Obesity is a long-term (chronic) health condition that progresses over time. Obesity is defined by excess body fat (adipose tissue) that may impair health.

Body mass index (BMI) is a calculation that takes a person’s weight and height into account to measure body size. Doctors typically use it as a screening tool for obesity.

In adults, obesity is often defined as having a BMI of 30 or more, according to the Centers for Disease Control and Prevention (CDC). Obesity is associated with a higher risk of developing serious diseases, including:

  • type 2 diabetes
  • heart disease
  • cancer

While BMI tends to relate to the level of body fat, it has some limitations as a measurement.

According to the CDC, “Factors such as age, sex, ethnicity, and muscle mass can influence the relationship between BMI and body fat. Also, BMI doesn’t distinguish between excess fat, muscle, or bone mass, nor does it provide any indication of the distribution of fat among individuals.”

Despite these limitations, BMI continues to be widely used as a way to measure body size. This is because it’s less expensive than other methods.

It’s worth noting that the effects of weight discrimination can also contribute to negative health effects.

Obesity is common. The CDC estimates that 41.9% of people in the United States had obesity from 2017 to March 2020.

Keep reading to learn more about obesity causes, risk factors, and treatment.

Language matters

You’ll notice that the language used to share stats and other data points is pretty binary, fluctuating between the use of “men” and “women.”

Although we typically avoid language like this, specificity is key when reporting on research participants and clinical findings.

Unfortunately, the studies and surveys referenced in this article didn’t report data on, or include, participants who were transgender, nonbinary, gender nonconforming, genderqueer, agender, or genderless.

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There are no specific symptoms associated with obesity. A doctor may diagnose obesity based on the following factors:

  • excess amounts of abdominal (visceral) fat that are higher than the amounts of body fat in other areas
  • a waist circumference of greater than 40 inches for men or 35 inches for women
  • a BMI over 30

The following classes are used for adults who are at least 20 years old:

18.5 or underunderweight
18.5 to <25“normal” weight
25 to <30overweight
30 to <35class 1 obesity
35 to <40class 2 obesity
40 or overclass 3 obesity (also known as morbid, extreme, or severe obesity)

What is childhood obesity?

For a doctor to diagnose obesity in a child over 2 years old or a teen, their BMI has to be at or above the 95th percentile. A BMI at or above the 95th percentile is higher than the BMI of 95% of people with the same age and sex assigned at birth.

Percentile range of BMIClass
5% to <85%“normal” weight
85% to <95%overweight
95% or overobesity

From 2015 to 2016, 18.5% (or about 13.7 million) of U.S. youth between 2 and 19 years old were considered to have clinical obesity.

Taking in more calories than you burn in daily activity and exercise — on a long-term basis — can lead to obesity. Over time, these extra calories add up and cause weight gain.

But it’s not always just about calories in and calories out or having a sedentary lifestyle. While those are indeed causes of obesity, some causes you can’t control.

Common specific causes of obesity include:

  • genetics, which can affect how your body processes food into energy and how fat is stored
  • growing older, which can lead to less muscle mass and a slower metabolic rate, making it easier to gain weight
  • not sleeping enough, which can lead to hormonal changes that make you feel hungrier and crave certain high calorie foods
  • high stress, which may trigger the production of hormones that cause you to eat more and store more fat
  • pregnancy, as weight gained during pregnancy may be difficult to lose and might eventually lead to obesity

Certain health conditions can also lead to weight gain, which may lead to obesity. These include:

A complex mix of factors can increase a person’s risk of obesity.


Some people have genes that make it more likely for them to gain weight and body fat.

Environment and community

Your environment at home, at school, and in your community can all influence how and what you eat, as well as how active you are.

You may be at a higher risk of developing obesity if you:

Psychological and other factors

Depression can sometimes lead to weight gain, as some people may turn to food for emotional comfort.

Having disturbed sleeping patterns can make you eat more during the day, especially foods high in fat and carbohydrates.

If you smoke, quitting smoking is beneficial to your health, but quitting may lead to weight gain too. In some people, it may lead to excessive weight gain. For that reason, it’s important to focus on diet and exercise while you’re quitting, at least after the initial withdrawal period.


Certain medications can also raise your risk of weight gain. These medications can include:

BMI provides a rough calculation of a person’s weight in relation to their height.

Other more accurate measures of body fat and where body fat is located include:

A doctor may also order certain tests to help diagnose obesity-related health risks. These may include:

A measurement of the fat around your waist is also a good predictor of your risk of developing obesity-related diseases.

