabetes, cardiovascular disease) is increased.

These are general tools most health practitioners can use to gauge risk. Other tools provide more precise information but may be expensive or impractical for many patients:

• Bioelectrical impedance analysis (BIA). A noninvasive electrical current is sent through the body and conductivity is measured. The higher the conduction, the more muscle and lean tissue.
  
  
• Bodpod. A test that uses a computerized, oval chamber to measure a person’s volume and mass. The person’s whole-body density can then be calculated, including fat and muscle percentages.
  
  
• DEXA (dual energy x-ray apsortiometry) x-ray. Often used to measure bone density, the DEXA x-rays also show the percentage of body fat, as well as where and how much fat a person has.
  
  
• Skinfold measurements. Skin calipers measure the thickness of skin and subcutaneous fat just beneath the skin on the back of the arm, waist or thigh.
  
  
• Underwater weighing (hydrostatic weighing). The patient is weighed underwater, showing how much lean body mass and body fat a person has.

A diagnosis of obesity comes from calculating BMI. A BMI equal to or greater than 30 is considered obese, which is defined as having an excess amount of body fat.

People who are obese usually can gain significant health benefits by losing weight. The key is to change the person’s energy balance so that energy output exceeds the energy intake. When this occurs the size of the person’s fat cells shrinks, leading to modest but significant weight loss.

Even losing as little as 5 percent to 7 percent of total body weight can result in a dramatic decrease in risk for health conditions such as diabetes, cardiovascular disease and stroke. A relatively small weight loss can also lower blood pressure, total and low-density lipoprotein (LDL, so-called “bad”) cholesterol levels, triglyceride levels and blood sugar. In addition, modest weight loss may cause high-density lipoprotein (HDLs, so-called “good”) cholesterol to increase.

It is important to note that people who are obese often have trouble losing significant amounts of weight and keeping the weight off. When an obese person loses weight, the size of fat cells shrinks, but the number of fat cells does not diminish. Over time, many of the shrunken fat cells may fill again, leading to weight gain. It is for this reason that people with an average number of fat cells tend to have a better chance of maintaining weight loss than those who have higher numbers of fat cells.

In addition, research indicates that each person has an inner mechanism that establishes an individual’s weight or body composition at a “set point.” When a person gains or loses weight, the body’s metabolism adjusts to restore the person’s weight to its original level.

Still, people who are obese can often maintain modest weight loss by adhering to a healthier diet and engaging in regular physical activity. Modest weight loss of one-half to 2 pounds a week (0.23 to 0.91 kilograms) is often easier to maintain over time than sudden and massive weight loss. And even people who have trouble losing weight can often prevent further weight gain by adopting healthier lifestyle habits. These habits can reduce the disease risks associated with obesity.

Most physicians agree that management and treatment of obesity requires permanent changes to the diet. In order to lose weight, numerous options are available, including:

• A well-balanced diet. For weight loss to occur, people must take in less energy (the calories taken in from the diet) than they use or burn (the calories used in daily activities and exercise). To get rid of one pound of body fat requires using or burning an additional 3,500 calories. For example, a deficit of 500 calories a day can result in losing a pound a week. For most people, a balanced diet containing no more than 2,000 calories a day for women and 2,500 for men is recommended. Crash diets are not recommended as they can end up leading to additional health problems including nutrition deficiencies.
  
  
• Regular exercise. Exercise without an associated change in diet will likely reduce weight by only a few pounds. However, people who exercise regularly are more likely to adhere to a change in diet, both of which help to prevent further weight gain. Physical exercise of at least 150 minutes a week along with a balanced diet is crucial for long-term weight loss. Aerobic exercise, such as walking, swimming or stair climbing, can help to keep weight off while at the same time providing many other benefits to an individual’s overall health and well-being. Even if people exercise regularly and do not lose weight, they can reduce abdominal obesity and the chances of developing cardiovascular disease, insulin resistance and high blood pressure.
  
