Although there are genetic, behavioral, metabolic and hormonal influences on body weight, obesity occurs when you take in more calories than you burn through normal daily activities and exercise. Your body stores these excess calories as fat.

In the United States, most people’s diets are too high in calories — often from fast food and high-calorie beverages. People with obesity might eat more calories before feeling full, feel hungry sooner, or eat more due to stress or anxiety.

Many people who live in Western countries now have jobs that are much less physically demanding, so they don’t tend to burn as many calories at work. Even daily activities use fewer calories, courtesy of conveniences such as remote controls, escalators, online shopping and drive-through banks.

Risk factors

Obesity usually results from a combination of causes and contributing factors:

Family inheritance and influences

The genes you inherit from your parents may affect the amount of body fat you store, and where that fat is distributed. Genetics may also play a role in how efficiently your body converts food into energy, how your body regulates your appetite and how your body burns calories during exercise.

Obesity tends to run in families. That’s not just because of the genes they share. Family members also tend to share similar eating and activity habits.

Lifestyle choices

• Unhealthy diet. A diet that’s high in calories, lacking in fruits and vegetables, full of fast food, and laden with high-calorie beverages and oversized portions contributes to weight gain.
• Liquid calories. People can drink many calories without feeling full, especially calories from alcohol. Other high-calorie beverages, such as sugared soft drinks, can contribute to significant weight gain.
• Inactivity. If you have a sedentary lifestyle, you can easily take in more calories every day than you burn through exercise and routine daily activities. Looking at computer, tablet and phone screens is a sedentary activity. The number of hours spent in front of a screen is highly associated with weight gain.

Certain diseases and medications

In some people, obesity can be traced to a medical cause, such as Prader-Willi syndrome, Cushing syndrome and other conditions. Medical problems, such as arthritis, also can lead to decreased activity, which may result in weight gain.

Some medications can lead to weight gain if you don’t compensate through diet or activity. These medications include some antidepressants, anti-seizure medications, diabetes medications, antipsychotic medications, steroids and beta blockers.

Social and economic issues

Social and economic factors are linked to obesity. Avoiding obesity is difficult if you don’t have safe areas to walk or exercise. Similarly, you may not have been taught healthy ways of cooking, or you may not have access to healthier foods. In addition, the people you spend time with may influence your weight — you’re more likely to develop obesity if you have friends or relatives with obesity.

Age

Obesity can occur at any age, even in young children. But as you age, hormonal changes and a less active lifestyle increase your risk of obesity. In addition, the amount of muscle in your body tends to decrease with age. Generally, lower muscle mass leads to a decrease in metabolism. These changes also reduce calorie needs and can make it harder to keep off excess weight. If you don’t consciously control what you eat and become more physically active as you age, you’ll likely gain weight.

Other factors

• Pregnancy. Weight gain is common during pregnancy. Some women find this weight difficult to lose after the baby is born. This weight gain may contribute to the development of obesity in women.
• Quitting smoking. Quitting smoking is often associated with weight gain. And for some, it can lead to enough weight gain to qualify as obesity. Often, this happens as people use food to cope with smoking withdrawal. In the long run, however, quitting smoking is still a greater benefit to your health than is continuing to smoke. Your doctor can help you prevent weight gain after quitting smoking.
• Lack of sleep. Not getting enough sleep or getting too much sleep can cause changes in hormones that increase appetite. You may also crave foods high in calories and carbohydrates, which can contribute to weight gain.
• Stress. Many external factors that affect mood and well-being may contribute to obesity. People often seek more high-calorie food when experiencing stressful situations.
• Microbiome. Your gut bacteria are affected by what you eat and may contribute to weight gain or difficulty losing weight.

Even if you have one or more of these risk factors, it doesn’t mean that you’re destined to develop obesity. You can counteract most risk factors through diet, physical activity and exercise, and behavior changes.

Complications

People with obesity are more likely to develop a number of potentially serious health problems, including:

• Heart disease and strokes. Obesity makes you more likely to have high blood pressure and abnormal cholesterol levels, which are risk factors for heart disease and strokes.
• Type 2 diabetes. Obesity can affect the way the body uses insulin to control blood sugar levels. This raises the risk of insulin resistance and diabetes.
• Certain cancers. Obesity may increase the risk of cancer of the uterus, cervix, endometrium, ovary, breast, colon, rectum, esophagus, liver, gallbladder, pancreas, kidney and prostate.
• Digestive problems. Obesity increases the likelihood of developing heartburn, gallbladder disease and liver problems.
• Sleep apnea. People with obesity are more likely to have sleep apnea, a potentially serious disorder in which breathing repeatedly stops and starts during sleep.
• Osteoarthritis. Obesity increases the stress placed on weight-bearing joints, in addition to promoting inflammation within the body. These factors may lead to complications such as osteoarthritis.