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Obesity affects more than one-third of adults and nearly one-fifth of children in the United States. It is a condition in which a person has a high proportion of body fat for his or her height. Over time, excess body fat can contribute to a variety of serious health conditions, including coronary artery disease, hypertension, type 2 diabetes, high cholesterol, obstructive sleep apnea, osteoarthritis of the knees and other joints, and fertility problems. In addition, obesity increases the likelihood of needing surgery to repair weight-related health conditions, such as worn joints, hernia, and narrowed blood vessels.

Excess weight and fat accumulate when there is an imbalance in the amount of calories—the energy derived from food—that a person consumes and the amount burned through normal body functions and physical activity. Most often, this imbalance occurs in people who overeat and do not—or cannot—exercise enough, causing the body to store extra calories as fat.

Obesity tends to run in families. This is partly due to genetic factors. It can also be explained by the diet and exercise habits of parents, who tend to influence their children’s habits.

Other factors—such as the widespread availability of fast food and processed foods; being more sedentary at work and at home; and not having enough time to exercise or cook healthy, nutritious meals—have also contributed to higher rates of obesity in the United States in recent decades.

People with a medical condition that limits physical activity, such as osteoarthritis, an orthopedic injury, or muscular dystrophy, may be more likely to gain weight and have a harder time losing it. Certain hormone conditions can also lead to excessive weight gain, including hypothyroidism, in which the thyroid gland does not produce enough thyroid hormone; Cushing's disease, which increases the production of the hormone cortisol; and polycystic ovary syndrome, which can contribute to the body’s resistance to the hormone insulin.

Some medications, such as corticosteroids, can disrupt the body’s hormone balance, which can lead to weight gain. Others, such as antidepressants, block signals in the brain that tell people when they’re full.

Body Mass Index

To diagnose obesity, doctors at �ٺ���Ƶ use a widely recognized formula called body mass index, or BMI. This is calculated based on a person’s height-to-weight ratio. For most people, BMI is a reliable indicator of body fat.

A BMI of 18.5 to 24.9 is considered healthy, whereas a BMI of 25 to 29.9 indicates that a person is overweight. People with a BMI of 30 or greater are obese. People who are seriously obese have a BMI of 40 or greater.

Fat Distribution

Doctors at �ٺ���Ƶ also measure waist circumference, which gives more information about a person’s risk of developing obesity-related health conditions. Research has shown that people who are “apple-shaped” and carry most of their body fat around the abdomen are at greater risk for conditions such as heart disease.

This is because abdominal fat is thought to break down more easily into fatty acids, which increase the production of LDL, or “bad,” cholesterol, and triglycerides, a type of fat in the blood that contributes to heart disease. Abdominal fat can also cause insulin, a hormone that controls blood sugar, to be less effective, which raises the risk of developing diabetes.

If you receive a diagnosis of obesity, you and your doctor can come up with a weight loss goal and create an action plan for achieving it. If your doctor determines that other medical conditions may be contributing to weight gain, he or she can refer you to other �ٺ���Ƶ specialists who can help.