  
• Counseling. Several studies have confirmed that obese patients who receive nutritional counseling from a dietitian achieve better weight-loss outcomes, including better metabolic control and reduced waist circumferences. In addition, some people have trouble controlling food intake not because of hunger, but because of emotional needs. People who use food to fill a feeling of emptiness, to comfort themselves or to gain a sense of control may benefit from speaking to a qualified counselor about the situation. Inpatient programs are available, in which people stay overnight as they relearn to eat only when hungry and to satisfy emotional needs through other means.
  
  
• Diet pills. Some diet pills are available over the counter, and others require a physician’s prescription. Over-the-counter medications may contain ingredients that increase the heart rate and/or blood pressure or cause other side effects. In addition, their ability to help reduce body weight has not been proven. Anyone considering use of these drugs should discuss the potential risks with a physician. To date, benzocaine is the only over-the-counter diet pill approved by the U.S. Food and Drug Administration (FDA) for weight loss. It anesthetizes the tongue, reducing taste sensations. Herbal supplements that claim to promote weight loss also appear to be largely ineffective. Prescription diet pills may be recommended for some patients. However, most people who use them regain the weight when they stop taking the medication. Also, the long-term effects of taking these drugs are not known.
  
  
• Other medications. If a healthy diet, exercise, stress management and counseling are ineffective in managing weight, medications may be prescribed. One drug (sibutramine) used to treat obesity affects levels of certain hormones (e.g., serotonin and noradrenaline) in areas of the brain associated with food intake and satisfaction. This temporarily decreases the urge to eat and, with appropriate diet and exercise, supports weight loss efforts. The prescription medication orlistat is used in combination with a low-fat diet to help lose weight and maintain the loss by blocking the absorption of dietary fat. There are some drawbacks to obesity medications. These drugs can produce serious side effects, such as high blood pressure in the blood vessels of the lungs and valvular heart disease. The medications should be used by patients whose treatments are closely monitored by a qualified physician.
  
  
• Surgical intervention. In certain cases, surgery may be used to treat people with clinically severe obesity who have not lost weight by other medical therapies. One surgical technique is gastric bypass surgery, in which part of the small intestine is bypassed as food passes out of the stomach. The gastric bypass involves significant changes in how much food and drink can be consumed, and people are encouraged to learn all about the surgery before having it done. For all surgical candidates, an integrated medical program must be in place to provide guidance on diet, physical activity and support before and after the surgery. Not all people are good candidates for surgery.
  
  
• Avoiding smoking and alcohol use. These unhealthy strategies are often used to relieve stress. They do not solve whatever problem is at hand, and often make situations worse (e.g., by causing new health problems). By using healthy stress management techniques instead of using alcohol excessively or smoking, individuals can reach weight loss goals more quickly.

Prevention of obesity requires a healthy lifestyle, including a well–balanced diet that is high in fiber, and low in fat and refined sugars. Time after time, studies have shown that weight loss and exercise can significantly reduce the risk of developing serious diseases.

Lifelong patterns of eating and activity are often established during childhood. Therefore, parents are urged to provide their children with healthy meals and to encourage an active lifestyle. Parents often underestimate the health effects of obesity on children, according to the American Obesity Association. In many cases, simply increasing the amount of physical activity can help children to achieve a healthy weight without dieting, which can deprive them of nutrients and calories that are essential to growth and development.

Preparing questions in advance can help patients to have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to obesity:

1. What is my body mass index? What should it be?
   
   
2. Am I overweight or obese?
   
   
3. What is my ideal body weight?
   
   
4. What are the risks of being too heavy?
   
   
5. How can I best achieve and maintain my ideal weight?
   
   
6. Should I follow a special diet?
   
   
7. Are there risks if I lose weight too fast or with certain diets?
   
   
8. How much weight should I be losing each week or month?
   
   
9. How much exercise should I get?
   
   
10. Which activities should I do, and are there any I should avoid?
    
    
11. Is my condition genetic? What steps can I take to protect my children?