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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.