Body mass index (BMI) is a calculation that takes a person’s weight and height into account to measure body size.

In adults, obesity is defined as having a BMI of 30.0 or more, according to the Centers for Disease Control and Prevention (CDC).

Obesity is associated with a higher risk for serious diseases, such as type 2 diabetes, heart disease, and cancer.

Obesity is common. The CDC estimates that 42.4 percent of Americans 20 years old and older had obesity in 2017 to 2018.

But BMI isn’t everything. It has some limitations as a metric.

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.

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

18.5 or underunderweight
18.5 to <25.0“normal” weight
25.0 to <30.0overweight
30.0 to <35.0class 1 obesity
35.0 to <40.0class 2 obesity
40.0 or overclass 3 obesity (also known as morbid, extreme, or severe obesity)

For a doctor to diagnose a child over 2 years old or a teen with obesity, their BMI has to be in the 95th percentile for people of their same age and biological sex:

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

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

Eating 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
  • 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 for obesity.


Some people have genes that make it difficult for them to lose weight.

Environment and community

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

You may be at a higher risk for obesity if you:

Psychological and other factors

Depression can sometimes lead to weight gain, as some people may turn to food for emotional comfort. Certain antidepressants can also increase the risk of weight gain.

Quitting smoking is always a good thing, 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.

Medications, such as steroids or birth control pills, can also raise your risk for weight gain.

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

Other more accurate measures of body fat and body fat distribution include:

Your 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 for obesity-related diseases.

Obesity can lead to more than simple 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 is thought to be a risk factor for cancer. Obesity is also a major risk factor for type 2 diabetes.

Obesity has been linked to a number of health complications, some of which can be life threatening if not treated:

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

Your 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.

Your 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.

Your healthcare team can educate you on food choices and help develop a healthy eating plan that works for you.

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

Counseling or support groups may also identify unhealthy triggers and help you cope with any anxiety, depression, or emotional eating issues.

Lifestyle and behavioral changes are the preferred weight loss methods for children, unless they’re extremely overweight.

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

Medications are usually prescribed only if other methods of weight loss haven’t worked and if you have a BMI of 27.0 or more in addition to obesity-related health issues.

Prescription weight loss medications either prevent the absorption of fat or suppress appetite. The following are approved for long-term use (at least 12 weeks) by the Food and Drug Administration (FDA):

  • phentermine/topiramate (Qsymia)
  • naltrexone/bupropion (Contrave)
  • liraglutide (Saxenda)
  • orlistat (Alli, Xenical), the only one that’s FDA-approved for use in children 12 years old and older

These drugs can have unpleasant side effects. For example, orlistat can lead to oily and frequent bowel movements, bowel urgency, and gas.

Your doctor will monitor you closely while you’re taking these medications.

Weight loss surgery is commonly called bariatric surgery.

This type of surgery works by limiting how much food you can comfortably eat or by preventing 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 for comorbidities.

Types of weight loss surgery include:

  • Gastric bypass surgery. In this procedure, your 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 35.0 (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 30.0 up to 35.0 (class 1) who:

  • have related comorbidities, especially type 2 diabetes
  • haven’t seen sustained results from nonsurgical treatments, such as eating and lifestyle modifications

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

People will often have to lose some weight before undergoing surgery. Additionally, they’ll normally undergo counseling to ensure that they’re both emotionally prepared for the surgery and willing to make the necessary 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.

There’s been a dramatic increase in obesity and in obesity-related diseases in the last couple decades. This is the reason why communities, states, and the federal government are putting an emphasis on healthier food choices and activities to help turn the tide on obesity.

On a personal level, you can help prevent weight gain and obesity by making healthier lifestyle choices: