Obesity refers to having an excess amount of body fat and is most commonly diagnosed using a body mass index (BMI) score. A BMI score is calculated by dividing a person’s weight in kilograms by the square of their height in meters. A BMI score above 30 corresponds with a diagnosis of obesity.

Obesity is complex and has many causes. These include genetic factors, dietary choices, certain health conditions, hormonal imbalances, stress, lack of sleep, limited mobility, low access to health care, a history of repeatedly losing and regaining weight, older age, pregnancy, and taking certain medications.

It’s important to note that while science is good at finding correlations, or apparent relationships, between factors and health conditions, correlation does not prove that the factor causes the condition. Behavior, genetics, and environmental factors can influence a person’s risk of developing obesity, but it’s difficult to know how influential any one factor is on an individual’s weight.

Older Age

Age is an important factor in body size for many people. Although people of any age — even children — can be diagnosed with obesity, weight gain becomes more common with age. As people get older, their hormone levels change and they often become less active.

Behavioral Factors

A lifestyle that includes high-calorie foods and little physical activity has been associated with higher body weight. Foods high in sugar and fat keep calorie intake high. A sedentary lifestyle — spending most of your time sitting, driving, or lying down — uses few of those calories. It’s important to note that not everyone has access to safe places and free time for physical activity — or the ability and energy to exercise regularly. Similarly, food choices may be limited by one’s finances, schedule, or access to healthy, fresh options.

Getting too little sleep and facing a lot of stress have also been identified as factors in weight gain.

Quitting smoking can also lead to weight gain. A gain of 5 to 10 pounds within months of quitting is common, but some people gain more. If you stop smoking, it lowers the risk of many serious diseases, so it’s better for your health regardless of your weight.

Hereditary Factors

Scientists have identified more than 400 genes believed to influence body weight. Your genes help determine your appetite, your metabolism (how quickly the body burns calories), the foods you crave, and how much food it takes to make you feel full. About 85 percent of people have “thrifty” genes that are geared to store fat and conserve energy. Although thrifty genes likely helped our ancestors survive during times of famine, they make it easier to gain weight in the modern world.

Researchers think genetic factors make some people more likely to have a BMI in the obese range than others. For some people, their genes make up 25 percent of the risk for obesity. In others, genetic inheritance may contribute between 70 percent and 80 percent of what leads to obesity. If you have been in a larger body for most of your life and one or both of your parents or close relatives have similar body types, your genes may be a significant contributor to your body weight.

A person’s sex and ethnic background also influence obesity. According to the National Institute of Diabetes and Digestive and Kidney Diseases, men are more likely to be overweight than women, but women are more likely to have BMI scores in the severely obese range than men. In the U.S. in 2020, 50 percent of non-Hispanic Black adults, 46 percent of Hispanic adults, 41 percent of non-Hispanic white adults, and 16 percent of non-Hispanic Asian adults were considered obese.

Environmental Factors

The family and culture in which a person grows up often set the stage for how they eat and pass their time as adults. If your family consumed a lot of high-calorie foods and was not regularly active, you might develop similar habits.

Socioeconomic factors also influence body size. It’s difficult to eat a nutritious and balanced diet without access to a grocery store with fresh produce and other healthy choices. Fresh foods can be more expensive than processed foods and can take longer to cook. Without education about nutrition or skills in meal planning and cooking, it can be daunting to choose and prepare foods that support good health.

A history of weight loss efforts and food restriction can also play a role. Some research shows that weight cycling from repeated dieting or disordered eating can cause some people to gain weight over time.

If you have chronic health conditions that cause fatigue, pain, or mobility problems, it can be challenging to get enough physical activity. Where you live also matters. It’s difficult to get exercise if there are no sidewalks or parks in your neighborhood, if you can’t afford a gym membership, or if it’s dangerous to go for a walk or ride a bike. These factors can all influence body size.

Stress can be a factor in weight gain. Hormones released during chronic stress have been shown to stimulate hunger and encourage the choice of high-calorie foods, which act on the brain to lessen the feeling of stress. This cycle, known as the hypothalamic-pituitary-adrenal axis, can lead to eating as a way to cope with stress.

Marketing and advertising can make it confusing to figure out which foods are actually healthy. For instance, many products advertised as low-fat contain extra sugar that makes them even higher in calories than the full-fat version. Many products marketed by trendy diets promise weight loss and other benefits they fail to deliver. Meanwhile, sugary soft drinks, fast food, and high-calorie snacks are often convenient and inexpensive.

Which Health Conditions Can Cause Weight Gain?

Weight gain is a common symptom of several chronic health conditions. Your doctor may look for one or more of these conditions if you have other related symptoms:

• Hypothyroidism (underactive thyroid gland)
• Cushing’s syndrome
• Depression
• Polycystic ovary syndrome
• Prader-Willi syndrome

Which Medications Can Cause Weight Gain?

Some medications, including those taken long-term for serious chronic health conditions, can cause weight gain as a side effect. These include:

• Corticosteroids, such as prednisone and methylprednisolone
• Insulin and some diabetes medications
• Some antidepressants, such as amitriptyline, citalopram (Celexa), fluoxetine (Prozac), and sertraline (Zoloft)
• Seizure and migraine drugs, such as carbamazepine, gabapentin (Neurontin), and pregabalin (Lyrica)
• High blood pressure medications, such as metoprolol, nisoldipine (Sular), and propranolol
• Hormonal contraceptives
• Antipsychotic drugs, including haloperidol, quetiapine (Seroquel), and risperidone (Risperdal)

Find Your Team

On MyObesityTeam, the social network and online support group for people with obesity and their loved ones, more than 54,000 members come together to share experiences, seek advice, and offer support. Have you been diagnosed with obesity? What steps do you take to manage your health? Join the conversation today to share your experience and connect with others on MyObesityTeam.