Overweight and obesity together make up one of the leading preventable causes of death in the U.S. Obesity is a chronic disease that can seriously affect your health.

Overweight means that you have extra body weight. Obesity means having a high amount of extra body fat. Being overweight or having obesity raises your risk for health problems. These include:

• Coronary heart disease.
• Type 2 diabetes.
• Asthma.
• High cholesterol.
• Osteoarthritis.
• High blood pressure.
• Sleep apnea.
• Some types of cancer.

Public health experts agree that overweight and obesity have reached epidemic proportions in this country and around the world. More than one-third of U.S. adults have obesity. People ages 60 and older are more likely to have obesity than younger adults, according to the most recent data from the National Health and Nutrition Examination Survey. And the problem also affects children. Approximately 20% of U.S. children and teens ages 2 to 19 have obesity.

Overweight and obesity are different points on a scale that ranges from being underweight to being morbidly obese. Where you fit on this scale is determined by your body mass index (BMI).

BMI is a measure of your weight as it relates to your height. BMI often gives you a good idea of the amount of body fat you have. Your doctor uses BMI to find out your risk for obesity-related diseases. Sometimes some very muscular people may have a BMI in the overweight range. But these people are not considered overweight because muscle tissue weighs more than fat tissue.

In general, a BMI from 20 to 24.9 in adults is considered ideal. A BMI between 25 and 29.9 is considered overweight. A person is considered to have obesity if the BMI is 30 or higher.

In general, after the age of 50, the weight of a person who is biologically male tends to stay the same and often decreases slightly between ages 60 and 74. In contrast, the weight of a person who is biologically female tends to increase until age 60, and then begins to decrease.

Obesity can also be measured by waist-to-hip ratio. This is a measurement tool that looks at the amount of fat on your waist, compared with the amount of fat on your hips and buttocks. The waist circumference tells the amount of stomach fat. Increased stomach fat is linked to type 2 diabetes, high cholesterol, high blood pressure, and heart disease. A waist circumference of more than 40 inches in people who are biologically male and more than 35 inches in people who are biologically female may increase the risk for heart disease and other diseases tied to being overweight.

Talk with your doctor if you have questions about healthy body weight.

In many ways, obesity is a puzzling disease. Experts don't know exactly how your body regulates your weight and body fat. What they do know is that a person who eats more calories than they use for energy each day will gain weight.

But the risk factors that determine obesity can be complex. They are often a combination of your genes, socioeconomic factors, metabolism, and lifestyle choices. Some endocrine disorders, diseases, and medicines may also affect a person's weight.

Factors that may affect obesity include the following.

Studies show that the likelihood of developing obesity is passed down through a family's genes. Researchers have found several genes that seem to be linked with obesity. Genes, for instance, may affect where you store extra fat in your body. But most researchers think that it takes more than just 1 gene to cause an obesity epidemic. They are continuing to do more research to better understand how genes and lifestyle interact to cause obesity. Because families eat meals together and share other activities, environment and lifestyle also play a role.

How your body uses energy is different from how another person's uses it. Metabolism and hormones differ from person to person. And these factors play a role in how much weight you gain. One example is ghrelin, the hunger hormone that regulates appetite. Researchers have found that ghrelin may help set off hunger. Another hormone called leptin can decrease appetite. Another example is polyendocrine metabolic ovarian syndrome (PMOS), a condition caused by high levels of certain hormones. A person with PMOS is more likely to have obesity.

How much money you make may affect whether you have obesity. This is especially true for those who are biologically female. Those who are poor and of lower social status are more likely to have obesity than those of higher socioeconomic status. This is especially true among minority groups.

Overeating and a lack of exercise both contribute to obesity. But you can change these lifestyle choices. If many of your calories come from refined foods or foods high in sugar or fat, you will likely gain weight. If you don't get much if any exercise, you'll find it hard to lose weight or maintain a weight that's healthy for you.

Medicines like corticosteroids, beta-blockers, some antidepressants, and antiseizure medicines can cause you to gain some extra weight.

Emotional eating (eating when you're bored or upset) can lead to weight gain. Too little sleep may also contribute to weight gain. People who sleep fewer than 5 hours a night are more likely to have obesity than people who get 7 to 8 hours of sleep a night.

Obesity has a far-ranging negative effect on health. Each year in the U.S., obesity-related conditions cost more than $150 billion and cause premature deaths. The health effects linked with obesity include:

Excess weight needs more blood to circulate to the fat tissue. It can also causes the blood vessels to become narrow (coronary artery disease). This makes the heart work harder because it must pump more blood against more resistance from the blood vessels and can lead to a heart attack (myocardial infarction). More circulating blood and more resistance also means more pressure on the artery walls. Higher pressure on the artery walls increases the blood pressure. Excess weight also raises blood cholesterol and triglyceride levels and lowers HDL (good) cholesterol levels, adding to the risk of heart disease.

Obesity is the major cause of type 2 diabetes. Obesity can make your body resistant to insulin, the hormone that regulates blood sugar. When obesity causes insulin resistance, your blood sugar level rises. Even moderate obesity dramatically increases the risk for diabetes.

Atherosclerosis, or hardening of the arteries, happens more often in people who have obesity. Coronary artery disease is also more common in people who have obesity because fatty deposits build up in arteries that supply the heart. Narrowed arteries and reduced blood flow to the heart can cause chest pain called angina or a heart attack. Blood clots can also form in narrowed arteries and travel to the brain, causing a stroke.

Obesity can affect the knees and hips because extra weight stresses the joints. Joint replacement surgery may not be a good choice for a person who has obesity. That's because the artificial joint has a higher risk of loosening and causing more damage.

Sleep apnea causes people to stop breathing for brief periods during sleep. Sleep apnea interrupts sleep. It causes sleepiness during the day. It also causes heavy snoring. Sleep apnea is also linked to high blood pressure, increased risk for heart disease, stroke, diabetes, and is linked to early death. Breathing problems tied to obesity happen when added weight of the chest wall squeezes the lungs. This restricts breathing.

Being overweight or having obesity increases your risk for a variety of cancers, according to the American Cancer Society. Among people who have obesity, the risk increases for cancer of the endometrium or the lining of the uterus. People who have obesity also have and increased risk for breast cancers in those who have gone through menopause. People who are biologically male and who are overweight have a higher risk for prostate cancer. People who have obesity are at increased risk for colorectal cancer.

The National Cholesterol Education Program says that metabolic syndrome is a risk factor for cardiovascular disease. Metabolic syndrome has several major risk factors. These are stomach obesity, high blood triglyceride levels, low HDL cholesterol levels, high blood pressure, and insulin resistance (severe type 2 diabetes). Having at least 3 of these risk factors confirms the diagnosis of metabolic syndrome.

People who are overweight or who have obesity can have problems socially or psychologically. This is because the culture in the U.S. often values a body image that's overly thin. People who are overweight or who have obesity are often blamed for their condition. Other people may think of them as lazy or weak-willed. It's not uncommon for people who are overweight or who have obesity to earn less than other people. Or to have fewer or no romantic relationships. Some people's disapproval and bias against those who are overweight may lead to discrimination and even bullying. Depression and anxiety are more common in people who are overweight or who have obesity.