Obesity can lead to more than weight gain.

Having a high ratio of body fat to muscle puts strain on your bones as well as your internal organs. It also increases inflammation in the body, which may be a risk factor for cancer. Obesity is also a major risk factor for type 2 diabetes.

Researchers have linked obesity to many health complications, some of which can be life threatening if not treated:

If you have obesity and would like to lose weight but have been unable to do so on your own, medical help is available. Start with a primary care physician, who may be able to refer you to a weight specialist in your area.

A doctor may also want to work with you as part of a team helping you lose weight. That team might include a dietitian, therapist, or other healthcare staff.

The doctor will work with you on making needed lifestyle changes. Sometimes, they may recommend medications or weight loss surgery as well. Learn more about treatment for obesity.

Doctors typically recommend lifestyle and behavioral changes to help children with obesity lose weight. In some cases, doctors may prescribe medication.

A healthcare team can recommend food choices and help develop a nutritious eating plan that works for you. The National Institute of Diabetes and Digestive and Kidney Diseases suggests that an eating plan that you can stick to long-term may help you lose weight and maintain a moderate weight.

A structured exercise program and increased daily activity — between 150 and 300 minutes a week — will help build up your strength, endurance, and metabolism.

Counseling or support groups may also identify triggers and help you cope with issues relating to:

A doctor may also prescribe certain prescription weight loss medications in addition to eating and exercise plans.

Doctors may prescribe medications if other methods of weight loss haven’t worked and if you have a BMI of 27 or more in addition to obesity-related health issues.

Prescription weight loss medications either prevent the absorption of fat or suppress appetite. The Food and Drug Administration (FDA) has approved the following medications for short-term use (up to 12 weeks):

These drugs may have unpleasant side effects for some people but may have more serious side effects for others. For example, orlistat can lead to the unpleasant side effects of oily and frequent bowel movements, bowel urgency, and gas. But liraglutide and semaglutide may increase the risk of thyroid cancer for people with a personal or family history.

It’s important to inform the prescribing doctor about your medical history before beginning any of these medications. The doctor will typically monitor you closely to manage side effects and reduce your risks.

Weight loss surgery is commonly called bariatric surgery.

This type of surgery limits how much food you can comfortably eat or prevents your body from absorbing food and calories. Sometimes it can do both.

Weight loss surgery isn’t a quick fix. It’s a major surgery and can have serious risks. Afterward, people who undergo surgery will need to change how they eat and how much they eat, or they risk getting sick.

However, nonsurgical options aren’t always effective at helping people with obesity lose weight and reduce their risk of comorbidities.

Types of weight loss surgery include:

  • Gastric bypass surgery: In this procedure, a surgeon creates a small pouch at the top of your stomach that connects directly to your small intestine. Food and liquids go through the pouch and into the intestine, bypassing most of the stomach. It’s also known as Roux-en-Y gastric bypass (RYGB) surgery.
  • Laparoscopic adjustable gastric banding (LAGB): LAGB separates your stomach into two pouches using a band.
  • Gastric sleeve surgery: This procedure removes part of your stomach.
  • Biliopancreatic diversion with duodenal switch: This procedure removes most of your stomach.

Candidates for surgery

For decades, experts recommended that adult candidates for weight loss surgery have a BMI of at least 35 (classes 2 and 3).

However, in 2018 guidelines, the American Society for Metabolic and Bariatric Surgery (ASMBS) endorsed weight loss surgery for adults with BMIs of 30 up to 35 (class 1) who:

  • have related comorbidities, especially type 2 diabetes
  • haven’t seen long-term results from nonsurgical treatments, such as eating and lifestyle changes

For individuals with class 1 obesity, surgery is most effective for those between the ages of 18 and 65 years.

Some people may have to lose weight before surgery can be approved. Additionally, they’ll normally undergo counseling to ensure that they’re both emotionally prepared for the surgery and the lifestyle changes that it’ll require.

Only a few surgical centers in the United States perform these types of procedures on children under 18 years old.

Due to a dramatic increase in obesity and obesity-related diseases, communities, states, and the federal government are putting an emphasis on healthier food choices and activities to help prevent obesity and treat people with overweight and obesity.

On a personal level, you can help prevent weight gain and obesity by:

Obesity is a chronic medical condition caused by excess visceral fat. It may raise your risk of developing multiple health conditions, including heart disease and diabetes.

Treatment can include lifestyle changes and medication. In some cases, it can involve surgery.